Spin polarized La0.7Sr0.3MnO3 thin films on silicon

La_0.7Sr_0.3MnO₃ polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiO_x amorphous native oxide. Curie temperatures of about 325 K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiO_x and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed.     

(BiFeO3)25/(La0.7Sr0.3MnO3)25 多层膜的光学和电学性质

采用射频磁控溅射方法在(001)SrTiO₃衬底上制备(001)取向的(BiFeO₃)₂₅/(La_0.7Sr_0.3MnO₃)₂₅多层膜.光学测试结果表明,1.3-2.1 eV范围内,相对于衬底而言多层膜光吸收增强; BiFeO₃的带隙为2.7 eV. 另外,结合绝缘介质导电模型分析了所测得的电流-电压数据,在所测试的温度及电压下,所制备的(BiFeO₃)₂₅/(La_0.7Sr_0.3MnO₃)₂₅多层膜的导电机理由空间电荷限制电导主导. 关键词： 多层膜 吸光度 空间电荷限制电导     

Electrical behavior of nano-polycrystalline (La1−yKy)0.7Ba0.3MnO3 manganites

We present a study of the structural and electrical behavior of nano-polycrystalline mixed barium and alkali substituted lanthanum-based manganite, (La_1−yK_y)_0.7Ba_0.3MnO₃ with y=0.0-0.3. The samples were synthesized by the polymerization complex sol-gel method. The powder X-ray diffraction (XRD) data of the samples show a single-phase character with space group. The magnetic and electrical transport properties of the nano-polycrystalline samples have been investigated in the temperature range 50-300 K and a magnetic field up to 10 kOe. The metal-insulator transition temperature T_p of all the samples decreased with potassium doping, and also, it increased slightly with the application of magnetic field. The low field magnetoresistance, which is absent in the single-crystalline perovskite, was observed and increased with decreasing temperature. Comparing the experimental resistivity data with the theoretical models shows that the high temperature electrical behavior of these samples is in accordance with the adiabatic small polaron-hopping model. In the metal-ferromagnetic region the resistivity is found to be quite well described by ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5.     

Strain effects in epitaxially grown La0.7Sr0.3MnO3 thin films

We report here on an X-ray absorption study of La_0.7Sr_0.3MnO₃ films epitaxially grown on SrTiO₃ substrate. The local organization around Mn in oriented films with 600 Å in thickness was investigated by polarized Extended X-ray Absorption Fine Structure. The angle between electric field vector and film surface was set equal to 5° and 70° to investigate almost independently the contribution of the manganese neighbors situated in and out of the film plane. The first neighboring shell oxygen is found to be the same in both geometries, but small changes in the next neighboring contribution are observed. These changes are associated with variation in the Mn–Mn bond length. A small in-plane elongation (3%) is observed in the constrained films with respect to the unconstrained case.     

Magnetotransport and magnetoelastic effects in Co-doped La0.7Sr0.3MnO3 nanocrystalline perovskites

La_0.7Sr_0.3Mn_1−xCo_xO₃ (x=0, 0.05, 0.1) nanoparticles, prepared by sol-gel method, were studied by means of X-ray diffraction, transmission electron microscopy, resistivity, magnetoresistance, thermal expansion and magnetostriction measurements. Results show that partial substitution of Mn by Co leads to a reduction in lattice parameters, enhancement of resistivity and room temperature magnetoresistance MR, decrease of metal-insulator transition temperature T_MI and T_C, an increase in thermal expansion coefficient, volume magnetostriction and anisotropic magnetostriction. The latter increases about one order of magnitude with 10% Co substitution. In comparison with Mn ions, the Co ions possess higher anisotropy energy, larger magnetostriction effect, smaller ionic size and spin state transitions with increase in temperature and magnetic field; this suggests that Co substitution leads to double-exchange interaction weakening, resulting in suppression of ferromagnetic long-range order and metallic state and increase of magnetic anisotropy. Furthermore, our samples have a relatively lower T_MI and T_C, higher resistivity and MR, compared with the reported values for similar compounds with larger particle sizes. This is attributed to the nanometric grain size and spin-polarized tunneling between neighboring grains.     

Magnetocaloric effect in polycrystalline La0.65Ba0.3M0.05MnO3 (M=Na, Ag, K) manganites

The effects of monovalent doping on the crystallographic, magnetic and magnetocaloric properties of La_0.65Ba_0.3M_0.05MnO₃ (M=Na, Ag, K) powder samples, elaborated using the solid state reaction method at high temperature, have been investigated. In our three samples the Mn⁴⁺ amount remains constant equal to 40%. The Rietveld refinement of the X-ray powder diffraction shows that all our synthesized samples are single phase and crystallize in the distorted rhombohedral system with R3¯c space group. All our studied samples undergo a paramagnetic–ferromagnetic transition with decreasing temperature. Using the Arrott plot, the second-order transition Curie temperature T_C for M=Na, Ag and K is found to be 310, 300 and 290 K, respectively. The magnetic entropy change, deduced from isothermal magnetization curves, exhibits a maximum |ΔS_M^Max| of about 2.65, 2.82 and 2.66 J/kg K for M=Na, Ag and K, respectively, in a magnetic applied field change of 5 T. Although these values are modest, the magnetocaloric effect extends over a large temperature range leading to an important value of the relative cooling power (RCP). The RCP values exhibit a nearly linear dependence with the magnetic applied field. The refrigeration capacity in a magnetic applied field of 1 T is found to be 28.8, 27.8 and 25.6 J/kg for M=Na, Ag and K compounds.     

