Progreso de la investigación en nanoestructuras de óxido de manganita y perovskita dopadas con tierras raras

Physical Properties of Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Rare Earth-Doped Perovskite Manganite Oxide Nanoparticles

Magnetic Properties

Superconducting quantum interference device (SQUID) magnetometer is the most powerful, sensitive, and widely used instrument for magnetic characterization in material science. This device works on the principal of quantum interference produced using Josephson junctions. This measurement system is used for d.c. magnetization and M vs. H measurements of the samples. For d.c. magnetization, a small external field is applied and χ is measured as a function of temperature at constant applied field. For M-H measurements, magnetization is measured at a constant temperature while magnetic field is varied up to a certain value of positive and negative applied field. The most common units for the magnetic moment is emu. The natural unit of the magnetization is thus emu/g or emu/cm ³ . If one can estimate the number of atom in the sample, then one can also calculated the magnetic moment per atom in μ_B .

The perovskite manganite oxide nanoparticles of La_1-x Sr_x MnO₃ are one of the most attractive rare earth-doped perovskite manganites, which exhibit a metallic nature, large bandwidth, and the Curie temperature (T _C ) as high as 300–370 K [199]. Their magnetic properties are influenced by many factors; the key ones include chemical compositions, the type and the degree of defectiveness of the crystal lattice, the particle size and morphology, the interactions between the particle, and the surrounding matrix and/or the neighboring particles. By changing the nanoparticle size, shape, composition ,and structure, one can control to an extent the magnetic characteristics of the nanoparticles. For example, Baaziz et al. [148] synthesized La_0.9 Sr_0.1 MnO₃ nanoparticles by the citrate-gel method and annealed them at 600 °C (H6), 800 °C (H8), 1000 °C (H10), and 1200 °C (H12), respectively. Their magnetization (M) versus temperature (T) curves measured under the applied magnetic field of 500 Oe are shown in Fig. 19a. The T _C was obtained from the inflection points in dM/dT as a function of temperature for all samples in the inset (Fig. 19i)). As shown in M–T curve, all samples exhibit a PM to FM transition at T _C upon cooling. The T _C dependent upon the particle sizes (d ) is shown as an inset (Fig. 19ii) in Fig. 19a, where it is clearly observed that T _C decreases from 250 to 210 K with increasing the particle size from 45 to 95 nm. The decrease of the Curie temperature with increasing the particle size can be ascribed to the strain effects of grains induced by the distortion at grain boundaries and the orthorhombic strains caused by the strong J–T coupling. It was also found that the saturation magnetizations (M_sat ) of the La_0.9 Sr_0.1 MnO₃ nanoparticles were increased with increasing the particle sizes, as shown in Fig. 19b. The reduction of M_sat in the small particles may be attributed to the loss of long-range ferromagnetic (FM) order in the smaller particle sized samples since the surface contribution is larger in this case. This can be explained in terms of a core-shell model developed for nanoparticles [58, 200, 201], where ideally the core part retains the bulk-like physical properties, but the outer shell (with thickness t ) can be considered as a disordered magnetic system whose magnetization may be considered to be zero in the absence of the magnetic field (see the inset of Fig. 19b). This shell is named as the dead layer, which does not have any spontaneous magnetization. As the particle size decreases, the shell thickness t increases, which enhances the inter-core separation between two neighboring particles, resulting in a decrease in the magnetic exchange energy. That is the reason why the reduction in the saturation magnetization with decreasing the particle size. In order to confirm this, M_sat value versus the inverse of the crystallite sizes (1/d ) is plotted in Fig. 19b, which reveals a quasi-linear relationship between the M_sat and 1/d . Similarly, the particle size effect on the magnetic properties of La_0.8 Sr_0.2 MnO₃ nanoparticles synthesized by the microwave irradiation process was also observed [64].

un Temperature dependence of magnetization in the La_0.9 Sr_0.1 MnO₃ muestras. Insets:(i) the dM/dT vs. T plots, and (ii) dependence of T_C on the average of grain size (d ). b Saturation magnetization M_sat values versus the inverse of the crystallite sizes (1/d ). Insert in (b ) is a schematic diagram of the core-shell model for ferromagnetic nanoparticle. Reproduced with permission [148]. Copyright 2014, Elsevier Ltd and Techna Group S.r.l

Beside the particle size and the synthesis process, the magnetic properties of La_1-x Sr_x MnO₃ nanoparticles are also dependent upon the Sr-doping level. Tian et al. [20] synthesized La_1-x Sr_x MnO₃ ( x =0, 0.3, 0.5, 0.7) nanoparticles by a facile molten salt synthetic route. Their magnetic properties were modified by the Sr-doping levels. The Curie temperatures (Tc) deduced from the magnetization (M )-temperature (T ) curves were 208, 252, 257, and 275K for x =0, 0.3, 0.5, and 0.7, indicating that the Tc values were increased with the Sr-doping levels. Recently, Xia et al. [39] synthesized (La_1-x Pr_x )_0.67 Ca_0.33 MnO₃ (LPCMO, x =0.0–0.5) nanoparticles via sol-gel process. Their T _C values were measured to be 233, 228, 180, and 171 K for the LPCMO samples (x =0.1, 0.2, 0.3, and 0.4), respectively, which were decreased with increasing the Pr-doping concentration. That is ascribed to that the double-exchange interactions in the LPCMO nanoparticles became weakened due to the narrower bandwidth and the reduced mobility of e_g electrons. Similarly, in the La_1-x Ba_x MnO₃ ( x =0.3, 0.5, and 0.6) nanocubes synthesized via hydrothermal methods, the low-temperature saturation magnetization was also decreased with increasing the Ba-doped content [46]. However, in the Ca_1-x Sm_x MnO₃ (CSM, x =0.0–0.20) nanoparticles, the T _C value was first abruptly decreased with increasing the Sm-doping concentration up to 0.05, but with a further increase in the doping level, it has monotonically augmented and approached a plateau above x =0.1, as shown in Fig. 20 [202]. The decrease in magnetic transition temperature demonstrates that the strength of the super-exchange interaction is reduced due to the dilution of the Mn ⁴⁺ lattice by Mn ³⁺ spins. While at moderate and larger doping regime, the magnetic behavior of nanograins are dominant by double exchange Mn ³⁺ –O–Mn ⁴⁺ interactions and the strong inter/intragrain coupling. The M_sat was also increased linearly from 0 to 0.03, and then it was increased abruptly in the 0.03–0.05 doping level, approaching a plateau above x =0.1 (seen in Fig. 20). Besides, as a popular system, Ca-doped lanthanum manganite, especially the magnetic properties of Ca-doped lanthanum manganite nanoparticles with different Ca doping levels were investigated by several groups [203,204,205,206]. The reduction of the M_sat due to the particle size reduction is attributed to the increase of magnetic dead layers. With decreasing the particle size, the finite-size effect causes the decrease of T _C .

Variation in saturation magnetization (M _sat ) and magnetic transition temperature (T _C ) of the Ca_1-x Sm_x MnO₃ nanopowders as a function of the Sm-doping concentrations. Solid and broken lines just show the trends. Reproduced with permission of [202]

To obtain information on the dynamical properties of magnetic nanoparticles, ac magnetic susceptibility is measured on cooling or heating the nanoparticle samples. The ac susceptibility (χ_ac ) has two components:one is in-phase (χ') with the excitation while the other is a dissipative out of phase (χ") component. Figure 21 shows the ac susceptibility of the La_0.8 Sr_0.2 MnO₃ nanoparticles (with particle size of 20 nm) versus temperature at an applied magnetic field of 1 mT and different frequencies (33.3, 111, 333.3, and 1000 Hz) [64]. In χ′(T) and χ″(T), a frequency-dependent peak near T_b =237 K (blocking/freezing temperature) was observed, which shifted to a higher temperature as increasing the frequency. The frequency dependence of the ac magnetic susceptibility and appearance of the irreversibility temperature in the field cooling (FC) and zero-field cooling (ZFC) magnetization patterns are the signature for super-paramagnetic/spin glass (SPM/SG) regime in both the interacting and non-interacting nanoparticles [207, 208]. Similar phenomenon was also reported for other perovskite manganite nanoparticles such as spin glass or super-spin glass behavior in La_0.67 Sr_0.33 MnO₃ [209] and La_0.6 Sr_0.4 MnO₃ nanoparticles [210], and SPM behavior in La_2/3 Sr_1/3 MnO₃ nanoparticles [211]. To reveal the dynamic behavior of magnetic nanoparticles and the nature of the T_b peak (SPM or SG) in of the La_0.8 Sr_0.2 MnO₃ nanoparticles calcined at 600 °C for 3 h, three well-known phenomenological models (e.g., Neel-Brown model, Vogel-Fulcher model, and critical slowing down model) have been used to fit the experimental data of ac susceptibility of the sample. The best fitting results from the critical slowing down model indicate that there exists a strong interaction between the LSMO magnetic nanoparticles. However, in the La_0.67 Sr_0.33 MnO₃ nanoparticles (with average particle size of 16 nm) prepared by sol-gel method, Rostamnejadi et al. [212] found that the experimental data of ac susceptibility was best fitted by the Vogel-Fulcher model, whereas the fitting the experimental data with Neel-Brown model and critical slowing down model give out unphysical value for the relaxation time. In addition, the unusually large value for the dynamic critical exponent and smaller value for relaxation time constant obtained from the fitting of data by critical slowing down model indicate that the spin-glass phase transition does not take place in this system of nanoparticles.

Ac susceptibility versus temperature for the La_0.8 Sr_0.2 MnO₃ sample (particle size of 20 nm) at different frequencies. Inset is the imaginary part as a function of the temperature at different frequencies. Reproduced with permission of [64]

Magnetocaloric properties

Recently, the large magnetocaloric effect (MCE) in perovskite manganites has been widely studied [213, 214]. MCE originates from the heating or the cooling of magnetic material due to the application of magnetic field, which is characterized by the magnetic entropy change. The magnetic entropy change (ΔS) can be estimated from the M(H) curves and the use of the Maxwell’s relation

where M is the magnetization, H is the magnetic field, and T is the temperature. The relative cooling power (RCP) proposed by Gschneidner et al. [215] is also an important parameter for selecting potential substances for magnetic refrigeration, which is described as the refrigeration capacity of magnetic refrigerant for magnetic refrigeration. It is evaluated using the relation

where δT is the full width at half maximum of a -ΔS(T) curve.

