Improved electrical and magnetic transport properties of La0.8Ba0.2MnO3 thin films by oxygen annealing

正Dear Editors,The properties of mixed-valence manganites are sensitive to the synthesis conditions[1].For oxide films grown by pulsed laser deposition(PLD),the physical properties are affected obviously by the growth oxygen pressure and annealing treatment[2-5].Using the PLD method,manganite oxides are usually grown at different oxygen pressures to investigate the     

Percolation nature of the metal-to-insulator transition in La0.8Ba0.2MnO3

For a La_0.8Ba_0.2MnO₃ single crystal, the temperature dependences of its resistivity, magnetization, and differential magnetic susceptibility are studied in the vicinity of the metal-to-insulator transition and near the ferromagnetic-paramagnetic phase transition. The critical temperatures corresponding to these transitions are determined independently from the experiment. These temperatures are found to coincide within 2–3%. The results are discussed within the framework of percolation theory.     

Kinetic effects in an La0.8Ba0.2MnO3 single crystal

Results are presented of a complex study of the magnetic and resistive properties, the Hall effect, the thermal emf, and the longitudinal Nernst-Ettingshausen effect of an La_0.8Ba_0.2MnO₃ single crystal at temperatures between 77 and 400 K. A maximum was observed near the Curie temperature T _c on the temperature dependences of the resistivity, the thermal emf, and the normal Hall coefficient. It was established that the Hall mobility remains constant near T _c. It is shown that these anomalies in the kinetic properties are attributable to a change in the position of the mobility edge relative to the Fermi level. A semiphenomenological theory is put forward to quantitatively describe the temperature and magnetic-field dependences of the resistivity and thermal emf of lanthanum manganites near the phase-transition temperature.     

Temperature evolution of the cluster state in La0.8Ca0.2MnO3 and La0.8Ca0.2CoO3

The results of investigations of the transport and magnetic properties (ac linear and nonlinear (second- and third-order) susceptibilities) of La_0.8Ca_0.2MnO₃ and La_0.8Ca_0.2CoO₃ single crystals have been presented. It has been found that both compounds in the paramagnetic phase contain ferromagnetic clusters with close magnetic characteristics. At high temperatures, ferromagnetic clusters nucleate in preferred sites associated with chemical inhomogeneities. Cooling below a specific temperature is accompanied by homogeneous nucleation of clusters. These two stages are observed in both compounds. In the doped cobaltite, the coalescence of clusters begins to develop at the third stage, whereas in the manganite, their behavior changes due to the development of ferromagnetic ordering of the matrix. These features indicate that the cluster state in doped manganites and cobaltites has a common nature. The difference in the behavior of ferromagnetic clusters is a consequence of the magnetically active character of the matrix in the case of manganites and the neutral character of the matrix in the case of cobaltites.     

Exchange bias tuned by cooling field in La0.2Ca0.8MnO3 nanoparticles

We report the magnetic properties in the nanosized charge ordering manganite La_0.2Ca_0.8MnO₃ with an average particle size ～50 nm. The sample exhibits ferromagnetism at low temperatures. The exchange bias phenomenon is observed when the sample is cooled down in an external magnetic field. Moreover, the exchange bias field is dependent on the cooling field and shows a maximum of ～520 Oe under a cooling field ～5 kOe. The exchange bias effect can be attributed to the exchange coupling between the ferromagnetic shell and antiferromagnetic core. The decrease of exchange bias field in high cooling field can be attributed to the growth of ferromagnetic component under high cooling field.     

Mn的价态对La0.8Ba0.2MnO3电磁性能的影响

在2-390K温度之间研究了La0.8Ba0.2MnO3的磁矩、磁电阻与温度的关系.发现以不同价态的Mn元素引入得到的La0.8Ba0.2MnO3,性能虽然都存在金属-绝缘体转变,以及在磁场作用下居里温度附近电阻率变化非常显著的特点,但是价态对磁性转变温度TC,金属-绝缘体转变温度Tmi,以及磁电阻极大值温度TMR的影响都非常显著.三种价态相比较,使用二价Mn的电阻率最低以及磁性转变温度更接近室温.认为影响材料性能的主要因素是材料制备时引进的Mn元素的价态,由于原料价态的不同而形成的氧空位浓度变化,进而影响了Mn4+/Mn3+的比.     

