磁电阻效应的研究进展

介绍了磁电阻效应的研究状况和进展,总结了铁磁金属的磁电阻效应、磁性多层膜和颗粒膜的巨磁阻效应、磁隧道电阻效应及氧化物铁磁体的超大磁阻效应的理论模型,并简要分析了磁电阻效应的物理机制。     

磁电子学讲座 第二讲 掺杂稀土锰氧化物的巨磁电阻效应

介绍了掺杂稀土锰氧化物的巨磁电阻效应研究概况和最新进展．在综合目前实验和理论研究结果的基础上，对在掺杂稀土锰氧化物材料中引起巨磁电阻效应的物理机制进行了探讨．对这一材料的应用前景和需要做的工作进行了讨论     

自旋输运和巨磁电阻--自旋电子学的物理基础之一

介绍磁性纳米结构和锰氧化物中电子的自旋极化输运和巨磁电阻效应，它们是新近发展的自旋电子学的物理基础之一．着重讨论的是以下三方面的基本物理图像：磁多层结构的巨磁电阻，铁磁隧道结的隧穿磁电阻，掺杂锰氧化物的庞磁电阻效应．     

掺杂稀土锰氧化物的巨磁电阻效应

介绍了掺杂稀土锰氧化物的巨磁电阻效应研究概况和最新进展，在综合目前实验和理论研究结果的基础上，对在掺杂稀土锰氧化物材料中引起巨磁电阻效应的物理机制进行了探讨，对这一材料的应用前景和需要做的工作进行了讨论。     

Co SiO2颗粒膜的巨磁电阻效应

采用离子束溅射方法在玻璃基片上制备了一系列的Co－SiO\-2颗 粒膜样品,并对样品的巨磁电阻效应进行了研究.在Co35(SiO\-2)65(体积 百分比)颗粒膜样品中,观测到室温下近4%的巨磁电阻效应.研究了不同基片温度对巨磁电 阻效应的影响并发现,随着基片温度的升高样品的巨磁电阻效应下降.根据样品的电阻率温度关系曲线分析,在铁磁金属- 非磁绝缘介质颗粒膜中,除了电子自旋相关隧穿效应外, 可能还存在其他的导电机制. 关键词：     

高度取向石墨的巨磁电阻效应

石墨是准二维半金属材料，然而在通常细晶粒、无取向的石墨中并没有发现很大的磁电阻效应.在高度取向的石墨中发现了巨大的正磁电阻效应，在8.15 T的外磁场中，4.2，300 K温度下的磁电阻分别高达85300%和4950%.生产这一巨磁电阻效应的机制除正常磁电阻效应外，可能源于磁场诱导的类金属-绝缘体的转变 关键词： 磁电阻效应 石墨     

稀土掺杂锰氧化物庞磁电阻效应

过去十多年来,具有庞磁电阻效应的稀土掺杂锰氧化物成为了凝聚态物理研究的重要领域。锰氧化物的载流子自旋极化率高,且在居里温度附近表现出很大的磁电阻效应,因此在自旋电子学中有潜在的应用前景。另一方面,锰氧化物是典型的强关联电子体系,它对目前有关强关联体系的认识提出了很大挑战。本文综述了锰氧化物的各种性质及其物理原因。全文首先概述了锰氧化物的庞磁电阻效应及其晶格和电子结构,简单介绍了其他一些庞磁电阻材料;随后综述了锰氧化物的电荷/轨道有序相及其输运性质;在第四部分简单介绍了锰氧化物中庞磁电阻效应的机制;最后讨论了锰氧化物的一些可能的应用,如低场磁电阻效应、磁隧道结、磁p＿n结以及全钙钛矿的场效应管和自旋极化电子注入装置等。     

用脉冲激光方法在Si(100)上沉积的Cox-C1-x颗粒膜及其磁电阻效应

利用脉冲激光沉积方法在Si(100)上制备了Cox-C1-x颗粒膜,并研究了其正磁电阻效应.实验结果表明,样品在室温下的正磁电阻效应要远远高于低温下的正磁电阻效应;Co0.02-C0.98样品具有最大的室温正磁电阻效应,在外加磁场B=1T时,其磁电阻率MR=22%;随着Co含量的增加,Cox-C1-x颗粒膜的正磁电阻效应呈减小趋势.样品的MR-B的曲线与传统的多层膜及颗粒膜结构有很大的不同,这一现象表明在实验样品中可能存在着一种新的输运机制.     

