Growth and characterization of epitaxial (La2/3Sr1/3)MnO3 films by pulsed laser deposition

High quality epitaxial (La_2/3Sr_1/3)MnO₃ (0 0 1) thin films were grown by pulsed laser deposition on SrTiO₃ (0 0 1) substrate at optimized growth parameters. The films quality was confirmed by both structural and physical properties characterization. Channeling Rutherford Backscattering Spectrometry characterization showed the minimal channeling coefficient as low as 4%. The LSMO thin films growth on SrTiO₃ substrate follows the island growth model. The Curie temperature of LSMO films is around 360 K, which is the one of the highest reported in literature. The resistivity of LSMO films showed the metal-insulate transition temperature coincides with the Curie temperature. This high quality LSMO is suitable for room temperature magnetic devices application.     

Improved electrochemical performance of La0.7Sr0.3MnO3 and carbon co-coated LiFePO4 synthesized by freeze-drying process

Carbon coated LiFePO₄ particles were first synthesized by sol-gel and freeze-drying method. These particles were then coated with La_0.7Sr_0.3MnO₃ nanolayer by a suspension mixing process. The La_0.7Sr_0.3MnO₃ and carbon co-coated LiFePO₄ particles were calcined at 400 °C for 2 h in a reducing atmosphere (5% of hydrogen in nitrogen). Nanolayer structured La_0.7Sr_0.3MnO₃ together with the amorphous carbon layer forms an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high-resolution transmission electron microscopy. X-ray diffraction results proved that the co-coated composite still retained the structure of the LiFePO₄ substrate. The twin coatings can remarkably improve the electrochemical performance at high charge/discharge rates. This improvement may be attributed to the lower charge transfer resistance and higher electronic conductivity resulted from the twin nanolayer coatings compared with the carbon coated LiFePO₄.     

Magnetic and electric properties of wrinkled manganite films derived by sol‐gel method

Flat and wrinkled La_0.7Sr_0.3MnO₃ (LSMO) thin films were prepared by sol‐gel method, respectively, on Si (001) substrates by adjusting heating rate at drying stage. Wrinkled film has larger grains than flat film. Coercive field (about 27 Oe) of wrinkled film is higher than that of flat film, which is much low as around 5 Oe. Compared with flat films, wrinkled films have larger magnetization, higher Curie temperature (334 K) and peak resistivity temperature (243 K), and lower resistivity (0.18 Ohm·cm at 300 K). The introducing of wrinkles is an efficient way to induce compressive stress in sol‐gel derived polycrystalline LSMO films and enhance the magnetic and electric properties.     

Electrochemical properties of the single crystal La0.7Pb0.3MnO3 electrode

Electrochemical properties of the single crystal La_0.7Pb_0.3MnO₃ were studied. This oxide is stable only at the anodic potential region in alkaline solution. It was found that the catalytic activities for the oxygen reduction and evolution reactions are relatively high due to the effect of the B site Mn cation on the oxide surface. However, the mechanisms of the oxygen reduction and evolution reactions are different from the case of La_1-xSr_xMnO₃ electrodes and it was presumed that these differences are assigned to the influence of the A site Pb cation in La_0.7Pb_0.3MnO₃ and that of the (100) crystal plane.     

A-site Ca/Sr co-doping to optimize room-temperature TCR of La0.7Ca0.3-xSrxMnO3 films

Based on the analysis of crystal structure, Mn³⁺/Mn⁴⁺ pairs, distortion of MnO₆ octahedron, and electrical transport properties of La_1-xCa_xMnO₃ and La_1-xSr_xMnO₃ materials, room-temperature coefficient of resistivity (TCR) of La_0.7Ca_0.3-xSr_xMnO₃ (LCSMO) films was optimized by Ca/Sr co-doping at the A-site. LCSMO films are successfully fabricated on LaAlO₃ (100) substrates via facile spin coating technology. The microstructure of LCSMO films is characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray photoemission spectroscopy. Results reveal that A-site Ca/Sr co-doping significantly influenced crystal structure, formation of Mn³⁺/Mn⁴⁺ pairs, and distortion of MnO₆ octahedron. The correlation between microstructure and electrical transport properties was explained through the phenomenological percolation model, double-exchange mechanism and Jahn-Teller effect. Furthermore, the TCR reached 10.2% K^-1 at 296.1 K in La_0.7Ca_0.18Sr_0.12MnO₃ films.     

