Transmission electron microscopy study of multilayer p-n hetero-junction La0.9Sr0.1MnO3/SrNb0.05Ti0.95O3 thin films

We have fabricated multilayer p-n hetero-junction structure La_0.9Sr_0.1MnO₃/SrNb_0.05Ti_0.95O₃ colossal magneto-resistivity thin films. The observations of transmission electron microscopy showed that the interfacial structure between La_0.9Sr_0.1MnO₃ and SrNb_0.05Ti_0.95O₃ was of great epitaxy and atomic level smooth. There was a superstructure within La_0.9Sr_0.1MnO₃ thin films which was clarified in terms of the ordered partial substitution of La with Sr at center body of unit cell. Its chemical composition and distribution information investigated by electron energy loss spectrum and element mapping in the multilayer thin films were discussed.     

Preparation and colossal magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3 device by dc magnetron sputtering

Epitaxial trilayer films of La_0.67Sr_0.33MnO₃ (LSMO)/La_0.75MnO₃ (L0.75MO)/La_0.67Sr_0.33MnO₃ (LSMO) have been prepared on (0 0 1) oriented LaAlO₃ substrates by dc magnetron sputtering. The structure and MR are studied. All as-deposited trilayer films exhibit a semiconductor to metal transition at temperature ranging from 116 to 185 K. The MR is also shown to be dependent on the thickness of the middle oxide layer. A maximum MR value of 32% (ΔR/R₀) has been obtained at 132 K under 0.4 T magnetic field for a LSMO (300 nm)/L0.75MO (70 nm)/LSMO (300 nm) trilayer film. The MR of trilayer film prefers to that of both LSMO and L0.75MO single layer films.     

Substitution induced B-cation ordering in solid oxide fuel cell ceramics: (La0.8Sr0.2)(M0.9Ni0.1)O3 (M = Mn, Cr)

Perovskites are important materials in a number of important technological applications, including solid oxide fuel cells, catalysis, and giant magneto-resistance materials. For many of these purposes, a mixture of B-cations can be used to tune the desired properties, e.g., oxygen reduction, ionic conductivity. For a solid oxide fuel cell, two particular ceramic components are of critical importance and have been extensively studied, the cathode (La_0.8Sr_0.2)MnO_3−x and the interconnect material (La_0.8Sr_0.2)CrO₃. In this study, we examined the mixed B-cation perovskites (La_0.8Sr_0.2)(M_0.9Ni_0.1)O₃ (M = Mn, Cr). All materials were synthesized using the glycine-nitrate method, followed by air annealing. The structures were determined using powder neutron diffraction methods. Refinement of the data showed that even at this low concentration, the compounds have monoclinic symmetry (P2₁/n) and that the nickel had a strong preference for the smaller of the two octahedral sites. This small amount of nickel substituted on the B-site resulted in a symmetry reduction when compared to the unsubstituted (LaSr)MnO₃ or (LaSr)CrO₃ materials. Although this structural type has been seen previously in heavily substituted perovskites, these materials show that even at this low level of substitution a segregation of the metals in a manner similar to the double perovskites A₂BB′O_6−x can be detected. This may have implications involving material stresses on cycling that may result as the temperature is raised or lowered through this crystallographic transition.     

Photoconductivity in BiFeO3/La0.7Sr0.3MnO3 heterostructure

In this letter, an oxide heterostructure has been fabricated by successively growing La_0.7Sr_0.3MnO₃ (LSMO) and BiFeO₃ (BFO) layers on LaAlO₃ (100) by pulsed laser deposition. Analysis of the leakage current at different temperature demonstrated that the Poole-Frenkel dominated the leakage current mechanism. Additionally, the BiFeO₃/La_0.7Sr_0.3MnO₃ heterostructure exhibits a positive colossal magnetoresistance (MR) effect over a temperature range of 50-320 K. The maximum MR values are determined to be about 45.32% at H = 0.5 T and 28.34% at H = 0.3 T. At last, we report photoconductivity in BiFeO₃/La_0.7Sr_0.3MnO₃ film under illumination from 160 mW/cm² and 200 mW/cm² green-light source, and photoconductivities increase with the intensity of light enhanced.     

