高温磁致冷工质的新进展

简要报道磁性钙钛矿化合物具有比钆更大的磁熵变，其居里温度可在甚宽的范围内改变，有可能成为一类新型的高温磁致冷工质。     

磁性薄膜研究的现状和未来

概括介绍了近十几年来磁性薄膜研究的主要成果、应用和可能的发展情况,主要内容有：钙钛矿结构氧化物薄膜的磁性和庞磁电阻效应,磁性金属多层 膜的层间耦合和巨磁电阻效应及磁电阻磁头应用情况,光存储技术纳米点阵存储技术,磁电子学。     

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。     

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。     

自旋电子学材料、物理和器件设计原理的研究进展

文章介绍了作者所在实验室在巨磁电阻(GMR)、隧穿磁电阻(TMR)、庞磁电阻(CMR)和反铁磁钉扎薄膜材料以及单晶金属氧化物、高自旋极化率材料、P-N异质结和纳米环磁随机存储器原理型演示器件设计等研究方面取得的一些重要研究成果和进展.例如:在Al-O势垒磁性隧道结材料体系里,获得室温磁电阻超过80%的国际最好结果;获得两种高性能层状反铁磁钉扎材料体系;发现具有大的电致电阻效应的CMR薄膜材料,并可期望用于电流直接进行磁信息写和读操作的磁存储介质;发现双势垒磁性隧道结中的量子阱态共振隧穿和磁电阻振荡效应,以及纳米器件体系中自旋翻转长度的观测新方法,可用于新型自旋电子学材料及相关器件的人工辅助设计;利用电子自旋共振谱探测和研究了金属氧化物的微观自旋结构和各向异性;在[CoFe/Pt]n磁性金属多层膜中,观测到超高灵敏度的反常霍尔效应;利用纳米环状磁性隧道结作为存储单元,研制出一种新型纳米环磁随机存储器MRAM原理型演示器件.     

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。     

La0.8-xCa0.2MnO3纳米颗粒的居里温度与磁热效应

采用溶胶凝胶法制备了系列La_0.8-xCa_0.2MnO₃多晶样品,用X射线衍射分析确定了样品的钙钛矿结构,用透射电子显微镜观察了样品的形貌及粒径分布情况,用PAR155型振动样品磁强计测量了样品的磁性随外场和温度的变化,确定样品的居里温度并计算了各样品的磁熵变.磁测量及计算结果表明制备的各样品的居里温度在180—260K的范围内且随焙烧温度和La³⁺离子空位浓度的不同而变化,不同温度焙烧的样品均有较大的磁熵变值,其中1100℃焙烧的La_0.77Ca_0.2MnO₃,多晶样品在240.5K,H=1.0T的外场下的磁熵变达3.76J/kg·K,对实验结果做了定性的分析.该材料具有较高的居里温度和较大的磁熵变,所需外场强度适中,电阻率高,性能稳定,适合做高温磁制冷材料. 关键词： 钙钛矿 居里温度 磁热效应     

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．     

磁电子学中的若干问题

本文综述了自旋极化输运过程中巡游电子的自旋极化、自旋相关的散射及自旋弛豫等三方面的内容；全面总结了铁磁金属的磁电阻效应（ＡＭＲ）、磁性金属多层膜和颗粒膜的巨磁电阻效应（ＧＭＲ）、氧化物铁磁体的特大磁电阻效应（ＣＭＲ）以及磁隧道结的巨大隧道电阻效应（ＴＭＲ）研究中具有代表性的实验结果及理论模型；简单介绍了新生的磁电子器件—磁电阻型随机存取存储器（ＭＲＡＭ）和全金属自旋晶体管的工作原理和工作过程。     

Ce2Fe16Al化合物在居里温度附近的磁性和磁熵变

研究了具有菱方Th2Zn17型结构的Ce2Fe16Al化合物在居里温度TC附近的磁性和磁熵变.实验结果表明，在居里温度附近样品的磁特性符合二级相变规律，样品的居里温度为2758K.通过磁化强度与磁场和温度关系的测量，计算出临界指数β=044±001，γ=130±001，δ＝383±001，临界指数β，γ，δ基本满足标度率方程γ＝β（δ-1)，但偏离三维Heisenberg模型的理论值.Ce2Fe16Al化合物的磁熵变在居里温度处达到峰值，2T外磁场下的最大磁熵变为195J/kg K. 关键词： Ce2Fe16Al化合物 临界指数 磁熵变     

Effects of Fe－Fe bond length change in NaZn13-type intermetallic compounds on magnetic properties and magnetic entropy change

In this paper the effects of Fe－Fe bond length change on magnetic properties and magnetic entropy change have been investigated on LaFe_{12.4-x}Si_xCo_{0.6} and LaFe_{12.3-x}Al_xCo_{0.7} intermetallic compounds. According to the analyses of Fe－Fe bond length change, the variation of Curie temperature and the unusual magnetic phase transition which results in the large magnetic entropy change were explained. The effects of the substitution of Co and Si for Fe on magnetic entropy change and field-induced itinerant-electron metamagnetic transition in LaFe_{12.4-x}Si_xCo_{0.6} compounds were also studied and the considerable magnetic entropy change has been achieved.     

