Anomalous acoustoelectric effect and the transport properties of thin La0.67Ca0.33Mno3 films

A complex of studies of the physical properties of thin lanthanum manganite La_0.67Ca_0.33MnO₃ films were performed for a monolithic layered structure consisting of a LiNbO₃ substrate and a La_0.67Ca_0.33MnO₃ thin epitaxial film. For the first time, not only ordinary acoustoelectric (AE) current of charge carriers dragged by a surface acoustic wave, but also longitudinal anomalous AE current, which flowed in a distinguished direction independent of the direction of the surface acoustic wave propagation, was observed. The anomalous AE effect predominated close to the metal-insulator transition, whereas the odd AE effect predominated at high and low temperatures. The sign of the ordinary odd AE effect corresponded to hole conduction of the film. A theoretical analysis showed that the anomalous AE effect is due to a strong film conduction modulation caused by deformation created by the surface acoustic wave. The theoretical results were in close agreement with experiment. The temperature dependences of resistivity ρ(both in the absence of a magnetic field and in fields of up to 3 T), high-frequency magnetic susceptibility, and thermoelectric power were studied.     

Charge-carrier density collapse in La0.67Ca0.33MnO3and La0.67Sr0.33MnO3 epitaxial thin films

We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse. Received 22 July 1999 and Received in final form 7 October 1999     

Prediction of thermomagnetic properties of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3

The dependence of magnetization on the variation of temperature for La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃ perovskite-type ceramic materials under an external magnetic field was modelled. Application of phenomenological model was performed, showing a good agreement with the experimental data. Temperature dependence of change of both magnetic entropy and specific heat under magnetic field 0.05?T was predicted for La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃. Predicted values of maximum magnetic entropy change, full-width at half-maximum, relative cooling power and maximum specific heat under 0.05 T were evaluated.     

La0.67Ca0.33MnO3的磁特性研究

采用高温固态反应法制备了具有钙钛矿结构的锰氧化物La0.67Ca0.33MnO3,利用X射线衍射、超导量子干涉仪等实验技术研究其结构和磁性质。应用标度律对起依赖于磁场的始磁化强度在t→0时的行为用临界指数β随磁场变化进行描述。临界指数β随着磁场的增大而减小,说明在临界点附近,磁场的增大导致自旋有序度的增加。     

The Hall effect in La0.67Ba0.33MnO3

The Hall effect in polycrystalline barium-substituted lanthanum manganite La_0.67Ba_0.33MnO₃ has been investigated in the temperature interval 298<T<355 K. It is found that the anomalous Hall coefficient in this material is two orders of magnitude greater than the normal coefficient. At T ₀=333 K the normal Hall coefficient changes sign, which indicates a change in the type of conductivity. The temperature dependence of the normal Hall coefficient, electrical conductivity, and magnetoresistance is explained on the basis of the concept of motion of the mobility edge attendant as the temperature changes. Zh. éksp. Teor. Fiz. 113, 981–987 (March 1998)     

La0.67Ca0.33MnO3纳米多晶环的时效现象

利用脉冲激光沉积(PLD)方法在多孔Al2O3模板上制备了纳米结构La0.67Ca0.33MnO3(LCMO)多晶膜.温度为80K时,LCMO纳米多晶环样品的电阻随时间连续下降,表现出明显的时效特征.时效后,样品具有保持低电阻状态的性质.该时效行为可用扩展的指数函数拟合.通过拟合可以得到相应条件下的样品平衡态电阻、样品的特征驰豫时间等信息.利用Core-Shell模型以及自旋玻璃相的存在对观察到的时效行为及拟合结果进行了解释.     

Magnetotransport parameters of La0.67Ca0.33MnO3 films grown on neodymium gallate substrates

Weakly mechanically stressed 40-nm-thick La_0.67Ca_0.33MnO₃ films have been grown coherently on a (001)NdGaO₃ substrate by laser evaporation. The electrical resistivity ρ of the La_0.67Ca_0.33MnO₃ film reaches a maximum at a temperature T _C ≈ 255 K. At temperatures below 0.6T _C, the temperature dependences of ρ are well approximated by the relation ρ = ρ_def + C ₁ T ² + C ₂ T ^4.5, in which the first term on the right-hand side accounts for the contribution of structural defects to electrical resistivity, and the second and third terms stand for those of the electron-electron and electron-magnon interactions, respectively. The parameters ρ_def ≈ 1 x 10^?4 Ω cm and C ₁ ≈ 7.7 × 10^?9 Ω cm K^?2 do not depend on temperature and magnetic field H. The coefficient C ₂ decreases with increasing H to reach about 4.9 × 10^?15 Ω cm K^?4.5 at μ₀ H = 14 T.     