Magnetic coupling in La0.67Ca0.33MnO3/La0.67Sr0.33CoO3/La0.67Ca0.33MnO3 sandwiches

La_0.67Ca_0.33MnO₃ (LCMO)/La_0.67Sr_0.33CoO₃ (LSCO)/LCMO trilayer films are fabricated on single-crystal substrates NdGaO₃ (110) and the interlayer coupling are investigated. Compared with LCMO single layer, sandwiches showed the enhanced metal-insulator transition temperature of LCMO layers. The magnetoresistance is dependent on spacer thickness and the peak value dramatically decreases when LSCO layer is thick enough because of shorting by the LSCO layer. The magnetic coercivity H_C shows a nonmonotonic behavior with changing spacer layer thickness and the waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom LCMO layers across the spacer layer.     

Electrical properties of Pb0.97La0.02(Zr0.95Ti0.05)O3 antiferroelectric thin films on TiO2 buffer layer

Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ antiferroelectric thin films with thickness of 500 nm were successfully deposited on TiO₂ buffered Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) and Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates via sol-gel process. Microstructure of Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ thin films was studied by X-ray diffraction analyses. The antiferroelectric nature of the Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ thin films was confirmed by the double hysteresis behaviors of polarization and double buffer fly response of dielectric constant versus applied voltage at room temperature. The capacitance-voltage behaviors of the Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ films with and without TiO₂ buffer layer were studied, as a function of temperature. The temperature dependence of dielectric constant displayed a similar behavior and the Curie temperature (T_c) was 193 °C for films on both substrates. The current caused by the polarization and depolarization of polar in the Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ films was detected by current density-electric field measurement.     

Influence of ZnO additions to 0.8(Mg0.95Co0.05)TiO3–0.2Ca0.6La0.8/3TiO3 ceramics on sintering behavior and microwave dielectric properties

The effects of ZnO addition on the microstructures and microwave dielectric properties of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics were investigated. ZnO was selected as liquid phase sintering aids to lower the sintering temperature of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics. With ZnO additives, the densification temperature of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ can be effectively reduced from 1450 to 1200–1325 °C. The crystalline phase exhibited no phase difference at low addition levels (0.25–2 wt.%). It is found that low-level doping of ZnO (0.25–2 wt.%) can significantly improve the density and dielectric properties of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ ceramics. The quality factors Q × f were strongly dependent upon the amount of additives. Q × f values of 36 000 and 13 000 GHz could be obtained at 1200–1325 °C with 1 and 2 wt.% ZnO additives, respectively. During all additives ranges, the relative dielectric constants were significantly different and ranged from 23.1 to 27.96. The temperature coefficient varies from 14.1–24.3 ppm/°C.     

Effect of A-site deficiency on electrical transport properties of YSZ doped manganites

The electrical transport properties of the composite samples: A-site stoichiometry (1−x)La_2/3Ca_1/3MnO₃+xYSZ and A-site deficiency (1−x)(La_2/3Ca_1/3)_0.95MnO₃+xYSZ with different YSZ percentage x were investigated, respectively. The similar metal-insulator transition at different temperature T_P was observed in the two types of composite samples: With increasing of the YSZ percentage, the zero-field resistivity of the composite samples increases and T_P shifts to the low temperature for the range of x<2%, but zero-field resistivity decreases and T_P increases for the range of x>2%. For the same YSZ percentage x, T_P of (1−x)La_2/3Ca_1/3MnO₃+xYSZ is lower than that of (1−x)(La_2/3Ca_1/3)_0.95MnO₃+xYSZ. The different effects between the two type composites were also observed in the nonlinear current-voltage (I-V) behaviors. The results suggest that A-site deficiency and YSZ dopant can be used to adjust purposely the metal-insulator transition temperature and resistivity of composites.     

Effect of high temperature post-annealing of La0.7Sr0.3MnO3 films deposited by radio frequency magnetron sputtering on SiO2/Si substrates heated at low temperature

La_0.7Sr_0.3MnO₃ thin films were deposited on SiO₂/Si substrates by RF magnetron sputtering under different oxygen gas flow rates with a sputtering power of 100 W. During deposition, the substrate was heated at 623 K. To investigate post-annealing effects, the as-deposited La_0.7Sr_0.3MnO₃ thin films were thermal-treated at 973 K for 1 h. The effects of oxygen gas flow rate and post-annealing treatment on the physical properties of the films were systematically studied. X-ray diffraction results show that the growth orientation and crystallinity of the films were greatly affected by the oxygen gas flow rate and substrate heating during deposition. The sheet resistance of the films gradually decreased with increasing oxygen gas flow rate, while the post-annealed films showed the opposite behavior. The temperature coefficient of resistance at 300 K of La_0.7Sr_0.3MnO₃ thin films deposited at an oxygen gas flow rate of 40 sccm decreased from − 2.40%/K to − 1.73%/K after post annealing. The crystalline state of the La_0.7Sr_0.3MnO₃ thin films also affected its electrical properties.     