Structural,magnetic and magnetocaloric study of La0.7−xEuxSr0.3MnO3 (x=0.1, 0.2 and 0.3) manganites

This paper reports the structural, magnetic and magnetocaloric properties of La_0.7−xEu_xSr_0.3MnO₃ (x=0.1, 0.2 and 0.3) polycrystalline manganites elaborated using the solid-state reaction at high temperatures. The X-ray powder diffraction shows that all the prepared compounds are single phase. La_0.6Eu_0.1Sr_0.3MnO₃ is crystallized in the rhombohedral symmetry, whereas a structural transition towards orthorhombic system is observed for x≥0.2. Eu doping was found to induce a decrease of the Curie temperature T_C from 343 K (x=0.1) to 272 K (x=0.3). All compounds undergo a large magnetocaloric effect and have consequently potential applications in magnetic refrigeration domain around room temperature.     

Synthesis of La1−xSrxMnO3 powders by polymerizable complex method: Evaluation of structural, morphological and electrical properties

Lanthanum strontium manganite (La_1−xSr_xMnO₃, LSM) powders were synthesized by polymerizable complex method, based on complexation of metal ions (MI) with citric acid (CA) and polyesterification between CA and ethylene glycol (EG). Firstly, the effect of the molar ratio of CA:MI (=1–3) was investigated on the synthesis of La_0.7Sr_0.3MnO₃ powders, which were characterized by thermal analysis (TGA/DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results indicated that the molar ratio CA:MI = 3 is adequate for a good crystallization of pure perovskite phase after calcination, with nanometric crystallite sizes and porous microstructure. For the La_0.7Sr_0.3MnO₃ sample synthesized with CA:MI ratio of 3, it was investigated the effect of calcination temperature, showing that the perovskite structure is better crystallized at 900 °C, without secondary phase formation. Using this same CA:MI ratio and calcination temperature, powders with different Sr content (x = 0.2–0.4) were synthesized, with surface areas of 4–10 m² g⁻¹. These powders were sintered at 1100 °C to produce porous pellets. The porosity of the sintered pellets and the electrical conductivity, measured by two-probe technique, increased with increasing Sr content.     

Polarization-induced electro resistance and magneto resistance in La0.67Ca0.33MnO3/BaTiO3 composite film

La_0.67Ca_0.33MnO₃/BaTiO₃ composite films have been grown on Nb-doped SrTiO₃ substrates by the sol–gel method. The magnetic and ferroelectric properties in the composite films are investigated. A three-state memory is formed by applying a vertical electric field across the La_0.67Ca_0.33MnO₃/BaTiO₃ heterostructure, this behavior is attributed to the polarization-mediated resistive switching effect. In addition, the transport properties of La_0.67Ca_0.33MnO₃ thin film can be modulated by an external magnetic field, a 10.3 K shift of the metal insulator transition temperature is obtained with the change of applied magnetic field from 0 T to 6 T. Consequently, in La_0.67Ca_0.33MnO₃/BaTiO₃ heterostructure, the resistance behavior can be modulated by piezoelectric effect, ferroelectric polarization and magnetic field simultaneously.     

Effects of Particle Size on Electromagnetic and Microwave Absorption Properties of La0.7Sr0.3MnO3±δ‐Epoxy Composite

La_0.7Sr_0.3MnO_3±δ powders were fabricated by solid‐state reaction method at 1473 K for 4 h. The precursors were prepared by ball‐milling raw materials for 3, 6, 9, and 12 h, respectively. The crystal structures, particle size, and morphologies of precursors and prepared La_0.7Sr_0.3MnO_3±δ were characterized by XRD, laser particle size analyzer and SEM, respectively. It is found that La_0.7Sr_0.3MnO_3±δ possessed large particle size by ball‐milling raw materials for a long time. Results indicated that La_0.7Sr_0.3MnO_3±δ, synthesized by ball‐milling raw materials for 3 h, exhibited the optimal microwave absorption properties. The maximum reflection loss was ?28.8 dB, and the ?6 dB absorption bandwidth was 5.80 GHz.     