Wang y col. [38] investigated the magnetocaloric effect in the Ln_0.67 Sr_0.33 MnO₃ (Ln =La, Pr and Nd) nanoparticles prepared by using the sol-gel method. Figure 22a–c shows the temperature dependence of −∆S(T) under different changes of applied field from 1 to 5 T for LaSrMnO₃ , PrSrMnO₃ , and NdSrMnO₃ nanoparticles, respectively. Under a field changing from 0 to 5 T, the maximum values of isothermal entropy change are found to be 2.49, 1.94, and 0.93 J/kg K for the samples with Ln =La, Pr, and Nd, respectively, and the corresponding values of RCP reach 225, 265, and 246 J/kg. The RCP as a function of magnetic field is presented in Fig. 22d. It is seen that the RCP increases in almost a linear way as the field increases. These results suggest that those nanoparticles could be useful for magnetic refrigeration in a broad temperature range.

Isothermal entropy changes as a function of temperature with field changes of 1, 2, 3, 4, and 5 T, a for LaSrMnO₃ , b for PrSrMnO₃ , and c for NdSrMnO₃ , respectivamente. d Relative cooling power (RCP) as a function of magnetic field for the Ln_0.67 Sr_0.33 MnO₃ (Ln =La, Pr, and Nd) nanocrystalline samples. Reproduced with permission of [38]

Transport Properties

Transport property measurements of the manganite materials were carried out using standard four-terminal method on a Quantum Design PPMS system. At a constant applied field, resistance was measured as a function of temperature. Kumar y col. [154] synthesized the (La0.6Pr0.4)0.65Ca0.35MnO3 nanoparticles via a sol-gel route at different sintering temperatures, and measured their electrical transport properties. The electrical resistivities (ρ ) as a function of temperature for the (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ nanoparticles sintered at 600 °C, 800 °C, and 1000 °C was shown in Fig. 23. It was observed that the (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ system shows insulator type behavior at higher temperatures due to the development of charge-ordered states in the nanocrystalline system, and starts to behave as a metal at lower temperatures because double exchange interaction plays a dominant role in the transport behavior of the system. The insulator-metal transition temperature (T _IM ) and resistivity (ρ ) of nanoparticles are dependent upon the sintering temperature of the system (or the particle sizes). With increasing the sintering temperature, the particle (grain) size is increased; therefore, the effect of grain boundary is reduced and consequently the charge carriers in the nanocrystalline system face less scattering from the grain boundaries. This factor also improves the double exchange interaction mechanism and the system starts to show M–I transition at higher temperatures, and the resistivity of the system is also decreased significantly.

Variation of resistivity with the temperature for the (La0.6Pr0.4)0.65Ca0.35MnO3 nanoparticles sintered at 600 °C, 800 °C, and 1000 °C, respectively. Reproduced with permission of [154]

Zi et al. [44] prepared La_0.7 Sr_0.3 MnO₃ nanoparticles by a simple chemical co-precipitation route. To study the magnetoresistance (MR) effect, the magnetic field dependence of MR ratio at 10 K and 300 K by sweeping the applied magnetic field from − 20 to 20 kOe, was shown in Fig. 24. MR is defined as

Magnetic field dependence of the MR curves from − 20 to 20 kOe at 10 K and 300 K for the La_0.7 Sr_0.3 MnO₃ nanopartículas. Reproduced with permission of [44]

where ρ _H and ρ ₀ refer to the resistivity under the applied and zero field, respectively. It can be seen that MR drops abruptly with the increasing field in low-field region, which is called as low-field magnetoresistance (LFMR). LFMR values at 10 and 300 K are 22.3% and 2.9% at 5 kOe, respectively. Because of the small coercive field, the alignment of the magnetization in each LSMO grain to the applied magnetic field occurs in the low-field region. At a comparatively high-field region above 5 kOe, MR decreases linearly with the applied field, but with a much reduced slope. High-field MR (HFMR) ratios at 10 and 300 K are 29.2% and 6.5% at 20 kOe, respectively. HFMR can be attributed to the non-collinear spins at LSMO boundaries.

To investigate the particle size effect on the transport properties of La_0.7 Sr_0.3 MnO₃ (LSMO) nanoparticles, Navin and Kurchania [216] synthesized the LSMO nanoparticles with particle sizes of 20 nm (LSMO-1), 23 nm (LSMO-2), and 26 nm (LSMO-3), respectively. Figure 25 shows their temperature dependence of resistivity measured under (H =1 T) and without magnetic field in the temperature range of 10–300 K. It was found that the resistivity values of the LSMO-1 nanoparticles (Fig. 25a) were higher than that of the LSMO-2 (Fig. 25b) and LSMO-3 (Fig. 25c) nanoparticles. That is ascribed to the smaller particle size in the LSMO-1 sample. As the particle sizes become smaller, more grain boundaries in the samples acts as scattering centers to the charge carriers, resulting in the larger resistivity. Besides, it was found that the value of resistivity of all samples decrease under an external magnetic field of 1 T. The applied magnetic field gives rise to the increasing of spin ordering and the decreasing of the localization of the charge, which result in the reduction of resistivity. The LFMR property is related to the spin dependent scattering or spin dependent tunneling of the conduction electrons near the interfaces and grain boundaries. Figure 26a shows the temperature dependence of the MR of the samples LSMO-1, LSMO-2, and LSMO-3 at an applied magnetic field of 1 T. It was found that their MR values increase monotonically with decreasing the temperature. The LFMR at 1 T and 10 K for the samples LSMO-1, LSMO-2, and LSMO-3 was obtained as 32.3%, 28.4%, and 25.1% respectively. Obviously, the LFMR enhanced with decreasing the particle sizes. Figure 26b shows the magnetic field dependence of the normalized resistivity (ρ_H /ρ_o ) with applied magnetic field at temperatures 10 K and 300 K, where ρ₀ and ρ_H are the resistivity without and with magnetic field respectively. A sharp drop in the value of resistivity in low magnetic field region was also observed and the resistivity does not saturate up to a magnetic field of 4 T.

Temperature dependence resistivity of the La_0.7 Sr_0.3 MnO₃ (LSMO) nanoparticles with zero field and 1 T fitted by using ρ (T ) = ρ ₀  + ρ ₂ T ²  + ρ _4.5 T ^4.5 . un LSMO-1 particles (average size of 20 nm), b LSMO-2 particles (average size of 23 nm), and c LSMO-3 particles (average size of 26 nm). Inset shows the fitting of the resistivity data in the low temperature region by using \( {\rho}_L={\rho}_0-{\rho}_s\ln T+{\rho}_e{T}^{\frac{1}{2}}+{\rho}_p{T}^5 \). Reproduced with permission of [216]

un Temperature dependence of the magnetoresistance (MR %) at 1 T. b Normalized resistivity as a function of the applied field at 10 K and 300 K of the La_0.7 Sr_0.3 MnO₃ (LSMO) nanoparticles. LSMO-1, -2, and -3 particles with average particle sizes of 20 nm, 23 nm, and 26 nm, respectively. Reproduced with permission of [216]

The effects of doping levels on the electrical transport properties of perovskite manganite nanoparticles were also investigated. Thombare et al. [217] reported the electrical properties of Nd_1-x Sr_x MnO_3-δ (NSMO, 0.3 ≤ × ≤ 7) nanoparticles synthesized by glycine assisted auto combustion method. Figure 27a shows the resistivity for NSMO in the temperature range 5–300 K at zero magnetic field. It is found that all plots show high resistivity. The resistivity values slightly increases with the Sr concentration up to 100 K, whereas below that the rise in resistivity is steeper. The M–I transition is not observed in the present NSMO nanoparticles without applied magnetic field. However, under applied magnetic field (H) of 8 T, the M–I transition temperature (T_P ) was clearly observed around 12–48 K, as shown in Fig. 27b. These transition temperatures are lower than that in bulk counterpart. That may be due to the formation of small ferromagnetic clusters which are suffice for magnetic contribution but forbids conduction [218].

Temperature dependent resistivity for the Nd_1-x Sr_x MnO_3-δ nanoparticles under different external magnetic fields. un H =0 and b H =8 T. Reproduced with permission of [217]

Optical Properties

The optical study of perovskite manganites has interestingly shown that they are controlled by the electronic structure of perovskites. Kumar y col. [154] synthesized (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ nanoparticles by sol-gel method and post-annealed at 600 °C, 800 °C, and 1000 °C. To investigate the optical absorbance and evaluate the optical band gap of (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ , ultraviolet-visible (UV-Vis) spectroscopy measurements are carried out and the obtained UV-Vis spectra are shown in Fig. 28a. Obviously, there is a sharp absorption edge around 308 nm in ultraviolet region. The optical absorption edges can be analyzed as follows [219]:

un Ultraviolet-visible spectra of the (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ nanoparticles sintered at 600 °C, 800 °C, and 1000 °C. b Variation of (αhν ) ² versus photon energy hν plot for the (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ nanoparticles sintered at 600 °C, 800 °C, and 1000 °C. Reproduced with permission of [154]

donde E _g is the band gap energy, hν is the photon energy, and α is the absorption coefficient, depending upon the optical absorbance (A) and thickness (d). n can be equal to 1/2 (for direct transition process) or 2 (for indirect transition process). The variations of (αhν ) ² versus photon energy (hν ) for the (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ nanoparticles post-annealed at 600 °C, 800 °C, and 1000 °C are plotted in Fig. 28b. It is observed that (αhν) ² varies linearly for a very wide range of photon energy (hν ), indicating a direct type of transitions in these systems. The intercepts of these plots on the energy axis give the energy band gaps of the systems, which were determined to be 3.52, 3.46, and 3.42 eV, respectively, for the (La_0.6 Pr_0.4 )_0.65 Ca_0.35 MnO₃ nanoparticles post-annealed at 600 °C, 800 °C, and 1000 °C. These direct band gaps fall into the range of wide band gap semiconductors. The decrease of the band gap (red-shift) with increasing post-annealing temperature can be attributed to the increased particle sizes. Negi et al. [220] also investigated the optical properties of GdMnO₃ nanoparticles synthesized by the modified sol-gel route. The room temperature optical absorption spectrum of the GdMnO₃ nanoparticles measured in the range of 200–600 nm clearly shows that the absorbance is less in the range of 380–600 nm. The low absorbance in the entire visible region is an essential condition for nonlinear optical applications [221]. An extrapolation of the linear region of a plot of (αhν) ² on the y -axis versus photon energy (hν ) on the x -axis gives the optical band gap ∼ 2.9 eV.

1D Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Magnetic Properties

Chandra et al. [222] synthesized La_0.67 Ca_0.33 MnO₃ crystalline nanowires with the average diameter of 70 nm by using porous templates of anodized alumina combined with CSD technique. Their temperature dependence of magnetization (M) measured at ZFC and FC modes and under a magnetic field of 100 Oe demonstrates that these nanowires undergo a PM to FM transition at T _C =245 K, which is defined by the minimum in dM /dT . Similar feature was also reported for the La_0.67 Ca_0.33 MnO₃ nanotubes [87]. Datta et al. [157] also synthesized La_1-x A_x MnO₃ (where A =Ca, Sr; x =0.3 and 0.5) nanowires by hydrothermal method. All the nanowires undergo FM–PM phase transitions with increasing the temperature. Their T _C values are dependent upon the crystal structure as well as the Mn valence and oxygen stoichiometry.