Effects of chemical stability of heterogeneous oxides on transport properties of La0.67Ca0.33MnO3

Two kinds of composites of La_0.67Ca_0.33MnO₃/ZrO₂ and La_0.67Ca_0.33MnO₃/YSZ have been investigated, in which the heterogeneous oxide YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃ + 92 mol% ZrO₂). Their transport properties are measured in a temperature range of 10-300 K and a magnetic field range of 0-3 T. With increase in ZrO₂ doping level for the range of 0-2%, the metal-insulator transition temperature, T_P, decreases, but T_P increases with the doping level increase further for higher than 2%. Meanwhile, in the composite of La_0.67Ca_0.33MnO₃/ZrO₂, besides the intrinsic metal-insulator (ferromagnetic-paramagnetic) transition, a new kink and a widen transition temperature region are observed both in the temperature dependence of resistivity and magnetization curves. Compared to pure La_0.67Ca_0.33MnO₃ and the composite of La_0.67Ca_0.33MnO₃/YSZ, we assume that the different effects between ZrO₂ and YSZ on transport behavior of La_0.67Ca_0.33MnO₃ result mainly from their different chemical stability.     

A new low-loss microwave dielectric using (Ca0.8Sr0.2)TiO3-doped MgTiO3 ceramics

The microwave dielectric properties and the microstructures of the (1-x)MgTiO₃-x(Ca_0.8Sr_0.2)TiO₃ ceramic system prepared by the conventional solid-state route were investigated. (Ca_0.8Sr_0.2)TiO₃ was employed as a τ_f compensator and was added to MgTiO₃ to achieve a temperature-stable material. Ilmenite-structured MgTiO₃ and perovskite-structured (Ca_0.8Sr_0.2)TiO₃ were coexisted and the two-phase system was confirmed by the X-ray diffraction patterns and the energy-dispersive X-ray analysis. Although the ε_r of the specimen could be boosted by increasing amount of (Ca_0.8Sr_0.2)TiO₃, it would instead render a decrease in the Q × f. The τ_f value is strongly correlated to the compositions and can be controlled through the existing phases. In fact, τ_f could be adjusted to a near-zero value by mixing 94 mole% MgTiO₃ and 6 mole% (Ca_0.8Sr_0.2)TiO₃. A dielectric constant (ε_r) of 21.42, a high Q × f value of 83,700 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of − 1.8 ppm/°C were obtained for 0.94MgTiO₃-0.06(Ca_0.8Sr_0.2)TiO₃ sintered at 1300 °C for 4 h. It is proposed as a low-loss and low-cost dielectric material for microwave and millimeter wave applications.     

The demonstration of epitaxial and electronic-magnetic properties for La0.67Sr0.33MnO3−δ films with (1 1 1) orientation

La_0.67Sr_0.33MnO_3−δ films, fabricated on (1 1 1) LaAlO₃ single-crystal substrates using a direct current magnetron sputtering technique, are demonstrated by X-ray diffraction patterns and pole figures to be high quality epitaxial films and there is a perfect matching relationship between the films and the substrates. We observed an obvious difference of the electronic-magnetic transportation properties among films sputtered on (1 1 1), (1 0 0) and (1 1 0) LaAlO₃ substrates, respectively. A mechanism for the difference is discussed briefly.     

Broadening of the magnetic entropy change in La0.75Ca0.15Sr0.10MnO3

A broad table-like entropy change (ΔS) at room temperature has been observed in the ferromagnetic compound La_0.75Ca_0.15Sr_0.10MnO₃, which is analyzed in the concept of Landau theory and with critical exponent analysis obtained from the magnetization measurements. The change in entropy in La_0.75Ca_0.15Sr_0.10MnO₃ is discussed in the light of magnetoelastic coupling between the magnetization and the lattice distortion. Application aspects of this unusual broad magnetocaloric effect with relative cooling power of 107 J kg⁻¹ in an applied magnetic field of 1.6 T with an operating temperature range of 93 K around the room temperature are also discussed.     