Growth and structural characteristics of perovskite-wurtzite oxide ceramics thin films

Wurtzite ZnO thin films were grown on single-crystal perovskite SrTiO₃(STO) (1 0 0) substrates at various temperatures. The ZnO/STO thin films thus formed exhibit a preferred (1 1 0)-orientation at a growth temperature of 600-700 °C. A high growth temperature enhances not only the (1 1 0)-texture of ZnO/STO thin films but also the crystalline quality of the film. (La_0.7Sr_0.3)MnO₃ (LSMO) thin films were subsequently grown on ZnO(1 1 0)/STO(1 0 0) substrates with various thicknesses, and were polycrystalline. A thicker LSMO film has a stronger (0 0 l)-preferred orientation than the thinner one. The lattice distortion of LSMO decreases as the LSMO thickness increases. Magnetization vs. temperature curves show that both crystalline quality and lattice distortion influence the magnetic properties of LSMO thin films. The physical properties of the manganite oxide can be modulated by forming a heterostructure with wurtzite ZnO.     

Strain effects in epitaxial La-doped SrSnO3 films on lattice-matched PMN-PT substrates

Here we report the effect of the strain states on the structure, optical and electrical transport properties of the La_0.05Sr_0.95SnO₃ (LSSO) thin films grown epitaxially on (001)-oriented 0.70 Pb(Mg_1/3Nb_2/3)O₃-0.30PbTiO₃ (PMN-PT) substrates by pulsed laser deposition. X-ray diffraction results indicate that the films are fully strained up to at least 100 nm thickness, and the in-plane compressive strain gradually releases in thicker films. High-resolution transmission electron microscopy characterizations demonstrate that the LSSO films were grown coherently on PMN-PT(001) substrates. With varying the thicknesses of the fully strained films from 20 to 100 nm, the electrical transport properties are improved significantly. A lowest room-temperature resistivity of 1.88 mΩcm and the highest mobility of 28.1 cm²/Vs are obtained in the 100 nm film. The optical band gap determined from spectroscopic ellipsometry is found to increase from 4.58 to 4.88 eV with the film thicknesses varying from 20 to 500 nm. The results imply that the LSSO epitaxial films exhibit tunable electrical performances and optical band gaps through strain, which may have potential applications in optoelectrical devices.     

Orientation-modulated multiferroic properties of BCZT/LCMO bilayer heterostructures

In this paper, orientation-modulated multiferroic properties of Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃/La_0.67Ca_0.33MnO₃ (BCZT/LCMO) bilayer heterostructures grown on (001), (110), and (111)-oriented Nb-doped SrTiO₃ single-crystal substrates are investigated. Epitaxially grown heterostructures with sharp and clear interfaces are verified by TEM analysis. Results show the strong orientation dependence of multiferroic properties for the heterostructures. The optimal ferroelectric and dielectric properties lie in the (001) direction, whereas the improved saturation magnetization and piezoelectric response are achieved in (111)-oriented heterostructure. The maximum magnetoelectric coupling is obtained in (111)-oriented heterostructure with a α_E31 value of 207 mV/cm·Oe.     

Nitric Oxide Reduction Using Hydrogen Over Perovskite Catalysts with Promotional Effect of Platinum on Catalytic Activity

The catalytic activities of strontium substituted La_0.7Sr_0.3MnO₃ type perovskite catalysts for NO reduction using H₂ as reducing agent has been studied, which is further improved by incorporation of Pt outside (0.1 wt.%Pt/La_0.7Sr_0.3MnO₃) and inside (La_0.7Sr_0.3Mn_0.97Pt_0.03O₃) the perovskite lattice structure. Pt shows excellent enhancement in catalytic selectivity towards N₂ when supported on the perovskite. The catalysts were characterized using thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area. Catalysts evaluations were carried out using thermo-gravimetric analysis coupled with mass spectra (TG-MS).     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

Magnetic and magnetoresistive properties of La0.65−xEuxSr0.35MnO3

Eu-doped perovskites La_0.65−xEu_xSr_0.35MnO₃ (0.05 ≤ x ≤ 0.30) were synthesized by sol–gel method using citric acid and characterized by X-ray diffraction, magnetization, resistivity and magnetoresistance (MR) experiments. All samples had a single hexagonal perovskite structure. As x increased from 0.05 to 0.30, the Curie temperature T_C for the samples decreased from 352 to 242 K. It was found that two transition points appeared when the resistivity changed with increasing temperature, and upon an application of a magnetic field of 20 kOe the maximum magnetoresistivity of 18% for the La_0.65−xEu_xSr_0.35MnO₃ with x = 0.20 was obtained at room temperature 300 K. The mechanism of the transitions for the samples was explored.     