Wang and Fan [223] reported on the magnetic properties of electron-doped Ca_0.82 La_0.18 MnO₃ nanowires and nanoparticles, and compared them with their bulk counterpart. It is found that the Ca_0.82 La_0.18 MnO₃ bulk exhibits a strong charge ordering (CO) peak at T _CO =132 K followed by an AFM ground state, whereas the CO peak becomes weak in the nanowires (T _CO =124 K), and disappeared in the nanoparticles which exhibits a ferromagnetism with T _C =165 K. Chandra et al. [224] also reported on the magnetic properties of the single-crystalline La_0.5 Sr_0.5 MnO₃ nanowires with diameter of 20–50 nm and length of 1–10 μm synthesized by the hydrothermal technique. Figure 29a–d shows their temperature dependence of dc magnetization M (T) measured under different applied magnetic fields. As the temperature is lowered from 340 K, the nanowires exhibit a PM to FM transition at T _C ∼315 K followed by a peak at T _N ∼210 K, which is associated with the onset of the FM–AFM transition. As the applied magnetic field is increased, the irreversible temperature shifts to lower temperatures as shown in Fig. 29b–d. Figure 29e, f demonstrates the temperature dependences of the real (χ ′) and imaginary (χ ′′) parts of ac susceptibility in the temperature range of 10–340 K, respectively. The χ ′ (T ) curves show a maximum at T _N with no frequency dependence and a kink at T _L . In addition, the χ ′′ (T ) curves, which reveal insight into magnetic loss behavior, showing a peak at T _C , a broad shoulder at T _N , and a kink at T _L . The χ ′ (T ) peak shifts to a higher temperature as the frequency is increased, which is consistent with the results reported previously [225, 226].

un , b Zero-field-cooled (ZFC) (blue), field-cooled-cooling (FCC) (green), and field-cooled-warming (FCW) (red) magnetization curves of the La_0.5 Sr_0.5 MnO₃ nanowires measured under applied fields of a 100 Oe and b 1000 Oe. c , d ZFC and FCW magnetization curves obtained under applied magnetic field of c 2000 Oe and d 5000 Oe. e Real and f imaginary parts of linear ac susceptibility versus temperature plots for different frequencies. The inset of (e ) shows a magnified view of χ’(T ) for more frequencies. Reproduced with permission of [224]

Magnetocaloric properties

Kumaresavanji et al. [227] reported on the MCE in La_0.7 Ca_0.3 MnO₃ nanotube arrays, which were synthesized by template-assisted sol-gel method in temperatures ranging from 179 to 293 K and under magnetic fields up to 5 T. Their temperature dependence of −∆S _M at different fields for nanotube arrays and bulk is plotted in Fig. 30a, b. When compared with the bulk counterpart (4.8 J/kg K), the magnitude of the ∆S _M (1.9 J/kg K) is smaller for nanotube arrays. In addition, the temperature dependence of −∆S _M curves for bulk sample show a narrow peak at 258 K which become broader and shift to lower temperature for nanotube arrays. The refrigerant capacitance (RC), is also an important parameter for selecting potential substances for magnetic refrigeration, which is described as the amount of heat transferred between the hot and cold sinks in one ideal refrigeration cycle. It is evaluated using the relation

Temperature dependence of −∆S _M curves for a La_0.7 Ca_0.3 MnO₃ (LCMO) nanotube (NT) arrays and b LCMO bulk at different fields. c RC and δ T_FWHM with respect to the field of the LCMO NT arrays and LCMO bulk sample. d Temperature dependence of −∆S _M curves of the LCMO NT arrays and bulk at 5 T. Reproduced with permission of [227]

where T ₁ and T ₂ are the temperatures of cold and hot reservoirs, respectively, which correspond to the full width at half maximum (δT_FWHM ) of the ∆S _M curvas. The calculated δT_FWHM and RC values for nanotube arrays and bulk samples are depicted in Fig. 30c. The RC values vary linearly with H in both cases. Moreover, the RC value is reasonably large for bulk sample compared to nanotube arrays. However, the δT_FWHM values of nanotube arrays are nearly 54% larger than the observed one for bulk sample. The temperature dependence of ∆S _M curves of nanotube arrays and bulk at a field of 5 T are comparatively shown in Fig. 30d. The shadow part represents the δT_FWHM of nanotube arrays which is nearly three times larger than their bulk counterpart. From this figure, one can understand how the nanotube arrays provide an expanded working temperature range compared to the bulk one. Even though the nanotube arrays present a broader ∆S _M curve, the ∆S _M value is lower compared to the bulk sample. However, the higher surface to volume ratio together with the hollow structure and broader peaks of ∆S _M indicate that the manganite nanotubes could be a suitable material for magnetic refrigeration in nano-electromechanical systems. The magnetocaloric properties of La_0.6 Ca_0.4 MnO₃ nanotubes with diameter of 280 nm and wall thickness of 10 nm were also reported by Andrade et al. [228]. It is found that the decrease of ∆S is commonly accompanied by a broadening in the ∆S curva. The RCP of nanoparticles is decreased with decreasing the particle size, but they still possess a larger cooling power than the nanotubes of the same compound, due to the broadening of the magnetic transition observed in these samples. In this way, it is important to notice that the reduced maximum value of ∆S observed for nanosystems is often accompanied by a broad magnetic entropy change.

Transport Properties

Lei et al. [229] successfully synthesized single-crystalline MgO/La_0.67 Ca_0.33 MnO₃ core-shell nanowires (MgO core is ∼ 20 nm in diameter and the La_0.67 Ca_0.33 MnO₃ shell layer is ∼ 10 nm in thickness) by depositing epitaxial La_0.67 Ca_0.33 MnO₃ sheaths onto MgO nanowire templates through the PLD technique. Transport investigations were carried out by measuring the four-probe resistance of individual core-shell nanowires, as shown in Fig. 31. The SEM image of a typical device with a 5-μm-long nanowire and four uniformly distributed electrodes is shown in Fig. 31a. The four-probe resistance of an MgO/La_0.67 Ca_0.33 MnO₃ nanowire was recorded as a function of temperature under two different magnetic fields (0 and 1 T), as shown in Fig. 31b. This M–I transition occurred at ∼ 140 K under zero magnetic field, and the transition temperature shifted to ∼ 160 K when a magnetic field of 1 T was applied normal to the device substrate. This M–I transition and transition temperature shifting effect induced by magnetic fields strongly suggest a correlation between the ferromagnetism and the metallicity, which is ascribed to the double-exchange mechanism. This M–I transition associated with FM to PM transition also happened in the MgO/La_0.67 Sr_0.33 MnO₃ core-shell nanowires with T _MI ∼240 K at H =0 and the T _MI shifted to ∼ 250 K under a perpendicular magnetic field of 1 T (Fig. 31c). In addition, MR measurements were also performed with both MgO/La_0.67 Ca_0.33 MnO₃ (inset of Fig. 31b) and MgO/La_0.67 Sr_0.33 MnO₃ (inset of Fig. 31c) core-shell nanowires at their transition temperature by sweeping the perpendicular magnetic field between ± 2.0 T. By defining the MR ratio as [R(H) - R(0)]/R(0) × 100%, a value of MR =34% was achieved at T =140 K and H =2.0 T for La_0.67 Ca_0.3d3 MnO₃ (inset of Fig. 31b), and 12% was achieved at T =240 K and H =2.0 T for La_0.67 Sr_0.33 MnO₃ (inset of Fig. 31c).

un Top view SEM image of a core-shell nanowire device showing four Ag/Au contact electrodes with even spacing. b , c Four-probe resistance (R ) vs. temperature (T ) curves measured under two different magnetic fields B =0 T (red) and B =1 T (blue), with b MgO/LaCaMnO₃ (LCMO) nanowire and c MgO/LaSrMnO₃ (LSMO) nanowire. Insets of panels b , c magnetoresistance (MR) recorded at temperature T =140 K for panel b and T =240 K for panel c , respectivamente. Reproduced with permission of [229]

Optical Properties

Arabi et al. [80] synthesized the La_0.7 Ca_0.3 MnO₃ nanorods by hydrothermal method under different conditions (e.g., different mineralization agents KOH and NaOH; various alkalinity conditions (10, 15, and 20 M)). The UV–Vis absorption spectra of all the La_0.7 Ca_0.3 MnO₃ nanorods are shown in Fig. 32a, where three obvious peaks are observed due to optical response in these La_0.7 Ca_0.3 MnO₃ nanorods. The first peak was observed around 220 nm (5.6 eV) for all the samples. Strong absorption peak appeared at wavelengths about 325–380 nm (3.8–3.3 eV) and the third peak appeared around 950 nm (1.3 eV) in all the samples, as shown in inset of Fig. 32a. It is found that a decrease and broadening of the absorption peaks for the N-series samples is related to size reduction. Figure 32b shows the curves of (αhv ) ² versus hv , and the intercepts of these plots on the hν axis provide the optical band gaps. It can be obtained that the main band gaps are estimated to be 2.48, 2.39, and 2.19 eV for the samples K10, K15, and K20 as well as 2.18, 2.25, and 2.32 eV for the samples N10, N15, and N20, respectively. Recently, Arabi et al. [230] also investigated the optical properties of La_{0. 68} Ca_{0. 32} MnO₃ nanowires prepared by hydrothermal method. Their optical band gap is estimated to be about 2.13 eV.

un UV–Vis absorption response of the La_0.7 Ca_0.3 MnO₃ (LCMO) nanorods synthesized by hydrothermal method under different alkalinity conditions. b The variation of (αhv ) ² absorption versus hv (photon energy) for the different samples (N10, N15, N20, K10, K15, and K20). N (or K) means the NaOH (or KOH) mineralizer, 10 (or 15, 20) for the NaOH (or KOH) concentration. Reproduced with permission of [80]