非磁性材料磁电阻效应的研究进展

探讨了非磁性材料与磁性材料磁电阻效应的异同，分析了几种有代表性的非磁性材料的巨大磁电阻效应，概述了目前非磁材料磁电阻效应的研究和应用状况。     

稀土掺杂锰氧化物庞磁电阻效应

过去十多年来，具有庞磁电阻效应的稀土掺杂锰氧化物成为了凝聚态物理研究的重要领域。锰氧化物的载流子自旋极化率高，且在居里温度附近表现出很大的磁电阻效应，因此在自旋电子学中有潜在的应用前景。另一方面，锰氧化物是典型的强关联电子体系，它对目前有关强关联体系的认识提出了很大挑战。本文综述了锰氧化物的各种性质及其物理原因。全文首先概述了锰氧化物的庞磁电阻效应及其晶格和电子结构，简单介绍了其他一些庞磁电阻材料；随后综述了锰氧化物的电荷／轨道有序相及其输运性质；在第四部分简单介绍了锰氧化物中庞磁电阻效应的机制；最后讨论了锰氧化物的一些可能的应用，如低场磁电阻效应、磁隧道结、磁p-n结以及全钙钛矿的场效应管和自旋极化电子注入装置等。     

Connection between colossal magnetoresistance in La0.85Sr0.15MnO3 and magnetic ordering inhomogeneities of a sample

Resistive and magnetic measurements are made for La_0.85Sr_0.15MnO₃. The dependence of resistivity on the applied magnetic field (10, 20, 30, and 50 kOe) and temperature (200–310 K) is analyzed using the s-d model and the obtained experimental data. The physical features that should be contained in models proposed to explain the colossal magnetoresistance of manganites with activation-type conductivity are determined. It is shown that the proposed mechanism associating the colossal magnetoresistance effect with phase separation into ferromagnetic and paramagnetic microregions near the Curie temperature has the necessary features.     

Effect of water quenching process on the microstructure and magnetic property of cold rolled dual phase steel

The effects of water quenching process on the microstructure and magnetic property of cold rolled dual phase steel are investigated. Correlations of microstructure, magnetic properties and water quenching parameters are established. The results show that the microstructure of the dual phase steels mainly consists of the ferrite and martensite phase, the martensite volume fraction increases gradually on increasing the holding and quenching temperature. It is found that magnetic properties of dual phase steel are very sensitive to the quenching process. Based on the minor hysteresis loop results, the coercivity and hysteresis loss increase obviously with the increase of quenching temperature, while the remanent induction and the maximum permeability tend to decrease. Furthermore, the magnetic domain structure of the ferrite phase in the presented dual phase steel is observed by magnetic force microscopy. The mechanism of the magnetic property varying with the quenching process is also discussed.     

直接利用磁场和温度精确确定磁性材料La0.67Ca0.33MnO3和Pr0.7Sr0.3MnO3的电阻率

电阻率是研究钙钛矿结构锰氧化物磁性材料的重要参数之一,它与温度和外加磁场有密切关系.本文的工作之一是寻找合适的方法,确定在金属-绝缘体转换过程中,不同磁场情况下,材料La0.67Ca0.33MnO3和Pr0.7Sr0.3MnO3的电阻率随温度变化的数学解析关系.通过非线性数值拟合,找到了满足这一关系的函数为双曲正切修正的高斯函数.同时,获得金属-绝缘体转换时居里温度Tc满足的微分方程以及与该温度对应的最大电阻率ρmax.本文的另一个工作是寻求最大电阻率ρmax和磁场之间的函数关系,发现采用玻尔兹曼函数可以精确反映两者之间的数学联系.两项工作得到的数学拟合结果与实验数据之间的最小相关系数为0.998,最大平均相对误差4.35％,说明数据拟合的结果与实验结果十分符合.     

Electromagnetic field penetration through lanthanum manganites

The process of electromagnetic field penetration through lead-doped lanthanum manganites exhibiting colossal magnetoresistance has been studied. The measurements have been performed in a range of radio frequencies from 20 kHz to 10 MHz in the temperature interval containing a magnetic phase transition. Application of a constant external magnetic field leads to an increase in the transmission coefficient. Relative variations of the electromagnetic field transmission coefficient are several times as large as the relative change in the dc magnetoresistance. The temperature dependence of the relative change in the transmission coefficient has been studied. Variations of the transmission coefficient sharply decrease in the vicinity of the phase transition temperature, but they still remain rather large at temperatures above the Curie point.     