Effect of internal strain on magnetic properties of La0.7Sr0.3MnO3 films

Oriented La_0.7Sr_0.3MnO₃ (LSMO) films were grown on (100) LaAlO₃ (LAO) and SrTiO₃ (STO) substrates by the method of radio frequency magnetron sputtering. X-ray diffraction patterns of LSMO films deposited on LAO and STO substrates showed a slight shift of diffraction peaks as compared to the corresponding peaks of the bulk LSMO. The shift of diffraction peaks was believed to result from the in-plane lattice mismatch between the film and substrate. The distinguished strain effects of substrates were found to be effective on the magnetic properties and the surface morphology of LSMO films on both substrates. The results indicate that STO substrate may be the promising candidate for room-temperature applications of LSMO film.     

Large-distance rf- and dc-sputtering of epitaxial La1 − xSrxMnO3 thin films

Studies on large-distance sputtering as an effective alternative to molecular beam epitaxy, pulsed laser deposition or off-axis sputtering for the deposition of epitaxial La_1 − xSr_xMnO₃ (LSMO) thin films, are reported. The focus of this study is on the quality of the samples and their structural and magnetic properties. The dependence of the characteristics of the LSMO films on the sputtering mode (rf, dc) and the sputtering parameters, in particular on the oxygen partial pressure is established and discussed. It is shown that large-distance sputtering can provide high quality LSMO thin films without the need for post-annealing.     

Magnetoimpedance effect in semiconducting La0.4Sr0.6MnO3

In the present work, it was found that for La_0.4Sr_0.6MnO₃, the dc resistance decreases with increasing temperature, from 77 to 280 K. Different from the case of metallic La_0.65Sr_0.35MnO₃, the ac impedance of the semiconducting La_0.4Sr_0.6MnO₃ at room temperature decreases with increasing frequency, from 100 kHz to 12 MHz. The magnetoimpedance effect was observed in La_0.4Sr_0.6MnO₃ at room temperature. The value of impedance ratio (Z(0)−Z(H))/Z(0) at H=0.8 kOe reaches 5% at a frequency of 500 kHz.     

Effects of step-edge conditions on the magnetoresistance of La0.7Sr0.3MnO3 tunneling junctions

La_0.7Sr_0.3MnO₃ (LSMO) tunneling magnetoresistance (TMR) junctions have been successfully fabricated on step-edge (001) SrTiO₃ substrates with various step-edge conditions. We report the fabrication techniques of LSMO step-edge TMR junctions in which the standard photolithography and ion-beam etching techniques are used to control the step angle and step height. We study the influence of step-edge conditions on the resistance and MR behavior of LSMO TMR junctions. It is found that the magnitude of junction resistance increases with an increase of step angle or with a decrease of d/H ratio, where d is the film thickness and H the step height. Based on our results, we conclude that a suitable d/H ratio of ∼0.5-0.9 with a high step angle of φ≥70° can be regarded as the optimum conditions for the fabrication of step-edge TMR junctions.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

The tetragonal-orthorhombic transition and the associated changes in the properties of Pr0.5−xNdxSr0.5MnO3

The transition from the tetragonal (I4/mcm) to the orthorhombic (Imma) structure associated with a change in A- to CE-type antiferromagnetism in the Pr_0.5−xNd_xSr_0.5MnO₃ has been investigated by X-ray diffraction as well as magnetization and resistivity measurements. The transition is found to occur between x=0.1 and 0.2.     

Electron spin resonance and AC susceptibility studies on La0.9Pb0.1MnO3 single crystals

Phase separation has been recognized as important properties of manganite to create colossal magnetoresistance (CMR). Frequency dependence of AC susceptibility and electron spin resonance (ESR) studies confirm the phase separated glassy magnetism in the La_0.9Pb_0.1MnO₃ crystals. ESR spectra exhibit a gradual narrowing of the asymmetric FMR signals upon increasing the temperature and approaching the Curie temperature (T_c). The temperature dependence of line width (ΔH_pp), g-factor, double integrated (I) intensity of the resonance signals and frequency dependence peak temperatures are discussed in this paper to explain inhomogeneity in the La_0.9Pb_0.1MnO₃ single crystals.