2D Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Magnetic Properties

Shao et al. [106] fabricated La_0.325 Pr_0.3 Ca_0.375 MnO₃ single-crystalline disks with diameters from 500 nm to 20 μm to study the spatial confinement effect on EPS. By using electron beam lithography with a negative tone resist, the La_0.325 Pr_0.3 Ca_0.375 MnO₃ disks were derived from the epitaxial La_0.325 Pr_0.3 Ca_0.375 MnO₃ films with thickness of 60 nm grown on STO(001) substrates by PLD. Magnetic properties of these La_0.325 Pr_0.3 Ca_0.375 MnO₃ disk arrays are measured by SQUID and MFM. Figure 33 shows the transition from the EPS state to a single ferromagnetic metallic (FMM) state. Figure 33a shows the AFM images of the morphologies for the La_0.325 Pr_0.3 Ca_0.375 MnO₃ disks with different diameters. The corresponding MFM images of the La_0.325 Pr_0.3 Ca_0.375 MnO₃ disks acquired at different temperatures (10 K, 100 K, and 180 K) under a perpendicular magnetic field of 1 T are shown in Fig. 33b–d. In the color scale, the contrast below zero (red or black) represents FMM phase, while the contrast above zero (green or blue) represents non-ferromagnetic phase. Obviously, all the disks show distinct features of the EPS state (i.e., the coexistence of the FMM and charge order insulating (COI) phases), except for the 500 nm disk. The typical length scale of the EPS domains is around a micrometer. It was also found that with decreasing temperature, the portion of FMM phase was increased.

un AFM images of the La_0.325 Pr_0.3 Ca_0.375 MnO₃ (LPCMO) disks with diameter sizes of 500 nm, 1 μm, 2 μm, 3.8 μm, 5 μm, and 7 μm in diameter. b - d MFM images of the LPCMO disks under external magnetic field of 1 T (external magnetic field direction is pointing perpendicularly to the sample surface plane) taken at 10 K (b ), 100 K (c ), and 180 K (d ). The negative value in MFM image indicates attractive force and positive value indicates repulsive force. Reproduced with permission of [106]

Huijben et al. [231] grew ultrathin La_0.7 Sr_0.3 MnO₃ films with thicknesses from 3 to 70 unit cells on STO substrates by PLD method. Their magnetic properties are shown in Fig. 34. Figure 34a shows the M -H loops of all samples. It is observed that the saturation magnetization (M _S ) is increased with increasing the film thickness up to 13 unit cells (~ 48 Å), whereas the coercive field (H _C ) is decreased. The M -T curves for all the films with different thickness are displayed in Fig. 34b, from which the Curie temperature T _C está determinado. The thicknesses dependent of H _C and T _C is shown in Fig. 34c. It is found that H _C and T _C are nearly constant for thicknesses down to 13 unit cells. Further reduction in the film thickness results in a dramatic change in the magnetic properties, although the films remain ferromagnetic down to three unit cells (~ 12 Å).

Ferromagnetic properties of ultrathin La_0.7 Sr_0.3 MnO₃ films on SrTiO₃ (001). un Magnetic hysteresis loops measured at 10 K. The diamagnetic contribution to magnetization not shown has been attributed to the substrate and has been subtracted. b Temperature dependence of the magnetization measured at 100 Oe. All samples were field cooled at 1 T from 360 K along the [100] direction before the measurements were performed. c Layer thickness dependence on the coercive field H _C and the Curie temperature T _C . Reproduced with permission of [231]

Magnetocaloric Properties

Debnath et al. [232] reported on the magnetocaloric properties of an epitaxial La_0.8 Ca_0.2 MnO₃ /LaAlO₃ thin film grown by PLD method. The magnetic entropy changes in the La_0.8 Ca_0.2 MnO₃ thin film for different magnetic field directions are shown in Fig. 35a–c, respectively. |∆S _M | exhibits a peak with its maximum around 247 K near T _C . The maximum values of |∆S _M | were estimated to be 35.90, 27.50, and 24.97 mJ cm ⁻³ K ⁻¹ under a field change of 1 T for the different field directions, H//ab, H//45°, and H//c, respectively. The |∆S _M | peaks in all directions are significantly broadened over a wider temperature region. The RCP values of the thin film under different field directions are shown in Fig. 35d. Large RCP values (i.e., 1000 mJ/cm ³ for the ab plane and 780 mJ/cm ³ for the c-direction) are obtained, which are higher than those observed in other perovskite manganites and rare earth alloys [213, 233, 234]. Such higher entropy change value and higher RCP with no noticeable hysteresis loss will make the epitaxial La_0.8 Ca_0.2 MnO₃ films more attractive for use as a magnetic refrigeration with large useful temperature ranges.

a–c Magnetic entropy changes in the La_0.8 Ca_0.2 MnO₃ thin films as a function of temperature and external magnetic field in different directions. un H//ab , b H//45°, and c H//c . d Relative cooling power (RCP) of the thin film as a function of magnetic field in different field directions. Reproduced with permission of [232]

Giri et al. [235] deposited epitaxial Sm_0.55 Sr_0.45 MnO₃ thin films on LAO(001), LSAT(001), and STO(001) single crystal substrates by PLD technique, and the relationship between magnetocaloric effect and lattice strain induced by the substrates was investigated. The temperature-dependent |∆S _M | values at different magnetic field calculated from M -H data for the Sm_0.55 Sr_0.45 MnO₃ thin films grown on LAO (001), LSAT (001), and STO (001) single crystal substrates are shown in Fig. 36a–c, respectively. It is interesting to notice that the values of |∆S _M | can be modulated by the lattice strain induced by the different substrates. The maximum value of |∆S _M | was found to be ~ 10 J kg ⁻¹ K ⁻¹ for the Sm_0.55 Sr_0.45 MnO₃ films grown on STO under a field change of 6 T. The inset of Fig. 36a shows the M-H loop measured at 10 K with increasing and decreasing magnetic fields of the films grown on STO. The field hysteretic loss is very less, which is a well characteristic of magnetic refrigeration. This low-field large magnetic entropy change in the thin film is mainly due to the rapid change of magnetization near the transition temperature in the easy magnetization plane. The specific heat (C _P ) data of the Sm_0.55 Sr_0.45 MnO₃ film grown on LAO substrate is shown in Fig. 36d, which clearly shows a lambda-shaped anomaly close to T _C . This is mainly arisen due to second-order magnetic phase transition. The peak temperature of C _P of the film matches well with T _C determined by dc magnetization measurement. The values of relative cooling power (RCP) are usually calculated for both the cases (near T _C and around T p), and several methods have been used to calculate the value of RCP. For example, in the first method, RCP-1 is calculated from the product of maximum peak value |∆S _M | and the full width at half maximum, δT _FWHM , i.e., RCP-1 =\( \left|{\Delta S}_M^{Max}\right| \)×δT _FWHM . In second method, RCP-2 is estimated from the maximum value (area) of the product |∆S _M | ×ΔT under the |∆S _M | vs. T curva. The ΔS _M versus T curve of the Sm_0.55 Sr_0.45 MnO₃ films grown on STO at magnetic field of 6 T is shown in Fig. 36e. The bigger rectangular sketch and shaded area corresponds to RCP-1 and RCP-2, respectively. Inset in Fig. 36e shows the value of RCP-2 as a function of magnetic field for three different films. Figure 36f demonstrates the value of RCP-1 as a function of magnetic field for three different films. It can be observed that for both the temperature regime, the value of RCP increases with increasing the magnetic fields. An important observation is that the values of RCP are significantly larger around T _C than T _p . Therefore, a material in the same refrigeration cycle with higher RCP is preferred as it would confirm the transport of a greater amount of heat in an ideal refrigeration cycle. The epitaxial Sm_0.55 Sr_0.45 MnO₃ films grown on STO exhibit large MCE modulated by lattice strain; their larger |∆S _M | and enhanced RCP with almost zero hysteresis loss make them ideal for magnetic refrigeration, providing an alternative approach in searching for energy efficient magnetic refrigerators.

a–c Temperature dependent|∆S _M | at different magnetic field calculated from M–H data for the SSMO/STO, SSMO/LSAT, and SSMO/LAO films, respectively. Inset of (b ) and its inset shows the one cycle M–H curves of the SSMO/LSAT film in the temperature interval of 2 K and 1 K, respectively. Inset of (c ) shows one cycle M–H curves of the SSMO/LAO films in the temperature interval of 5 K. d C_P (T) of the SSMO/LAO film at μ ₀ H =0 T. e The ∆S _M vs. T curve of the SSMO/STO film at μ ₀ H =6 T. The bigger rectangular sketch and shaded area corresponds to RCP-1 and RCP-2, respectively. Inset shows the value of RCP-2 as a function of magnetic field for three different films. f The value of RCP-1 as a function of magnetic field for three different films. Sm_0.55 Sr_0.45 MnO₃ (SSMO) thin films deposited on LaAlO₃ (LAO), SrTiO₃ (STO), and (La_0.18 Sr_0.82 )(Al_0.59 Ta_0.41 )O₃ (LSAT) single crystalline substrates are named as SSMO/STO, SSMO/LSAT, and SSMO/LAO, respectively. Reproduced with permission of [235]

Transport Properties

Chen et al. [236] investigated the transport properties of (La_1-x Pr_x )_0.67 Ca_0.33 MnO₃ (0 ≤ × ≤ 0.35) films (with thicknesses from 9 to 60 nm) grown on NGO(110)_OR substrates by PLD. Their temperature coefficient of resistance (TCR, defined by (dρ/dT)/ρ, where ρ is the resistivity and T the temperature) is shown in Fig. 37a. The inset shows the corresponding ρ-T curves. The doping-level dependent T_C and TCR peak values are shown in Fig. 37b. Obviously, the monotonous reduction of T _C is accompanied by the increasing Pr-doping. TCR value greatly relies on the Pr-doping level x , which reaches the maximum value at 88.17% K ⁻¹ when doped at x =0.25. The (La_1-x Pr_x )_0.67 Ca_0.33 MnO₃ film at x =0.25 may give a suitable FMM clusters size and distribution, leading to a sharp MIT with an optimized TCR. Dhakal et al. [105] also reported on the electrical properties of the (La_1-y Pr_y )_0.67 Ca_0.33 MnO₃ (y =0.4, 0.5, and 0.6) films with thickness of 30 nm grown on NGO(110) and STO(100) substrates by PLD. They found that the M-I transition temperature T _MI was decreased with increasing the Pr-doping concentration due to the reduction of average A-site cation radii <r _A > . The transport properties of La_0.8-x Pr_0.2 Sr_x MnO₃ ( x =0.1, 0.2, and 0.3) manganite films were also reported by Solanki et al. [118]. Their results confirmed the effects of Sr-concentration on transport properties. As smaller sized Pr ³⁺ substituted at La-site in LaMnO₃ results in the reduction in Mn-O-Mn bond angle from 180° making superexchange competitive with Zener double exchange, while substitution of Sr ²⁺ in La_0.8-x Pr_0.2 Sr_x MnO₃ system results in the increase in lattice parameters and the Mn-O-Mn bond angle toward 180°. Increase in Sr-concentration (from x =0.1 to 0.3) enhances the e_g electron bandwidth due to larger size of Sr ²⁺ ion, promoting the motion of more itinerant electrons between Mn ³⁺ and Mn ⁴⁺ which in turn suppresses resistivity and enhances T_P (metal–insulator/semiconductor transition temperature). In addition, due to substitution of Sr ²⁺ (x) at La-site, average grain size increases and grain boundary density decreases resulting in the suppression in scattering of e_g electrons which in turn increases T _P .