Oxygenation-induced two peaks in the self-doped La0.9MnO3+δ compounds

A series of self-doped La_0.9MnO_3+δ compounds have been fabricated by the solid-state reaction method with different post-annealing and oxygenation processes. The temperature dependence of resistivity measured at several magnetic fields indicated that all of these samples have an insulator–metal transition with a large negative colossal magnetoresistance (CMR) ratio. The resistivity, maximum CMR ratio and peak position are dominated by the oxygen content. As the oxygen content increases, the resistivity decreases, the peak position shifts to higher temperature and the maximum CMR ratio increases, respectively; meanwhile the peak number in the R–T curve is changed from 1 to 2, then from 2 to 1. The temperature dependencies of magnetization and specific heat show the evidence of magnetic phase transition in these samples. We think that two peaks contributed by the different oxygen-induced ferromagnetic phases resulted in the phase separation of the sample.     

First-order antiferro-ferromagnetic transition in Fe(49)(Rh(0.93)Pd(0.07))(51) under simultaneous application of magnetic field and external pressure

A magnetic field-pressure-temperature (H-P-T) phase diagram for first-order antiferromagnetic (AFM) to ferromagnetic (FM) transitions in Fe(49)(Rh(0.93)Pd(0.07))(51) has been constructed using resistivity measurements under simultaneous application of magnetic field (up to 8 T) and pressure (up to 20 kbar). The temperature dependence of resistivity (ρ-T) shows that the width of the transition and the extent of hysteresis decreases with pressure and increases with magnetic field. By exploiting opposing trends of dT(N)/dP and dT(N)/dH (where T(N) is the first-order transition temperature), the relative effects of temperature, magnetic field and pressure on disorder-broadened first-order transitions has been studied. For this, a set of H and P values are chosen for which T(N)(H(1),P(1)) = T(N)(H(2),P(2)). Measurements for such combinations of H and P show that the temperature dependence of resistivity is similar, i.e. the broadening (in temperature) of transition as well as the extent of hysteresis remains independent of H and P. Isothermal magnetoresistance measurements under various constant pressures show that even though the critical field required for AFM-FM transition depends on applied pressure, the extent of hysteresis as well as transition width (in magnetic field) remains constant with varying pressure.     

一级相变磁制冷材料的基础问题探究

由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.     

Hall resistivity in ferromagnetic manganese-oxide compounds

The transport properties of manganese-oxides are studied using the spin correlation fluctuation scattering mechanism. It is shown that the Hall resistivity in a small magnetic field exhibits a maximum near the Curie point, and a strong field shifts the peak position to high temperature and suppresses the peak value; the dependence of the Hall resistivity on the magnetic field above T_c and below T_c is different. These results agree with the experimental curves qualitatively, but disagree quantitatively, which indicates that the spin correlation fluctuation scattering might not be the dominant mechanism of the colossal magnetoresistance. The double polaron mechanism due to strong electron-phonon and electron-spin coupling is proposed to be responsible for the colossal magnetoresistance in manganese-oxides.     

Metastable structure and properties of (La0.73Bi0.27)0.67Ca0.33MnO3

The electrical transport and magnetic properties of high Bi doped (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃ are studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T. Significant temperature and magnetic field hystereses are observed in both resistivity and magnetization measurements. Meanwhile, an enhanced magnetoresistance effect, within a wide temperature window, is obtained in the (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃. The hysteresis and enhanced magnetoresistance are discussed based on an inhomogeneous metastable structure related to the Bi dopant.     

Relevance of cooperative lattice effects and stress fields in phase-separation theories for CMR manganites

Previous theoretical investigations of colossal magnetoresistance (CMR) materials explain this effect using a "clustered" state with preformed ferromagnetic islands that rapidly align their moments with increasing external magnetic fields. While qualitatively successful, explicit calculations indicate drastically different typical resistivity values in two- and three-dimensional lattices, contrary to experimental observations. This conceptual bottleneck in the phase-separated CMR scenario is resolved here considering the cooperative nature of the Mn-oxide lattice distortions. This effectively induces power-law correlations in the quenched disorder used in toy models with phase competition. When these effects are incorporated, resistor-network calculations reveal very similar results in two and three dimensions, qualitatively modifying previous scenarios and solving the puzzle.