DC current effect on the magnetic phase and transport properties of polycrystalline La0.7Ca0.3MnO3

The influence of DC current on the resistivity and phase transition of polycrystalline La_0.7Ca_0.3MnO₃ has been investigated. The specific heat measurement found that charge carriers and ferromagnetic spin-wave contributions were changed after applied DC current. Applying high electric fields leads to the formation of ferromagnetic regions. The resistivity drops abruptly once the percolating current path is established. As current through the sample disappears, the larger ferromagnetic (FM) clusters, however, remain and are frozen in giving a measurable contribution to the specific heat of the system. The larger clusters should give rise to the value of spin-wave stiffness constant (D), as it is expected to increase the strength of the ferromagnetic coupling. The metallic ferromagnetic regions would make the charge carrier delocalization and attribute to specific heat linear term γ.     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

Hydrothermal synthesis and Magnetocaloric effect of La0.5Ca0.3Sr0.2MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.5Ca_0.3Sr_0.2MnO₃ have been studied. The La_0.5Ca_0.3Sr_0.2MnO₃ can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔS_M for the as-prepared sample is 0.88 J kg⁻¹ K⁻¹ at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg⁻¹ K⁻¹, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application.     

Domain wall pinning in polycrystalline perovskite La0.7Sr0.3MnO3

Reversible and irreversible domain wall (DW) motions have been investigated in La_0.7Sr_0.3MnO₃ ceramic samples using frequency-response complex permeability with various amplitudes of AC field. We also examine the effects of temperature in the range from 293 to 368 K and transverse DC magnetic field with a maximum of 4.40×10⁵ A/m on the real part of permeability (μ′). Two relaxations corresponding to reversible wall motions and domain rotations occur in low and high frequency regions, respectively. The irreversible DW displacements can be activated as the amplitude larger than the pinning field of 3 A/m, leading to an increase in μ′. The μ′ obeys a Rayleigh law at the temperature below 343 K or under DC field of less than 4.22×10⁴ A/m. The Rayleigh constant η increases from 5.45×10⁻² to 1.54×10⁻¹ (A/m)⁻¹ as the temperature rises from 293 to 343 K, and η decreases from 5.58×10⁻² to 3.67×10⁻² (A/m)⁻¹ with increasing DC field from 1.99×10³ to 4.22×10⁴ A/m.     

Low-temperature transport properties of chemical solution deposited polycrystalline La0.7Sr0.3MnO3 ferromagnetic films under a magnetic field

Polycrystalline La_0.7Sr_0.3MnO₃ (LSMO) films were prepared on SiO₂/Si (001) substrates by chemical solution deposition technique. Electrical and magnetic properties of LSMO were investigated. A minimum phenomenon in resistivity is found at the low temperature (<50 K) under magnetic fields from 0 T to 3 T. Kondo-like spin dependent scattering, which includes both spin polarization and grain boundary tunneling, was observed in the low-temperature electrical transport for the LSMO polycrystalline films. The temperature-dependent resistivity at low temperatures can be well fitted in the framework of elastic scattering, electron-electron (e-e) interaction, Kondo-like spin dependent scattering, and electron-phonon (e-ph) interaction.     

Phase separation and abnormal transport behaviours in La0.7-xGdxSr0.3MnO3 system

The magnetic and transport behaviours of the La_{0.7-x}Gd_xSr_{0.3}MnO_3 (0≤x≤0.70) system are investigated. The experimental results indicate that with increasing Gd doping content, the magnetism of the system changes from the long-range ferromagnetic order state to the cluster-spin glass state, then to the antiferromagnetic (AFM) state. It is interesting that the phase separation appears at x=0.30 and 0.40 and disappears for x≥0.50 where the AFM state occurs. At high doping content, the transport behaviours exhibit abnormality, e.g. there are two temperature ranges in which the ρ－T curves can be well fitted by a variable-range hopping (VRH) model. We suggest that the VRH does not come from the hopping of carriers between clusters, but from the different magnetic backgrounds in the clusters.     

电脉冲对多晶La0.7Ca0.3MnO3比热的影响

对电脉冲诱导的不同电阻态下La_0.7Ca_0.3MnO₃样品的比热进行了研究.实验结果表明,电脉冲导致La_0.7Ca_0.3MnO₃样品比热随电阻状态发生可逆变化.比热随电阻状态的减小而减小.低温比热拟合及不同电阻状态下的比热差与温度关系说明,声子对比热的贡献不随电阻状态变化,磁性和载流子对比热的贡献是导致La_0.7Ca_0.3MnO₃样品比热变化的原因.电脉冲诱导O离子沿一维扩展性缺陷的电化学迁移,导致材料中局部区域的O离子浓度发生变化.O离子浓度的变化导致载流子浓度的变化,同时载流子浓度的变化将使得低温下磁性耦合强度发生变化,从而导致比热发生变化.     