un Temperature dependence of TCR for the 30 nm (La_1-x Pr_x )_0.67 Ca_0.33 MnO₃ (LPCMO)/ NdGaO₃ (NGO) films at various doping levels (x =0–0.35) at zero field. The inset shows the corresponding ρ -T curvas. b T _C and TCR (peak value) as a function of the Pr doping level x, extracted from the ρ -T curves measured during cooling. Reproduced with permission of [236]

The thickness-dependent transport properties of the La_0.7 Pb_0.3 MnO₃ manganite films grown on LAO(100) single crystal substrates by CSD technique are also reported [115]. Figure 38a shows the ρ -T data of all the La_0.7 Pb_0.3 MnO₃ /LAO films under zero applied field. It is found that the ρ decreases and T _P increases up to 269 K (with thickness of 350 nm) as increasing the film thickness, which is ascribed to the total (in plane and out of plane) strain relaxation effect. Figure 38b shows low temperature MR behavior of various thickness films and bulk, the MR isotherms, at 5 K. It was found that bulk exhibits MR ~ 38% at 9 T, while the film only shows MR ~ 42% at room temperature. In addition, as the film thickness is increased, MR value at 5 K increases from 5% for 150 nm film to 18% for 350 nm film. This observation suggests the thickness-dependent microstructural effect on the transport and MR behavior of the La_0.7 Pb_0.3 MnO₃ /LAO films at low temperature under high fields. The thickness-dependent transport properties in the La_0.7 Sr_0.3 MnO₃ thin films are also reported [116, 117, 237,238,239].

un ρ-T plots for the La_0.7 Pb_0.3 MnO₃ (LPMO)/LaAlO₃ (LAO) thin films grown by chemical solution deposition (CSD) method with various thicknesses. Inset (a ):resistivity vs. temperature plot for bulk LPMO sample. Inset (b ):enlarged view of low temperature (below 80 K) dependent resistivity behavior for the LPMO/LAO thin films having various thicknesses. b MR vs. H isotherms recorded at 5 K for the CSD grown LPMO/LAO thin films with different thicknesses and bulk LPMO sample. Reproduced with permission of [115]

Optical Properties

Cesaria et al. [240] reported the optical response of 200-nm-thick La_0.7 Sr_0.3 MnO_3-δ films, which were deposited by PLD on amorphous silica substrates at nearly 600 °C under different oxygen pressures (0.1, 0.5, 1, 5, and 10 Pa). A blue-shift of the transmittance-curve edge was observed as the p(O₂ ) was increases from 1 to 10 Pa. That is ascribed to the changes of oxygen non-stoichiometry in the films, leading to larger Mn ⁴⁺ /Mn ³⁺ ratios under higher oxygen pressure. In order to in-depth understand the optical response of the deposited films, the nature (direct or indirect) of the optical transitions in the films were investigated by plotting (Eα ( E )) ⁿ versus the photon energy (E ) for n =1/2 and 2, where α ( E ) is the absorption coefficient. n is equal to 1/2 for indirect transition process or 2 for direct transition process. The graphs of (Eα ( E )) ² versus energy E for all the deposited films exhibit a linear regions, as shown in Fig. 39a, from which the direct energy gap values (E _g ) for the thin films can be estimated by extrapolation to the energy axis of the linear regions of the graphs. In the allowed direct transitions, the lowest energy one at nearly 1.0 eV was observed only for the films grown under oxygen pressures of 0.1 Pa and 0.5 Pa. That can be assigned to electronic excitations Mn ³⁺ (\( {\mathrm{e}}_g^1 \))→Mn ³⁺ (\( {\mathrm{e}}_g^2 \)) from a bound state (owed to the lattice distortion around the Mn ³⁺ ion) into a final state also bound by lattice distortions. The highest observed transition appeared at nearly 3.5 eV. Other transitions were yielded at intermediate energies for all the examined values of p(O₂ ):optical transitions of at least 2.45 eV were observed with increasing energy as p(O₂ ) was increased. The films grown under oxygen pressures of 10 Pa exhibited further transitions (at 3.04 eV and 3.50 eV), which can be assigned to transitions from O 2p states to the higher energy Mn ³⁺ (\( {\mathrm{e}}_g^2 \)) band. In the plots of (Eα ( E )) ^1/2 versus energy (E ) of the films grown under the oxygen pressure of 10 Pa and 0.5 Pa, linear regions were also observed, as shown in Fig. 39b, which may be assigned to indirect transitions (phonon assisted transitions, i.e., phonon absorption and phonon emission) or indicative of amorphous nature. The occurrence of two adjacent linear dependences cab be interpreted as corresponding to phonon absorption and phonon emission processes leading to an indirect band gap. Indirect transitions are assigned to the films grown under oxygen pressures of 10 Pa, 5 Pa, and 1 Pa, and amorphous nature is detected in the films grown under oxygen pressures of 0.5 Pa and 0.1 Pa. Tanguturi et al. [241] reported the optical properties of Nd_0.7 Sr_0.3 MnO₃ films grown on amorphous-SiO₂ sustrato. In the absorption coefficient spectra of the as-deposited and annealed films, a broad peak in the region hv < 2 eV was observed and beyond that they rose rapidly up to around 4 eV. Beyond 4 eV, no appreciable change in the spectrum was observed. The energy band gaps of the films were determined by plotting (αE ) ² as a function of energy (E ), which were determined to be 2.98 eV and 2.64 eV for the as-deposited and annealed films, respectively. Therefore, the amorphous film exhibits larger band gap as compared with the crystalline one. Such a large band gap value in amorphous phase is a known phenomenon [242].

un Plots of (Eα ( E )) ² versus photon energy (E ) of the LSMO films grown under different oxygen pressures. The direct transitions are identified by extrapolation of the straight line portions to energy axis. b Plots of (Eα ( E )) ^1/2 versus photon energy (E ) of the LSMO films grown under oxygen pressures of 10 Pa and 0.5 Pa. Linear regions occur that are indicative of indirect transitions in the LSMO films grown under oxygen pressure of 10 and the amorphous nature for LSMO films grown under oxygen pressure of 0.5 Pa. Reproduced with permission of [240]

3D Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Up to date, only a limited works on 3D rare earth-doped perovskite manganite oxide nanostructures are available. Here, an example of 3D rare earth-doped perovskite manganite oxide nanostructure is demonstrated, which is constructed by interlayering La_0.7 Sr_0.3 MnO₃ (LSMO)–CeO₂ -based epitaxial vertically aligned nanocomposite (VAN) thin films with pure CeO₂ (or LSMO) layers. This 3D strained framework nanostructures combine both the lateral strain by the layered structures and the vertical strain in the VAN, and thus maximize the 3D strain states in the systems, manipulating the electron transport paths in these systems. For example, in the 3D nanostructured LSMO–CeO₂ VAN systems, the electrical transport properties can be effectively tuned from a 3D insulating CeO₂ framework with integrated magnetic tunnel junction structures, to a 3D conducting LSMO framework by varying the types of the interlayers (i.e., CeO₂ or LSMO) and the number of interlayers from 1 to 3 layers [198]. Figure 40 shows the transport properties of these 3D framed nanostructures. The temperature-dependent resistance (R–T) curves at zero-field are shown in Fig. 40a for samples C0–C3, and Fig. 40b depicts the temperature dependence of the MR (%) in the 3D-framed nanostructures C0–C3. It is observed that in Fig. 40a, the resistance is decreased with increasing temperature, indicating typical semiconductor behavior in C0–C3 due to the large portion of CeO₂ introduced in the nanostructures (CeO₂ :LSMO ≥ 1:1 in C0–C3). The MR (%) of the films C0–C3 is increased at first and then reduced as the temperature increasing from low temperature to room temperature. Therefore, a MR peak is observed around 50 K. It is also noticed that the 3D CeO₂ frameworks could enhance the overall MR properties; for example, the MR peak value is increased from 40% (C0) to 51% (C3), 57% (C2), and maximized at 66% (C1). Such an enhancement can be ascribed to the 3D CeO₂ framework not only tailoring the out-of-plane strain of the LSMO phase but also building up the 3D tunneling framework for the electron transport. The relatively lower MR (%) in C2 and C3 samples compared to C1 is possibly related to the surface roughness observed in both samples where the 3D insulating framework might not be effective in the top layers. In contrast to the C1–C3 samples, a metallic behavior is observed in the L1–L3 samples with a 3D LSMO framework, as shown in Fig. 40c. The resistances are gradually increased from 10 to 350 K with a M–I transition temperature (T _MI ) at ~ 325 K. Such a metallic behavior is associated with the high composition of LSMO in L1–L3 and the 3D interconnected conductive LSMO frames built in the composite films L1–L3. Meanwhile, the resistance of the composite films L1–L3 decreases with inserting more lateral LSMO interlayers over the entire temperature regime. The LSMO interlayers interconnect with the vertical LSMO domains forming a conductive 3D frame in the film. Thus, the tunneling MR effect is effectively reduced. Figure 40d demonstrates the temperature dependence of MR for the nanocomposite thin films L0–L3 with the M–I transition temperature (T _MI ) marked for samples L1–L3. It is observed that such L1–L3 structures enable higher MR values at higher temperatures, e.g., 13% at 316 K in sample L2, which is a dramatic MR value improvement compared to C0–C3 and the previous reports at higher temperatures (e.g., near room temperature). Based on the above observations, it is clear that magnetic tunneling junctions (MTJ) of the LSMO/CeO₂ /LSMO and their geometrical arrangement in the 3D framework nanostructures are very important for enhancing the low-field MR properties. In C1–C3 samples, there are effective vertical and lateral MTJ structures integrated in the system by incorporating CeO₂ interlayers in the VAN system, such 3D insulating frameworks effectively maximize the 3D magnetic tunneling effect and lead to a record high MR% in the LSMO based systems. This 3D strain framework concept opens up a new avenue to maximize the film strain beyond the initial critical thickness and can be applied to many other material systems with strain-enabled functionalities beyond magneto-transport properties.

un R–T plots of 3D CeO₂ framed nanocomposite thin films C0—C3. b The temperature dependence of MR for the nanocomposite thin films C0–C3. c R–T plots of 3D La_0.7 Sr_0.3 MnO₃ (LSMO) framed nanocomposite thin films L0–L3. The arrows point out the metal-to-insulator transition temperature T_MI of L1–L3. d The temperature dependence of MR for the nanocomposite thin films L0 - L3 with the metal-to-insulator transition temperature T_MI marked for samples L1–L3. The single layer LSMO - CeO₂ VAN thin films are named as C0 or L0, without LSMO or CeO₂ as the interlayers. 3D CeO₂ interlayered thin films with 1, 2, and 3 interlayers inserted in VAN structures are named as samples C1, C2, and C3, respectively. Similarly, 3D LSMO interlayered thin films with 1, 2, and 3 interlayers inserted in VAN are named as sample L1, L2, and L3, respectively. Reproduced with permission of [198]

Applications of Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Rare Earth-Doped Perovskite Manganite Oxide Nanoparticles

Magnetic Refrigeration

Magnetocaloric effect (MCE) makes magnetic materials attractive for potential applications in magnetic refrigeration. As compared to the conventional gas compression technology, the magnetic refrigeration technology offers many advantages, such as no use of any gasses or hazardous chemicals, low energy consumption, and low capital cost [243,244,245].