Evidence of ferrimagnetism in ferromagnetic La0.67Ca0.33MnO3 nanoparticle

The present paper shows that bulk La_0.67Ca_0.33MnO₃ (LCMN) sample of micron size particles is a typical ferromagnet and magnetic properties of LCMN samples with particle size in the nanometer scale are better described in the frame work of a ferrimagnetic structure. Detailed analysis of the temperature dependence of the inverse of paramagnetic susceptibility has confirmed the signature of ferrimagnetism in the nano-sized particles of LCMN. We have explained the observed ferrimagnetism as an effect of modified magnetic properties in the core–shell spin structure of LCMN particles, irrespective of the nanocrystalline and amorphous phase of the material.     

Hall effect in La0.67−x CexSr0.33MnO3

An experimental study of the Hall effect and of magnetoresistance in polycrystalline La_0.67−x Ce_xSr_0.33MnO₃ samples (x=0, 0.07) is reported. It is shown that the normal Hall coefficient reverses sign at T ₀=360 K in both samples and that metallic conduction occurs for T<T ₀, while for T>T ₀ the conduction is by mechanisms operative in disordered materials. It has been established that doping with cerium brings about an increase in the metallic phase of the normal and anomalous Hall coefficients. A qualitative interpretation of the observed features is given. Fiz. Tverd. Tela (St. Petersburg) 40, 2085–2088 (November 1998)     

Transport parameters of granular La0.67Ca0.33MnO3 films grown on an R-plane sapphire

Precisely (100)-oriented, 200-nm thick La_0.67Ca_0.33MnO₃ films have been grown by laser ablation on a sapphire (R-plane) substrate covered by a (100)SrTiO₃/(001)Bi₂SrNb₂O₉/(001)CeO₂ trilayer buffer. The azimuthal misorientation of crystal grains (50–300 nm) in the La_0.67Ca_0.33MnO₃ films decreased by about 40% as the condensation temperature was increased ered from 760 to 810° C. The lattice parameter of the grown manganate films was reduced to 3.81–3.82 Å by enriching them with oxygen. The maximum in the temperature dependence of the electrical resistivity of the La_0.67Ca_0.33MnO₃ films grown was shifted toward lower temperatures by 20–50 K relative to its position for bulk ceramic samples of a stoichiometric composition. The largest magnetoresistance (MR=42% at H=0.4 T) was found in La_0.67Ca_0.33MnO₃ films with a Mn⁴⁺ concentration on the order of 50% (T=166 K).     

La0.33Pr0.34Ca0.33MnO3薄膜的应变效应

采用脉冲激光沉积法分别在(100)LaAlO3和(100)SrTiO3基片上生长了La0.33Pr0.34Ca0.33MnO3 薄膜,并通过磁测量和电输运测量对生长在不同基片上的La0.33Pr0.34Ca0.33MnO3薄膜的物性进行了研究.结果表明,基片和薄膜之间的压应力导致La0.33Pr0.34Ca0.33MnO3薄膜容易出现铁磁相,而电荷有序相则被抑制.基片和薄膜之间的张应力则导致La0.33Pr0.34Ca0.33MnO3薄膜容易出现电荷有序相,铁磁相被抑制.导致该现象的可能原因之一是应力使得晶格中Mn-O键角发生改变而引起双交换作用的改变.     