LaFe$lt;sub$gt;11.5$lt;/sub$gt;Si$lt;sub$gt;1.5$lt;/sub$gt;化合物氢化特性及稳定性的研究

系统研究了温度、时间、压力等因素对LaFe_11.5Si_1.5化合物吸氢过程的影响以及效果. 结果表明：吸氢处理会增大化合物的晶格常数,但不会改变其晶体结构. 在温度为423 K,氢气压力为0.0987 MPa时,可以制备出氢分布均匀的LaFe_11.5Si_1.5H_1.6间隙化合物. 氢化处理可以明显提高化合物的居里温度,降低热滞后,并且使磁熵变保持在较高值. LaFe_11.5Si_1.5H_1.6样品随着在空气中暴露时间的延长,居里温度和磁熵值的变化非常小,氢化合金的磁热性能具有良好的时间稳定性. 关键词： P-C-T关系曲线 居里温度 磁熵变 11.5Si_1.5氢化物')" href="#">LaFe_11.5Si_1.5氢化物     

Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe11.83Si0.94Al0.23 compounds

Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe_{11.83}Si_{0.94}Al_{0.23} compounds has been investigated by means of magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase with the cubic NaZn_{13}-type structure. Substitution of Co for Fe leads to an increase of Curie temperature of the material. The magnetic entropy changes in LaFe_{11.83}Si_{0.94}Al_{0.23} and LaFe_{11.03}Co_{0.80}Si_{0.94}Al_{0.23} compounds are 21.8J/(kg·K) to 16.9J/(kg·K) under a magnetic field change of 0－5T at Curie temperature, respectively. Giant magnetic entropy changes are attributed to the higher magnetization and the rapid change in magnetization at Curie temperature.     

Large magnetic entropy change and magnetic properties in La （Fel－xMnx）ll.TSil.3Hy compounds

Magnetic properties and magnetic entropy change in La(Fe_{1-x}Mn_x)_{11.7}Si_{1.3}H_y compounds have been investigated. A significant increase of the Curie temperature T_C and a small increase of the saturation magnetizations μ_S have been observed after the introduction of interstitial H, which caused a slight volume expansion. The first-order field-induced itinerant-electron metamagnetic (IEM) transition remains and brings about a large magnetic entropy change around room temperatures for the compounds. The maximal magnetic entropy change is about 23.4, 17.7 and 15.9J/kg·K under a magnetic field change from 0 to 5T for x=0.01, 0.02 and 0.03, respectively. Therefore, the compounds appear to be potential candidates for magnetic refrigerants around room temperatures.     

Magnetism and magnetic entropy change in LaFe11Al2Cx compounds around room temperature

Magnetism and magnetic entropy changes in LaFe11A12Cx(x=0.0, 0.2 and 0.5) compounds have been investigated.The Curie temperature TC is conveniently controlled from 200K to room temperature by varying the carbon concentration.Large magnetic entropy change is obtained over a wide temperature range due to the high magnetization and the drastic decrease in the magnetization around TC.The large magnetic entropy change in wide temperature range,low cost and the convenience of controlling TC suggest that the LaFe11Al2Cx compounds are promising candidates for magnetic refrigerants in the corresponding temperature range.     

Mn5Ge2.7M0.3 (M=Ga，Al，Sn) 化合物的磁性和磁熵变

研究了Mn₅Ge_2.7M_0.3(M=Ga，Al，Sn)化合物的磁性和磁熵变. x射线衍射实验表明，研究的化合物均呈六角Mn₅Si₃型结构. 三种原子对Ge原子的替代，使得平均Mn原子磁矩下降，但居里温度没有明显的变化. 由于磁矩的降低，导致磁熵变值的下降，在磁场变化为4.0×10⁶A·m^-1时，对应于M=Ga，Al和Sn的样品，最大磁熵变值ΔS^max_Ｍ分别为6.1，6.3和5.3J·kg^-1K^-1，但磁熵变峰值的半高宽ΔＴ_ＦＷＨＭ有所增加. 另外，Mn₅Ge_2.7M_0.3(M=Ga，Al，Sn)化合物在高于居里温度的Arrott曲线上出现了一个不连续点，即样品在一定温度下的顺磁磁化率在某一临界磁场下发生了突变，临界磁场与温度几乎呈正比关系.这可能是由于样品在加一定磁场时3d带的费米能级发生了变化，使得有效电子数的减少所致. 关键词： 居里温度 平均Mn原子磁矩 磁熵变 Arrott图     

Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Magnetic properties and magnetocaloric effects of Mn5G3-xGax

We report on the magnetic properties and magnetocaloric effects of Mn_5Ge_{3-x}Ga_x compounds with x=0.1, 0.2, 0.3, 0.4, 0.6 and 0.9. All samples crystallize in the hexagonal Mn_5Si_3-type structure with space group P6_3/mcm and order ferromagnetically. The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9). The average Mn magnetic moments increases with increasing Ga content, reaching a maximum value at x=0.6. The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn_5Ge_3. One is that the magnitude of the magnetic entropy change decreases, the other is that the MCE peak becomes broadened.