Mahato et al. [246] synthesized La_0.7 Te_0.3 MnO₃ nanoparticles with average particle size of 52 nm where a large magnetic entropy change of 12.5 J kg ⁻¹ K ⁻¹ was obtained near T _C for a field change of 50 kOe. These results confirmed the application for magnetic refrigeration. Yang y col. [247] reported that for La_0.7 Ca_0.3 MnO₃ nanoparticles with average size of 30 and 50 nm, their maximum magnetic entropy changes at 15 kOe applied field were 1.01 and 1.20 J kg ⁻¹ K ⁻¹ , respectively, indicating that the La_0.7 Ca_0.3 MnO₃ nanoparticles could be considered as a potential candidate for magnetic refrigeration applications at room temperature. Phan et al. [248] demonstrated strong enhancement of both the MCE and refrigerant capacity in the nanostructured mixed phase manganite of La_0.35 Pr_0.275 Ca_0.375 MnO₃ (with an average particle size of 50 nm). Compared to other candidates such as Pr_0.65 (Ca_0.7 Sr_0.3 )_0.35 MnO₃ (~ 67 nm) [249], the higher entropy change (\( -{\Delta \mathrm{S}}_{\mathrm{M}}^{\mathrm{M}\mathrm{ax}} \)) and RC (refrigerant capacity) values were achieved in the nanocrystalline sample.

Biomedical Applications

Magnetic nanoparticles offer some attractive possibilities in biomedicine. First, they have controllable sizes ranging from a few nanometers up to tens of nanometers, which places them at dimensions that are smaller than or comparable to those of a cell (10–100 μm), a virus (20–450 nm), a protein (5–50 nm), or a gene (2 nm wide and 10–100 nm long). This means that they can “get close” to a biological entity of interest. Indeed, they can be coated with biological molecules to make them interact with or bind to a biological entity, thereby providing a controllable means of “tagging” or addressing it. Second, the nanoparticles are magnetic, which means that they obey Coulomb’s law, and can be manipulated by an external magnetic field gradient. This “action at a distance,” combined with the intrinsic penetrability of magnetic fields into human tissue, opens up many applications involving the transport and/or immobilization of magnetic nanoparticles, or of magnetically tagged biological entities. In this way, they can be made to deliver a package, such as an anticancer drug, or a cohort of radionuclide atoms, to a targeted region of the body, such as a tumour. Third, the magnetic nanoparticles can be made to resonantly respond to a time-varying magnetic field, with advantageous results related to the transfer of energy from the exciting field to the nanoparticle. For example, the particle can be made to heat up, which leads to their use as hyperthermia agents, delivering toxic amounts of thermal energy to targeted bodies such as tumours; or as chemotherapy and radiotherapy enhancement agents, where a moderate degree of tissue warming results in more effective malignant cell destruction. These, and many other potential applications, are made available in biomedicine as a result of the special physical properties of magnetic nanoparticles [250].

Bhayani et al. [251] report, for the first time, immobilization of commonly used biocompatible molecules on La_1-x Sr_x MnO₃ nanoparticles, namely bovine serum albumin and dextran. Such bioconjugated nanoparticles have a tremendous potential application, especially in the field of biomedicine. Daengsakul et al. [61, 62] reported the cytotoxicity of La_1-x Sr_x MnO₃ nanoparticles with x =0, 0.1, 0.2, 0.3, 0.4, and 0.5 evaluated with cell NIH 3T3. The result showed that the La_1-x Sr_x MnO₃ nanoparticles were not toxic to the cells. This will be useful for medical applications. Similar studies about the toxicity of the nanoparticles were performed by Zhang et al. for safe biomedical applications [252].

Magnetic resonance imaging (MRI) represents a powerful imaging method commonly utilized in clinical practice. The method shows excellent spatial resolution, which is very suitable not only for examination of human bodies but also for detailed anatomical studies of animal models in vivo in biological research. On the other hand, the sensitivities of other techniques, such as optical methods, single-photon emission computed tomography or positron emission tomography, are much higher. Thus, the design and synthesis of so called dual or multimodal probes is an important field. The combination of both respective approaches utilizing only one dual probe, e.g., magnetic nanoparticles tagged with fluorescent moieties, establishes a very useful method for bioimaging. Moreover, fluorescent magnetic nanoparticles are promising materials for other medical applications, where the same tool might be used either for diagnostics or for therapy, like for magnetic hyperthermia and optically driven surgery. The positioning of the magnetic cores with the external magnetic field could be used in cell micromanipulation [253,254,255]. Kačenka et al. [256] reported the potential of magnetic nanoparticles based on the La_0.75 Sr_0.25 MnO₃ perovskite manganite for MRI. Fluorescent magnetic nanoparticles based on a perovskite manganite La_0.75 Sr_0.25 MnO₃ core coated with a two-ply silica layer were synthesized and thoroughly characterized in order to prepare a novel dual MRI/fluorescence probe with enhanced colloidal and chemical stability. Viability tests show that the complete particles are suitable for biological studies.

In recent years, magnetic nanoparticles have been used in magnetic hyperthermia, referring to the introduction of ferromagnetic or super-paramagnetic particles into the tumor tissue. The magnetic nanoparticles create heat that can be used to treat cancer when they are placed in alternating magnetic fields. The La_1-x Sr_x MnO₃ nanoparticles for hyperthermia applications are studied in details [257,258,259,260].

Catalysts

Research in environmental catalysis has continuously evolved over the last two decades owing to the necessity of obtaining worthwhile solutions to environmental pollution problems. The development of innovative environmental catalysts is a crucial factor toward the purpose of determining new sustainable manufacturing technologies. Rare earth perovskite manganites attract notable attention of explorers due to their high catalytic activity in numerous redox reactions [261, 262].

Oxygen electrocatalysis is one of the key processes limiting the efficiency of energy conversion devices such as fuel cells, electrolysers, and metal-air batteries. In particular, the oxygen reduction reaction (ORR) is commonly associated with slow kinetics, requiring high overpotentials, and high catalyst loadings. Celorrio et al. [263] reported the effect of tellurium (Te) doping on the electrocatalytic activity of La_1-x Te_x MnO₃ nanoparticles with an average diameter in the range of 40 - 68 nm toward the oxygen reduction reaction.

Carbon monoxide is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is toxic to humans and animals when encountered in higher concentrations. The catalytic oxidation of CO is utilized in various applications, e.g., indoor air cleaning, CO gas sensors, CO₂ lasers, and automotive exhaust treatment. The structural and catalytic properties of La_1-x (Sr or Bi)_x MnO₃ samples with x =0.0, 0.2 or 0.4 for CO oxidation, are investigated [264].

Volatile organic compounds (VOCs), emitted from many industrial processes and transportation activities, are considered as great contributors to the atmospheric pollution and dangerous for their effect on the human health [265]. From an economical point of view, compared to an incineration process, catalytic combustion is one of the most interesting technology for the destruction of emissions of VOCs. Blasin-Aubé et al. [266] reported that the La_0.8 Sr_0.2 MnO_3+x perovskite-type catalyst is highly active in the oxidative destruction of VOCs, especially for oxygenated compounds.

The possibility of catalytic activity enhancement in NO reduction is also studied [267]. Ran et al. synthesized the Ce-doped PrMnO₃ catalysts and investigated the effect of cerium doping on the catalytic properties of Ce-doped PrMnO₃ catalysts. Their results showed that in the case of the Ce-doped series with lower doping ratio, most of the Ce ⁴⁺ ions were introduced into the A-site to form perovskite-type oxides with some additional ceria. The oxidation state of manganese was more easily affected by the addition of cerium and more vacancies might arise at the A-site due to the structural limit of the oxide. High catalytic activity in NO reduction might be caused by the presence of oxygen vacancies and the relative ease of oxygen removal. Besides, ceria could also adsorb oxygen to sustain the reduction of NO.

Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have become of great interest as a potential economical, clean, and efficient means of producing electricity in a variety of commercial and industrial applications. Its major advantages include high efficiency, potential for cogeneration, modular construction, and very low pollutant emissions. Lanthanum manganite-based oxides, e.g., La_1-x Ca_x MnO₃ and La_1-x Sr_x MnO₃ , are promising materials as cathodes, because of their high electrical conductivity and good compatibility with yttria-stabilized zirconia (YSZ).

For example, nano-sized (La_0.85 Sr_0.15 )_0.9 MnO₃ and Y_0.15 Zr_0.85 O_1.92 (LSM–YSZ) composite with 100–200 nm in diameter was co-synthesized by a glycine–nitrate process (GNP) [268]. Alternating current impedance measurement revealed that the co-synthesized LSM–YSZ electrode shows lower polarization resistance and activation energy than the physically mixed LSM–YSZ electrode. This electrochemical improvement was attributed to the increase in three-phase boundary and good dispersion of LSM and YSZ phases within the composite. Lay et al. [269] synthesized the Ce-doped La/Sr chromo-manganite series (Ce_x La_{0.75- x} Sr_0.25 Cr_0.5 Mn_0.5 O ₃ with x =0, 0.10, 0.25, and 0.375) as potential SOFC anode or solid oxide electrolyzer cell (SOEC) cathode materials. All those materials are stable in both elaboration and operating conditions of an SOFC anode, and they are also stable in steam electrolysis of cathodic conditions in SOEC. Besides, the possibility of A_2-x A′_x MO₄ (A =Pr, Sm; A′ =Sr; M =Mn, Ni; x =0.3, 0.6) as a cathode of SOFC was investigated by Nie et al. [270].