Thickness‐induced anomalous angular‐dependent magnetoresistance of La2/3Sr1/3MnO3 thin films grown on SrTiO3

In this paper the effect of thickness on angular‐dependent magnetoresistance (MR) of La_2/3Sr_1/3MnO₃ (LSMO) thin films grown on SrTiO₃ is systematically investigated. In films thinner than 8 nm, we observe an anomalous weak peak in the MR curves when the magnetic field is parallel to the film surface. In films thicker than 10 nm, however, the weak peak disappears and a novel MR valley appears when the magnetic field is perpendicular to the film surface. The weak peak is thought to be induced by the formation of two‐dimensional electron gas (2DEG) at the interface, which is confirmed by density functional theory calculations. The disappearance of the peak and appearance of the valley in the films thicker than 10 nm is associated with the formation of misfit dislocations near the interface between film and substrate, which are clearly visible in high‐resolution transmission electron microscopy images. Our work could shed significant light on the influence of film thickness on 2DEG formation.     

Electrical and magnetic properties of La1−xSrxMnO3 (0.1 ≤ x≤ 0.25) ceramics prepared by sol–gel technique

La_1−xSr_xMnO₃ (0.1 ≤ x ≤ 0.25) high density ceramics were prepared by sol-gel method using methanol as solvent. X-ray diffraction analysis showed that all samples exhibited single perovskite structure and no second phase was detected. Scanning electron microscopy images exhibited good particle connectivity on the surface of sample, and grain size increased with the increase in Sr doping. Resistivity-temperature curves of samples were measured by standard four-probe method, and curves exhibited significant differences in studied range of Sr doping. Magnetic measurement results indicated that the variation of susceptibility of different samples was quite different, and the Curie temperature of samples increased with the increase in Sr content. For x = 0.2, temperature coefficient of resistance value of the sample was larger, and corresponding peak TCR temperature was 307.1 K, which is very close to room temperature. Thus, La_0.8Sr_0.2MnO₃ ceramics exhibited high TCR value close to room temperature. Combined with its excellent magnetic properties, La_0.8Sr_0.2MnO₃ ceramics may potentially act as effective candidates for uncooled radiation calorimeter and uncooled magnetic sensor. Applications of La_0.8Sr_0.2MnO₃ ceramics in uncooled infrared radiation calorimeter at room temperature will be highly beneficial.     

Microstructure and Dielectric Properties of Textured SrTiO3 Thin Films

We investigate the relationship between microstructure and dielectric properties of textured SrTiO₃ thin films deposited by radio-frequency magnetron sputtering on epitaxial Pt electrodes on sapphire substrates. The microstructures of Pt electrodes and SrTiO₃ films are studied by transmission electron microscopy, atomic force microscopy, and X-ray diffraction. SrTiO₃ films grown on as-deposited and annealed Pt electrodes, respectively, consist of a mixture of (111)- and (110)-oriented grains. Temperature-dependent dielectric measurements show that differences in texture and microstructure are reflected in the Curie–Weiss behavior of the SrTiO₃ films. Phenomenological models that account for the effects of thermal mismatch strain on the dielectric behavior are developed for different film textures. The models predict that at a given temperature, paraelectric (111)-oriented films of SrTiO₃ on tensile substrates will have a higher Curie–Weiss temperature and a greater dielectric constant than (110)-oriented films or bulk SrTiO₃. The experimental dielectric behavior is compared with the predictions from theory, and different contributions, such as interfacial layers, film stress, and microstructure, to the Curie–Weiss behavior are discussed.     

Crystallographic orientation dependent dielectric properties of epitaxial BaTiO3 thin films

Epitaxial BaTiO₃ ferroelectric films with (001), (110), and (111) orientations were, grown on SrRuO₃ buffered SrTiO₃ substrates via a radio-frequency magnetron sputtering method. The effects of the orientation on dielectric response were systematically investigated. We found that the (110)-oriented film exhibited the largest dielectric permittivity, tunability and phase transition temperature, whereas the (111)-oriented film demonstrated the best figure of merits if the dielectric loss was taken into consideration. The large anisotropic dielectric behavior, which is predominantly attributed to the differences of polarization states in these orientation engineered BaTiO₃/SrTiO₃ heterostructures, was observed. These results suggest the (111)-oriented film may be an attractive candidate for applications in dielectric tunable devices.