多晶La0.7Sr0.3MnO3的低温输运性质和磁电阻效应

详细研究了由纳米晶粒组成的块体多晶La_0.7Sr_0.3MnO₃(LSM)的电阻率和磁电阻效应,以及它们的温度依赖性.随着温度从室温降低,电阻率(ρ)在250K附近存在一最大值,低于该温度后,样品表现为金属导电特性,随后在50K附近存在一极小值.也就是说在低于50K的温度范围内,随着温度降低ρ反而升高,表现为绝缘体性的导电特性.经研究发现,这种随温度降低ρ反而增加的现象与隧穿效应的理论模型(lnρ∝T^-1/2)符合得很好 关键词： 0.7Sr_0.3MnO₃')" href="#">多晶La_0.7Sr_0.3MnO₃ 隧道效应 隧道磁电阻效应     

Role of the interfacial state: a comparison of Co/Al2O3/NiFe and La0.7Ca0.3MnO3/Al2O3/La0.7Ca0.3MnO3 tunnel junctions

Two junctions of Co/Al₂O₃/NiFe (J₁) and La_0.7Ca_0.3MnO₃/Al₂O₃/La_0.7Ca_0.3MnO₃ (J₂) were prepared to compare their tunneling magnetoresistance (TMR) in consideration of interfacial state effects. The structural and transport properties of the layered samples were characterized by X-ray and magnetic measurements, showing indeed an interfacial state dependence. The influences such as from a CoO sublayer in J₁ and from interfacial coherence in J₂ were discussed. The largest TMR observed amounts to 16% (290 K) for J₁ and 65% (40 K) for J₂.     

Thickness and microstructure dependent transport and MR in La0.7Pb0.3MnO3 manganite films

Thickness dependent modifications in the structure and microstructure and their effect on the transport and magnetotransport in chemical solution deposition (CSD) grown La_0.7Pb_0.3MnO₃ (LPMO) manganite thin films grown on single crystalline LaAlO₃ (LAO) (1 0 0) substrates have been studied. X-ray diffraction (XRD) measurements show the reduction in the microstrain at the film-substrate interface with increasing thickness. Increase in grain size and island like grain growth with the reduction in surface roughness as a function of film thickness has been observed from the microstructural studies using atomic force microscopy (AFM) and lateral force microscopy (LFM) measurements. Improvement in the surface to volume ratio (D⁻¹) resulting in the modifications in transport and magnetotransport properties of the LPMO films has been discussed in detail in the light of thickness dependent microstructural effects on the resistivity and magnetoresistance (MR) behavior. The variations in intrinsic and extrinsic MR with D⁻¹ show an interesting interplay between them, which can be explained on the basis of thickness dependent grain size, grain boundary density and grain boundary nature.     

Effects of crystallographic orientation on the oxygen exchange rate of La0.7Sr0.3MnO3 thin films

The oxygen surface exchange of La_0.7Sr_0.3MnO₃ (LSM) thin films was investigated using the electrical conductivity relaxation (ECR) method. Epitaxial (100)-, (110)-, and (111)-oriented LSM films were fabricated on corresponding SrTiO₃ (STO) substrates using pulsed laser deposition. The LSM films had well-controlled surface qualities, exhibited bulk-like steady-state electrical properties, and exhibited surface dominated responses in ECR. The chemical surface exchange coefficients (k_chem) were determined and varied from ≈ 1 × 10^− 6 to 65 × 10^− 6 cm/s, depending on temperature and orientation, with activation energies of between 0.8 and 1.2 eV. At 800 °C, a four fold variation is observed in the k_chem values, with (110)/(100) being the highest/lowest, explained well by the high activation energy for (110), ≈ 1.16 eV, and the low energy for (111) and (100), ≈0.83 eV.     

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO₃ content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO₃, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO₃. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.     

Strain induced phase separation on La0.5Ca0.5MnO3 thin films

The effect of epitaxial strain on La_0.5Ca_0.5MnO₃ films of various thicknesses grown on SrTiO₃, SrLaAlO₄, and SrLaGaO₄ substrates is studied by Raman spectroscopy, magnetic, and resistivity measurements. The transport and magnetic properties as well as Raman spectra of the films are affected by epitaxial strains. The energy of the A_g(2) mode and the tilting angle of the MnO₆ octahedra is affected by the strain imposed by the substrate. In the spectra of the films deposited on the (1 0 0) SrTiO₃ substrate strong Jahn-Teller (JT) modes appear, which couple with charge-ordering. In all other films these modes are suppressed and no additional Raman lines are present at low temperatures contrary to the bulk compound. The low frequency continuum scattering decreases at low temperatures indicating a coupling with both the charge and orbital transitions. Comparison of the Raman spectra with the magneto-transport properties suggests an interpretation in terms of a strain induced phase separation between ferromagnetic metallic and antiferromagnetic insulating states.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.