Besides being used as cathodes in solid oxide fuel cells, rare earth-doped perovskite manganite oxides (Ln_x A_1-x MnO₃ ) also exhibit high potential for being used as redox materials for solar thermochemical fuel production from thermochemical H₂ O/CO₂ splitting [271]. It was reported that substituted lanthanum manganite perovskite was one of the most suitable candidates among the perovskite family, owing to its unique redox properties [272]. To further improve the redox properties of these materials, Nair and Abanades [273] performed a systematic study to investigate the effects of synthesis methods on the redox efficiency and performance stability for CO₂ splitting. They synthesized single-phase La_x Sr_1-x MnO₃ (LSMO) by using various technical routes such as solid-state reactions, pechini process, glycine combustion, or glucose-assisted methods, and found that the materials synthesized by the pechini method exhibited the highest reactivity among the series, and a stable CO production of ~ 260 μmol g ⁻ 1 was achieved at x =0.5. They also found that the substitutions of Y/Ca/Ba at A-site and Al/Fe at B-site in (La,Sr)MnO₃ did not enhance the redox cycling capability as compared with LSMO. It was observed that Sr was the best A-site substituent and the presence of single Mn cation alone in B site was the most suitable option for promoting CO₂ -splitting activity. Furthermore, the addition of promotional agents and sintering inhibitor such as MgO and CeO₂ without altering the La_0.5 Sr_0.5 MnO₃ composition could improve the CO₂ -splitting activity. For an overview on the recent progress on solar thermochemical splitting of water to generate hydrogen, we refer to other review articles [ 274–276].

1D Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Catalysts

In the catalytic combustion of methane, the perovskite manganites nanoparticles generally lose their activities due to the severe sintering under such a high-output and high-temperature reaction. As a consequence, designing of highly reactive and stable catalysts has been an interesting research direction in heterogeneous catalysis. Recently, some results indicate that the catalytic properties of the catalysts could be improved by controlling their morphologies and structure. For example, SrCO₃ nanowires showed higher activity for ethanol oxidation than the nanoparticles [277]. Besides, CeO₂ nanorods were more reactive for CO oxidation than the corresponding nanoparticles [278]. However, catalytic properties of 1D Rare earth-doped perovskite manganite have been scarcely reported. Teng [279] reported the hydrothermal synthesis of La_0.5 Sr_0.5 MnO₃ nanowires, and the stability and the activity for methane combustion of La_0.5 Sr_0.5 MnO₃ nanowires were also investigated. The results showed that after being calcined for a long time, the nanowires showed a higher stability as compared with the La_0.5 Sr_0.5 MnO₃ nanopartículas. The nanowire catalyst maintained a higher catalytic activity for methane combustion. The photocatalytic activity of La_1-x Ca_x MnO₃ ( x ≈ 0. 3) nanowires synthesized by hydrothermal method was also investigated by Arabi et al. [230]. The results revealed that La_{0. 68} Ca_0.32 MnO₃ nanowires exhibited sufficient photocatalytic activity for degradation of methylene blue solution under visible-light irradiation.

Solid Oxide Fuel Cells

Up to now, various approaches have been suggested to fabricate LSM/YSZ composite cathodes for SOFCs. Several researches have shown that electrode microstructure (i.e., particle size, pore size, and porosity) has a strong influence on the value of the area specific resistance (ASR) [280,281,282]. Da and Baus synthesized La_0.65 Sr_0.3 MnO₃ (LSM) nanorods through a simple hydrothermal reaction. It is worth noting that the ASR values in this work are substantially lower than most of the former reports available in the literature [268, 283,284,285,286,287,288]. They pointed out that the promising performance of the nanostructured LSM cathodes was attributed to the optimized microstructure, i.e., high surface area, small grain size, and good inter-granular connectivity, which make it a potential candidate for intermediate temperature SOFC application. In addition, nano-tube structured composite cathodes were also investigated [283]. La_0.8 Sr_0.2 MnO_3-δ /Zr_0.92 Y_0.08 O ₂ (LSM/YSZ) composite nano-tubes are co-synthesized by a pore wetting technique as a cathode material for SOFCs. The as-prepared nanostructured composite cathode shows low ASR values of 0.17, 0.25, 0.39, and 0.52 Ω cm ⁻² at 850, 800, 750, and 700 °C, which is mainly due to small grain size, homogeneous particle distribution and fine pore structure of the material.

Magnetic Memory Devices

The elaboration of submicron MR read heads and high-sensitive elements of non-volatile memories (MRAM) passes through patterning processes that are commonly used in the semiconductor industry. The planar processes for thin-film patterning are based on two main steps:(i) the pattern definition in photon or electron sensitive polymer (resist) by lithography and (ii) the transfer of these nanostructures in the manganite film using dry etching [163]. Conventional UV lithography is traditionally used to get patterns higher than one micron in size; however, patterning at dimensions lower than 50 nm needs high-resolution techniques such as scanning electron beam lithography (SEBL), X-ray lithography (XRL), or NI [289, 290]. At present, ultimate resolution limits of SEBL and XRL are well known [291, 292] and the electron PMMA (polymethylmethacrylate) resist allows replications below 20 nm. After nanolithography, the pattern transfer can be achieved using direct etching with the resist as mask, or using a metallic lift-off process followed by etching. The lift-off process is the preferred method for manganite etching since these CMR oxides are very hard materials compared to metals.

A magnetic domain wall separates two oppositely polarized magnetic regions, and a number of data storage schemes based on domain walls in magnetic nanowires have been proposed [293, 294]. In the race-track memory, each magnetic domain wall represents a data bit [293]. During the write operation, the domain wall is moved by external magnetic field or spin transfer torque [295,296,297]. To read a bit, GMR or TMR type devices are used to detect the stray field from the domain wall. To utilize such a scheme, it is critical to controllably create domain walls. Magnetic nanowires with an artificial pinning center, such as notches [293], bent conduits [298], and narrow rings [299], can serve this purpose. In perovskite manganite nanostructures, various types of domain patterns such as stripes [300], bubbles [301], and checker-boards [302] have been reported. For example, Wu et al. [303] reported on the perpendicular stripe magnetic domains in La_0.7 Sr_0.3 MnO₃ nanodots. Takamura et al. [304] reported flower-shaped, flux closure domain, and vortex structures in patterned manganites created by Ar+ ion milling. Mathews et al. [305] reported successful fabrication of La_0.67 Sr_0.33 MnO₃ nanowires on NdGaO₃ substrate by using interference lithography. It was demonstrated that not only the shape anisotropy but also the substrate induced anisotropy play important roles in determining the magnetic easy axis in these manganite nanostructures. In spite of challenges in controlling magnetic domain walls in perovskite manganite oxide nanowires, several groups have reported current induced domain wall motion in perovskite manganite oxide materials such as La_0.7 Sr_0.3 MnO₃ and La_0.67 Ba_0.33 MnO_3-δ . By using FIB milling, Ruotolo et al. [306] and Céspedes et al. [139] patterned La_0.7 Sr_0.3 MnO₃ into nanowires containing notches as the domain wall pinning centers. The MR measurements confirm the current induced domain wall depinning with a critical current density of 10 ¹¹ A/m ² . Liu y col. [307] reported current dependent low-field MR effect in La_0.67 Sr_0.33 MnO₃ nanowires with constrictions and they ascribed this effect to the spin polarized bias current. In a similar constricted La_0.67 Ba_0.33 MnO_3-δ nanowire, Pallecchi et al. [308] observed magnetic field and DC bias current dependent asymmetric resistance hysteresis, which was also connected to the effect spin transfer torque. Surprisingly, the threshold current was found to be in the range of 10 ⁷ –10 ⁸ A/m ² , much smaller than the typical current (10 ¹¹ A/m ² ) needed for moving domain walls in metals [309]. A number of possibilities, such as stronger spin torque due to half metallicity, Joule heating assistance, and spin wave excitation, may contribute to such a drastic reduction in the threshold current.

Spintronic Devices

In terms of perovskite manganites, the large MR and the great tunability of CMR oxides are promising for magnetic recording, spin valve devices, and magnetic tunnelling junctions [310,311,312,313,314]. However, there are several obstacles related to perovskite manganites in nanodevice applications. First, the spin polarization of manganites decays rapidly with temperature. Second, the defect chemistry and the stoichiometry–property correlation in perovskite manganites are quite complex [315, 316]. Third, the physical properties of interfaces in manganite-based devices remain elusive [317, 318]. Finally, there is the urgent need for developing suitable device processing techniques.

The spintronic devices exhibit prominent advantages, such as nonvolatility, increased processing speed, increased integration densities, and reduced power consumption. The significance and value of spintronics lies in the study of active control of carrier spin, and then the development of new devices and technologies, for example, spin transistors, spin-FET, spin-LED, spin-RTD, spin filters, spin modulators, reprogramming gate circuit.

Nanowires have been successfully incorporated into nanoelectronics [319], and naturally they are envisaged as ideal building blocks for nanoscale spintronics. For perovskite manganite oxides, nanowires and related heterostructures hopefully can enhance the Curie temperature [82] and the low-field MR [75, 320]. Another promising research topic is anisotropic magnetoresistance (AMR) which is a result of spin-orbital interaction and has important applications in magnetic field detection and data storage [321]. Significant magnetic anisotropy has been observed in perovskite manganite oxide nanowires [322]. In addition, the morphology of nanowire can be modified by annealing or growing on engineered substrates [323], which could significantly affect their properties. It is expected that more efforts will be made to explore the anisotropic transport properties of perovskite manganite oxide nanowires and to gain deeper understanding of low-dimensional spin-dependent transport properties.

Spin valves based on perovskite manganite oxide nanowires are also reported. Gaucher et al. [324] successfully fabricated La_2/3 Sr_1/3 MnO₃ nanowires with the smallest width of 65 nm by combining electron beam lithography and ion beam etching. They showed that the electronic transport properties of these perovskite manganite oxide nanowires are comparable to those of unpatterned films.

2D Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Magnetic Memory Devices

Typically, micrometer-sized La_{0. 7} Sr_{0. 3} MnO₃ dots with smooth edge and surface are etched in the corresponding films by such lift-off process. Ruzmetov et al. [135] fabricated regular arrays of epitaxial perovskite La_2/3 Sr_1/3 MnO₃ magnetic nanodots by PLD combined with electron beam lithography and argon ions exposure. The diameters of these dots are less than 100 nm with a height about 37 nm. These perovskite magnetic nanodots maintain their crystallinity, epitaxial structure, and ferromagnetic properties after the fabrication process, which have promising applications in magnetic massive memory devices.