Ga掺杂对La0.67Ca0.33MnO3磁电阻的提高

本文研究了Ｇａ部分替代Ｍｎ位对巨磁阻材料Ｌａ０．６７Ｃａ０．３３ＭｎＯ３的磁性和输运性质的影响。实验结果表明Ｇａ掺杂破坏了双交换作用,使电阻率上升而磁有序转变推移到低温。但是,Ｇａ掺杂使磁电阻效应显著提高,证明Ｍｎ位元素替代可能是提高ＣＭＲ的一种有效途径。     

Electrical transport and magnetic properties of nanostructured La0.67Ca0.33MnO3

We report fabrication of nanostructured La_0.67Ca_0.33MnO₃ (NS-LCMO) by pulsed-laser deposition on the surface of porous Al₂O₃. The resistance peak temperature (T_p) of the NS-LCMO increases with increasing average thickness of the films, while their Curie temperatures (T_c) remain unchanged. The coercive field of the samples increases with decreasing film thickness and its temperature dependence can be well described by H_c(T)=H_c(0)[1-(T/T_B)^1/2]. A large magnetoresistance and strong memory effect were observed for the NS-LCMO. The results are discussed in terms of the size effect, Coulomb blockade and magnetic tunneling effect. This work also demonstrates a new way to get nanostructured manganites. PACS 75.70.Ak; 75.75.+a; 72.80.Ga     

EPR investigation of nanosized La0.67Ca0.33MnO3−δ manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La_0.67Ca_0.33MnO_3?δ manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.     

厚度与应变效应对La0.67 Ca0.33 MnO3薄膜电输运与居里温度的影响

在厚度为25-400nm范围内,系统地研究了(001)SrTiO3(STO),(001)LaAlO3(LAO)衬底上La0.67Ca0.33MnO3(LCMO)薄膜的电输运与居里温度TC随薄膜厚度及衬底的变化.结果表明,随薄膜变薄,电阻率ρ增加,TC降低.对于同一薄膜厚度,LCMO/STO薄膜的ρ大于LCMO/LAO基上的薄膜的ρ.TC与衬底的依赖关系则与ρ相反.分析表明,LCMO薄膜的低温区电阻温度(ρ-T)符合关系式ρ=ρ0+Bωs/sinh2(hωs/2/kBT)+CTn,其中ρ0为剩余电阻;等号右端第二项反映软光学模声子对电子散射的贡献;第三项包括其余可能散射机理在电输运过程中所起的作用;B,ωs(软光学模声子的平均频率)与C都为拟合系数.高温区的电输运则由小极化子跃迁模型ρ=DT×exp(Ea/kBT)描述(Ea为极化子激发能).根据ρ0,ωs,Ea以及TC变化,初步讨论了薄膜中的厚度与应变效应.进一步研究发现ωs,Ea的变化与TC相关,从而说明极化子效应为影响TC变化的主要因素.     

La0.67Ba0.33MnO3薄膜的激光辐照效应研究

利用脉冲激光沉积技术在LaAlO3(00l)单晶衬底上制备了La0.67Ba0.33MnO3薄膜,研究了CO2激光辐照对La0.67Ba0.33MnO3薄膜的微结构和磁电性能的影响.结果表明,经激光辐照后,La0.67Ba0.33MnO3薄膜的结晶性增强,薄膜应变减小;薄膜表面形貌由"岛状"结构变为"平原"结构,且粗糙度大大降低;同时,薄膜的饱和磁化强度、铁磁居里温度、金属-绝缘态转变温度和磁电阻增大,而矫顽场和电阻率减小.根据对传统退火效应的分析和理论计算,认为激光辐照导致的表面微结构的变化以及薄膜的氧含量和均匀性的提高对La0.67Ba0.33MnO3薄膜的磁电性能的改善与优化密切相关.     

Coherent spin precession via photoinduced antiferromagnetic interactions in La0.67Ca0.33MnO3

Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic La(0.67)Ca(0.33)MnO(3) thin films using the time-resolved magneto-optical Kerr effect. The precession amplitude monotonically decreases with increasing field, indicating that the coherent spin rotation may be triggered by a transient exchange field and not by demagnetization and/or anisotropy field modulation. We attribute the transient exchange field to emergent antiferromagnetic interactions due to charge transfer and modification of the kinetic energy of e(g) electrons under optical excitation.     

La0.67Pb0.33MnO3的Preisach分析

测量了居里点TC=360K的钙钛矿La0.67Pb0.33MnO3在2K关键词： Preisach模型 磁滞回线 磁化强度 磁化率     

La0.67 Pb0.33 MnO3 的Preisach分析

测量了居里点TC=36 0K的钙钛矿La0 6 7Pb0 33MnO3在 2K 相似文献