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρ _xx and transverse ρ _xy resistivities and magnetic susceptibility χ _ac of La_0.8Sr_0.2MnO₃ single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρ _xy ∼ρ _xx , which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χ _ac. As a result, the curve of ρ _xy versus ρ _xx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρ _xx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽T _C . The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials. Zh. éksp. Teor. Fiz. 116, 671–683 (August 1999)     

Co掺杂对钙钛矿锰氧化物La0.8Sr0.2MnO3磁性的影响

采用传统的高温固相反应法制备La0.8Sr0.2Mn1-xCoxO3（x=0,0.1）系列多晶样品,采用X射线衍射（XRD）测量样品的结构,并用物性测量系统（PPMS）探究样品的磁化强度随温度的变化曲线（M-T）,以及在不同的温度下磁化强度随外加磁场的变化曲线（M-H）。结果表明：当温度高于TG时,样品呈现出纯顺磁态,当TG相似文献   

Metastability in the resistance of polycrystalline La0.8Li0.2MnO3

Bulk polycrystalline La_0.8Li_0.2MnO₃ is found to switch between a low‐resistance state and a high‐resistance state on thermal cycling. The low‐temperature, high‐resistance state exhibits strong electroresistance whereas the high‐temperature, low‐resistance state does not. The change in resistance between the two distinct states is of two orders of magnitude. It is proposed that the observed metastability may serve as the basis for resistive thermal‐switching devices.

     

Effect of annealing treatment on the magnetic properties of mechanochemical synthesized La0.8Pb0.2MnO3 manganites

In this paper, we have investigated the effect of annealing on structural and magnetic properties of La_0.8Pb_0.2MnO₃ (LPMO) polycrystalline samples. Mechanochemical procedure has been used to produce LPMO nanocrystalline samples from oxide precursors. From the AC susceptibility measurements we found that the magnitude of susceptibility increases and the paramagnetic–ferromagnetic (PM–FM) transition width decreases by annealing the ball milled powders. Also, reentrant spin glass (RSG) behavior has been found in high temperature annealed samples. It seems that, due to the evaporation of PbO in high temperature annealed samples, vacancies created in perovskite structure and portion of the Mn³⁺–O–Mn⁴⁺ network is broken. So the FM double-exchange interaction is weakened due to the magnetic dilution and the antiferromagnetc (AFM) phase fraction increases in high temperature annealed samples. The competition between FM double-exchange interaction and AFM super-exchange interaction produces spin frustration and is responsible for the occurrence of RSG.     

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Magnetization of La_0.66Ba_0.34MnO₃ and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0<H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.     

Thermopower and electrical resistivity of La0.875Sr0.125MnO3 single crystals under hydrostatic pressure

The thermopower α and the electrical resistivity ρ of La_1?x Sr_xMnO₃ single crystals with a strontium content x=0.125, which corresponds to the stoichiometric composition of the new charge-ordered (CO) phase, are measured in the temperature range 77–300 K at pressures up to 12 kbar. The dependence α(T) exhibits two maxima. The first low-temperature maximum is associated with the formation of the charge-ordered phase. The second high-temperature maximum is attributed to the O → O′ structural transition between the orthorhombic phases and the formation of ferromagnetic clusters. It is found that the phase transition observed at P>9.2 kbar is accompanied by a substantial shift of both maxima toward the low-temperature range.     

Degradation of La0.8Ca0.2MnO3 single crystal surface: Electron spectroscopy studies

The electronic structure of degraded (as grown and chemically polished) and ion-sputtered La_0.8Ca_0.2MnO₃ single crystal surfaces has been studied by X-ray photoemission and Auger electron spectroscopy techniques. XPS spectra of deep core levels (Mn2p, O1s, La3d, La4d) were taken from as grown, chemically polished and cleaned in UHV by Ar⁺ ion-bombardment surfaces. XPS data show different oxidation state of Mn and La ions on the degraded and ion-sputtered surfaces of the manganite crystal. Mn2p_3/2 line width and energy position suggest mixed valence state of the manganese ions at the degraded and ion-sputtered crystal surfaces.     

Ambipolar gate effect and low temperature magnetoresistance of ultrathin La0.8Ca0.2MnO3 films

Ultrathin La(0.8)Ca(0.2)MnO(3) films have been measured in a field-effect geometry. The gate electric field produces a significant ambipolar decrease in resistance at low temperatures. This is attributed to the development of a pseudogap in the density of states and the coupling of localized charge to strain. Within a mixed phase scenario, the gate effect and magnetoresistance are interpreted in the framework of a "general susceptibility," which describes how phase boundaries move through a hierarchical pinning landscape.     