Liu y col. [325] demonstrated a new programmable metallization cell based on amorphous La_0.79 Sr_0.21 MnO₃ thin films for nonvolatile memory applications. The schematic diagram of the metallization cell is shown in Fig. 41, where amorphous La_0.79 Sr_0.21 MnO₃ thin films are deposited on the Pt/Ti/SiO₂ /Si substrates by rf magnetron sputtering. The Ag/amorphous La_0.79 Sr_0.21 MnO₃ /Pt cell exhibited reversible bipolar resistive with a R_OFF /R_ON ratio (> 10 ² ), stable write/erase endurance (> 10 ² cycles) with resistance ratio over 10 ² , and stable retention for over 10 ⁴ s. Such a sandwiched device may be a promising candidate for future nonvolatile memory applications.

Schematic of the Ag/La_0.79 Sr_0.21 MnO₃ ( a -LSMO)/Pt memory device. Reproduced with permission of [325]

Hoffman et al. [326] also tested the non-volatile memories using an electric-field-induced M–I transition in the PbZr_0.2 Ti_0.8 O ₃ /La_1-x Sr_x MnO₃ (PZT/LSMO), PZT/La_1-x Ca_x MnO₃ (PZT/LCMO), and PZT/La_1-x Sr_x CoO₃ (PZT/LSCO) devices. To study the switching speed of the Mott transition field effect devices, they fabricated a series of devices where the room temperature RC-time constants varied from 80 ns to 20 μs. They found the circuit RC- time constant limited the switching speed of devices down to 80 ns, offering the opportunity for faster operation though device scaling. Room temperature retention characteristics show a slow relaxation, with more than 75% of the initial polarization maintained after 21 days. These Mott transition field effect devices have promising potential for future non-volatile memory applications.

Spintronic Devices

Spin valves may be the most influential spintronic devices which have already found applications in magnetic data storage industry. Its basic working principle is the GMR effect [327, 328], where the resistance of a FM/NM (non-magnetic)/FM multilayers depends on the relative alignment between the two FM layers. Most GMR research focuses on transition metals, but spin valves have also been realized in oxides, in particular FM manganites [329, 330]. A related concept is the magnetic tunnel junction (MTJ) [331, 332], which also has vast applications in nonvolatile magnetic memory devices. The basic difference between spin valve and MTJ lies in the middle spacer layer, which needs to be insulating for MTJ, whereas for spin valve, this layer is conducting. A number of efforts have been devoted to create oxide based MTJ devices so far, especially due to the 100% spin polarization in several FM oxides, such as La_1-x Sr_x MnO₃ with x ~ 0.33, CrO₂ and Fe₃ O ₄ . Lu et al. [333] and Sun et al. [334] first fabricated all-oxide MTJ device with La_0.67 Sr_0.33 MnO₃ and SrTiO₃ as FM and insulating layer, respectively. Subsequently, a record tunneling magnetoresistance (TMR) ratio of 1850% was reported in 2003 by Bowen et al. [335]. Despite these promising results, a major issue for the oxide based MTJ devices is that the working temperature is often lower than the room temperature, generally ascribed to the degraded interfaces [336, 337].

Magnetic Sensors

The application of perovskite manganite CMR thin films to magnetic sensors at room temperature has been considered. Compared to magnetoresistive sensors using permalloy films, the field coefficient of resistance (dR/dH)/R is much smaller, typically of about (10 ⁻² –10 ⁻¹ )% per mT. However, they can operate over a wide field range and their characteristics should be maintained at submicron lateral size, since the CMR mechanism does not involve large scale entities such as magnetic domains and walls. By means of magnetic flux concentration with soft ferrite poles, the field coefficient of resistance can be increased up to 4% per mT [338]. Other applications of CMR of La_0.67 Sr_0.33 MnO₃ thin films to position sensors and to contact-less potentiometers are presently investigated [339]. The basic idea is to exploit the large resistance variation induced by the stray field of a permanent magnet in Sm-Co or Nd-Fe-B alloys.

In recent years, there has been growing demand for sensitive yet inexpensive infrared detectors for use in a variety of civilian, industrial, and defense applications such as thermal imaging, security systems and surveillance, night vision, biomedical imaging, fire detection, and environmental detection. The material for bolometric applications should possess a high temperature coefficient of resistivity (TCR), which enables small temperature variations caused by absorbed infrared radiation (IR) generate a significant voltage drop across the bolometer. High TCR in recently discovered CMR manganese oxides in the vicinity of metal-to-semiconductor phase transition makes them suitable for thermometer and bolometer applications. For example, Lisauskas et al. [340] reported that the epitaxial submicron thick perovskite manganite La_0.7 (Pb_0.63 Sr_0.37 )_0.3 MnO₃ films deposited on LAO single crystal by PLD exhibited high value of TCR (=7.4% K ⁻¹ @ 295 K). Choudhary et al. [341] synthesized the polycrystalline/amorphous films with mixed-valence manganites (e.g., La_0.7 Ca_0.3 MnO₃ , La_0.5 Sr_0.5 MnO₃ , La_0.5 Ba_0.5 MnO₃ , and (La_0.6 Pr_0.4 )_0.67 Ca_0.33 MnO₃ ) by PLD at low temperature (450 °C) on single crystal (001) silicon substrate. These films are evaluated for uncooled bolometric applications.

Solid Oxide Fuel Cells

Lussier et al. [342] identified a mechanism whereby the strain at an interface is accommodated by modifying the chemical structure of the SOFC material to improve the lattice mismatch and distribute the strain energy over a larger volume (thickness), concentrate on two particular manganite compounds, La_2/3 Ca_1/3 MnO₃ and La_1/2 Sr_1/2 MnO₃ thin films.

3D Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Recently, several 3D rare earth-doped perovskite manganite oxide nanostructures such as 3D (La_0.275 Pr_0.35 Ca_0.375 )MnO₃ nanobox array structures (145), 3D strained LSMO–CeO₂ VAN nanostructures (198) fabricated by PLD technique are reported. It was found that 3D (La_0.275 Pr_0.35 Ca_0.375 )MnO₃ nanobox array structures exhibited an insulator-metal transition at higher temperature than that in the corresponding thin film, which provided a new way to tune the physical properties of CMR oxide 3D nanostructures. This enables 3D (La_0.275 Pr_0.35 Ca_0.375 )MnO₃ nanobox array structures to find promising application in oxide nanoelectronics by making full use of the huge electronic/spintronic phase transition. The 3D framework of LSMO–CeO₂ VAN nanostructures combine not only the lateral strains from the layered structures but also the vertical strain from the VAN, and thus maximize the 3D strain states in the systems, controlling the electron transport paths. This new 3D framed design provides a novel approach in maximizing film strain, enhancing strain-driven functionalities, and manipulating the electrical transport properties effectively. At present, the applications of 3D rare earth-doped perovskite manganite oxide nanostructures in the fields of oxide nanoelectronics, spintronics, and solar energy conversion are still in their infancy; thus, many problems remain unsolved and technical challenges lie ahead. In this direction, there is a long way to walk on before the commercialization of rare earth-doped perovskite manganite oxide nanostructures.

Conclusions and Perspectives

In this work, we have discussed the recent advances in the fabrication, structural characterization, physical properties, and functional applications of rare earth-doped perovskite manganite oxide nanostructures. It is our aim that we have captured all the excitements achieved in the development of rare earth-doped perovskite manganite oxide nanostructures used for microelectronic, magnetic, and spintronic nanodevices, providing some useful guidelines for the future researches. In spite of great progress made in the past two decades, considerable effort is highly required to realize the practical applications of rare earth-doped perovskite manganite oxide nanostructures in the next generation of oxide nanoelectronics. While many fascinating physical properties of rare earth-doped perovskite manganite oxide nanostructures are originated from the interactions among the spin, charge, orbital, and lattice degrees of freedom, whereas there is still a long way to go for obtaining a full understanding of the interaction mechanisms among the spin, charge, orbital, and lattice degrees of freedom. It is expected that in the next years, further progress will be achieved in the experimental and theoretical investigations on rare earth-doped perovskite manganite oxide nanostructures. We believe that this review of the recent advances on rare earth-doped perovskite manganite oxide nanostructures will motivate their future researches and applications of not only in the fields of oxide nanoelectronics, but also in energy and biomedical fields.

Disponibilidad de datos y materiales

It is a review article that gives a comprehensive overview of the recent progress in the fabrication, structural characterization, physical properties, and functional applications of rare earth-doped perovskite manganite oxide nanostructures.

Abreviaturas

0D:

Zero-dimensional

1D:

One-dimensional

2D:

Two-dimensional

AAO:

Anodic aluminium oxide

AFM:

Antiferromagnetic

AFM:

Microscopía de fuerza atómica

CMR:

Colossal magnetoresistance

CSD:

Chemical solution deposition

CTAB:

Cetyltrimethylammonium bromide

CVD:

Deposición de vapor químico

EDS:

Energy Dispersive X-ray spectroscopy

EELS:

Electronic energy loss spectroscopy

EPS:

Electronic phase separation

FC:

Field cooling

FE-SEM:

Field-emission scanning electron microscopy

FIB:

Focus ion beam

FM:

Ferromagnetic

FTIR:

Fourier-transform infrared spectroscopy

HRTEM:

High-resolution TEM

LAO:

LaAlO₃

LCMO:

La_1-x Ca_x MnO₃

LPCMO:

(La_5/8-0.3 Pr_0.3 )Ca_3/8 MnO₃

LSMO:

La_0.7 Sr_0.3 MnO₃

MBE:

Epitaxia de haz molecular

MCE:

Magnetocaloric effect

M-H:

Microwave-hydrothermal

M–I:

Metal-insulator

MOCVD:

Metalorganic chemical vapor deposition

MR:

Magnetoresistance

MRI:

Magnetic resonance imaging

MSS:

Molten salt synthesis

MTJ:

Magnetic tunneling junctions

NGO:

NdGaO₃

PLD:

Pulsed laser deposition

PMMA:

Polymethylmethacrylate

PVD:

Physical vapor deposition

RCP:

Relative cooling power

SAED:

Difracción de electrones de área seleccionada

SEBL:

Scanning electron beam lithography

SG:

Spin glass

SOFCs:

Solid oxide fuel cells

SPM:

Super-paramagnetic

STEM:

Scanning transmission electron microscopy

STO:

SrTiO₃

TCR:

Temperature coefficient of resistivity

TEM:

Microscopía electrónica de transmisión

UHV:

Ultra-high vacuum

VOCs:

Volatile organic compounds

XPS:

X-Ray photoelectron spectroscopy

XRD:

Difracción de rayos X

XRL:

X-ray lithography

YSZ:

Yttria-stabilized zirconia

ZFC:

Zero-field cooling

Relaciones lógicas y dependientes de electrones de valencia fuertes de impurezas elementales en semiconductores binarios 2D:un caso de monocapa GeP3 de estudios ab initio Síntesis altamente eficiente de nanopartículas de fosfuro de molibdeno a base de carbono para la evolución electrocatalítica de hidrógeno