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     

The structural and magnetic properties of one-step mechanochemical route synthesized La0.8Pb0.2MnO3 manganites

Mechanochemical route has been used to produce La_0.8Pb_0.2MnO₃ (LPMO) nanocrystalline samples from oxide precursors. The samples were characterized using X-ray diffraction, scanning electron microscope and AC susceptibility measurements. The results showed that it is possible to produce LPMO perovskite powders after 10 h of ball milling. The crystallite size and microstrain were estimated using Williamson-Hall equation. The results showed that the crystallite size and microstrain increase initially and then decrease by the increase of milling time. By decreasing particle size the dislocation density (strain) increases and reaches to a saturation point at a particular particle size, further particle size reduction takes place through gliding motion along grain boundaries, which leads to a reduction of the strain. The dynamic properties of 15 h ball-milled sample were investigated by AC susceptibility using the Neel-Brown and Vogel-Fulcher law for superparamagnetism. The frequency dependence of blocking temperature is well described by the Vogel-Fulcher law, and fitting the experimental data with Neel-Brown law gives unphysical value for relaxation time.     

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.     

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.

     

Nd0.5Sr0.4Pb0.1MnO3的结构和磁性

通过室温下的中子衍射和磁性测量对多晶样品Nd0.5Sr0.4Pb0.1MnO3的结构和磁性进行了实验研究.中子衍射结果表明,该样品具有正交的钙钛矿结构,空间群是Pnma,即结构发生了晶场畸变.由M-T和R-T曲线可知,居里温度TC=273 K,其特征是随着温度的增加样品经历了从铁磁金属态转变到顺磁半导态,且转变温度Tp＝225 K;用锰氧化物晶场和双交换作用的竞争解释了其温度Tp以下的金属特性.     

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)     

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.     

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.     

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   

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.     

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.     

Effects of pressure and magnetic field on the electrical resistivity of La0.8Ba0.2MnO3 in rhombohedral and orthorhombic phases

A structural transition from the orthorhombic phase to the rhombohedral one is observed and investigated on a La_0.8Ba_0.2MnO₃ single crystal. In the region of the coexistence of two phases, the effects of magnetic field (up to 12 T) and hydrostatic pressure (up to 5 kbar) on the temperature dependences of the electrical resistivity are studied. It is shown that magnetic field and pressure reduce the transition temperature and produce qualitatively similar effects on the electrical resistivity in the region of the structural transition.     

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.     

Effect of substrate temperature on electrical and magnetic properties of epitaxial La1−x Pb x MnO3 films

Epitaxial La_1−x Pb _x MnO₃ (LPMO) thin films, grown on (100) SrTiO₃ substrates by laser ablation technique at different temperatures between 600 and 850°C, have been characterized for electrical and magnetic properties. The temperature dependence of resistivity showed that the metal-insulator transition temperature (T _MI) decreases with increasing substrate temperature, which has been attributed to decrease in Pb content in the filsm. The YBa₂Cu₃O _x /La_1−x MnO₃ heterostructures, exhibiting both superconductivity and ferromagnetism, have been fabricated.     

La0.2Ca0.8MnO3材料电荷有序转变附近弹性模量的研究

通过超声与低频切变模量测量,得到多晶锰氧化物La0.2Ca0.8MnO3材料纵向模量与切变模量随温度变化关系曲线,发现在电荷有序转变温度附近,纵向模量与切变模量都出现最小值.运用合作Jahn-Teller效应理论对实验数据进行了拟合,发现理论与实验曲线符合较好,表明Jahn-Teller效应是发生电荷有序状态转变的主要机制之一.     

Influence of annealing atmosphere on the properties of La0.5Sr0.5MnO3

The structure, magnetic and electrical transport properties of La_0.5Sr_0.5MnO₃ annealed in different atmosphere have been investigated. No evident change of structural symmetry and the Curie temperature is observed for the samples. The resistivity at zero magnetic field of the samples annealed in air and nitrogen exhibits a metal–insulator transition, while no metal–insulator transition is observed for the sample annealed in oxygen, and for which the resistivity decreases monotonously with increasing temperature. Surprisingly, when an external magnetic field is applied, a metal–insulator transition appears for the sample annealed in oxygen. It is suggested that the annealing atmosphere affects the competition between FM and AFM phases due to the change of Mn⁴⁺/Mn³⁺ ratio and the oxygen/cation vacancies, and has a great influence on the electrical transport properties of La_0.5Sr_0.5MnO₃.     

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.     

Magnetic properties and magnetic entropy changes of perovskite manganese oxide La0.8-xEuxSr0.2MnO3 (x = 0, 0.075)

Magnetocaloric effect and critical exponent analysis of La_0.8-xEu_xSr_0.2MnO_{3 (}x = 0, 0.075) manganites synthesized via a solid state reaction route have been explored. Rietveld analysis indicates that crystal structure of La_0.8-xEu_xSr_0.2MnO_{3 (}for x = 0.0) is in orthorhombic structure with pbnm space group. After the substitution of Eu, the structure changes from orthorhombic (for x = 0.0) to rhombohedral structure for (x = 0.075) with $\mathrm{R}\overline{3}c$ space group. The T_C is 287 K and 271 K for the samples with x = 0.000 and x = 0.075, respectively, and the magnetic entropy change is 2.76J K ^− 1•kg⁻¹ for x = 0.000 and 3.94J K ^− 1•kg⁻¹ for x = 0.075. The real cooling power (RCP) is 447.7 J•kg⁻¹ for the sample with x = 0.000 and 445 J•kg⁻¹ for the samples with x = 0.075. Both samples show a second order phase transition in the vicinity of T_C. The critical exponents were determined using modified Arrott plots and Kouvel-Fisher methods. The critical behavior of these two samples fit best with the mean field model.