Electrical transport properties and laser-induced voltage effect in La0.8Ca0.2MnO3 epitaxial thin films

La_0.8Ca_0.2MnO₃ (LCMO) thin films about 200 nm thickness were grown on untilted and tilted (5°, 10° and 15°) LaAlO₃ (100) single crystal substrates by pulsed laser deposition technique. Electrical properties of the epitaxial thin films were studied by conventional four-probe technique and the anisotropic thermoelectric properties of the films grown on the tilted substrates have been investigated by laser-induced voltage (LIV) measurements. X-ray diffraction analysis and atomic force microscopy results show that the prepared LCMO thin films have a single phase and high crystalline quality. The remarkably large temperature coefficient of resistance (TCR) values (above 11 %/K) are observed in the all films. TCR value reaches 18 %/K on the film grown on 10° tilted substrate. The intensity of LIV signals monotonously increases with the tilting angles, and the largest signal is 148 mV with the fast time response 229 ns for the film grown on 15° tilted substrate.     

Perovskites Bi0.8La0.2FeO3 and Bi0.8La0.2Fe0.95Cr0.05O3: Crystal structure and magnetic and charge states of iron ions

Perovskites of the Bi_0.8La_0.2Fe_{1 ? x} Cr _x O₃ system (x = 0, 0.05) were investigated by Mössbauer spectroscopy in the temperature range of 298–800 K. The samples were fabricated by solid-state synthesis and had a rhombic structure. Iron ions in Bi_0.8La_0.2FeO₃ and Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are situated in trivalent states. The magnetic transition temperatures (the Néel temperatures T _N ) T _N = 677.5 ± 2.5 K for Bi_0.8La_0.2FeO₃ and T _N = 647.6 ± 2.5 K for Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are measured. The substitution of trivalent iron ions from trivalent chromium ions in the amount x = 0.05 in Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ perovskite decreases the hyperfine magnetic field at nuclei ⁵⁷Fe in Fe⁺³-O-Cr⁺³ chains by 30 kOe.     

Internal friction evidence of uncorrelated magnetic clusters in electron-doped manganite Sr0.8Ce0.2MnO3

Electron-doped manganite Sr_0.8Ce_0.2MnO₃ has been systematically investigated by X-ray diffraction, electronic transport, magnetic, internal friction, and Young's modulus experiments. The X-ray diffraction result indicates that the compound remains tetragonal (I4/mcm) structure at room temperature. Due to the strong Jahn–Teller (JT) distortion, the ground state is antiferromagnetic (AFM) insulator. Below 20 K, a spin-glass (SG) state dominates at low temperatures. In the paramagnetic (PM) region, an internal friction peak at around 250 K, which is characteristic of relaxation, has been observed. Under applied magnetic field, the internal friction peak moves to higher temperature, which is suggested to originate from the formation of ferromagnetic (FM) clusters in PM region. In addition, the softening of Young's modulus in the vicinity of AFM transition temperature is interpreted in terms of the strong electron–phonon interaction.     

Effects of oxygen partial pressure on the ferroelectric properties of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films

The Ba_0.8Sr_0.2TiO₃ thin films were grown on the Pt–Si substrate at 700 °C by using a pulsed laser deposition technique at different oxygen partial pressure (PO₂) in the range of 1–20 Pa and their properties were investigated. It is observed that the PO₂ during the deposition plays an important role on the tetragonal distortion ratio, surface morphology, dielectric permittivity, ferroelectric polarization, switching response, and leakage currents of the films. With an increase in PO₂, the in-plane strain for the BST films changes from tensile to compressive. The films grown at 7.5 Pa show the optimum dielectric and ferroelectric properties and also exhibit the good polarization stability. It is assumed that a reasonable compressive strain, increasing the ionic displacement, and thus promotes the in-plane polarization in the field direction, could improve the dielectric permittivity. The butterfly features of the capacitance–voltage (C–V) characteristics and the bell shape curve in polarization current were attributed to the domain reversal process. The effect of pulse amplitude on the polarization reversal behavior of the BST films grown at PO₂ of 7.5 Pa was studied. The peak value of the polarization current shows exponential dependence on the electric field.     

Effect of the size factor on the magnetic properties of manganite La0.50Ba0.50MnO3

Nanocrystalline manganite La_0.50Ba_0.50MnO₃ was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ～30 nm to ～7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ～3.787 to ～3.904 Å with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ～30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T _C from ～264 to ～331 K and in the spontaneous magnetic moment σ _S from ～1.52 to ～3.31 μ_B/f.u. The anomalous behavior of the magnetic properties of the manganite La_0.50Ba_0.50MnO₃ obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn³⁺-O-Mn⁴⁺ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.