磁性金属纳米薄膜界面增强各向异性磁电阻效应的研究

Magnetic sensors based on the anisotropic magnetoresistance (AMR) of magnetic thin films are widely utilized in industry, and the issues about new materials and physics of AMR in magnetic films have attracted a great deal attention for years. In this project, we focus on the interfacial enhancement of AMR in nonmagnetic/ magnetic/ nonmagnetic trilayer films by taking advantage of the magnetic proximity effect, where the nonmagnetic layer is 4d and 5d elements and their alloys, and the magnetic layer mainly includes NiFe thin to a few nano-meters as well as half metals. Aimed at developing a new artificial structure of high AMR performance, our proposed routine to improve the AMR ratio of the NiFe thin films is tuning the spin-orbit coupling, disorder atomic scattering of the nonmagnetic layer by alloying different 4d and 5d elements with proximity induced magnetism. Physically, we will firstly investigate the prerequisite for magnetic proximity effect for 4d and 5d elements, and we will pursue the mechanism of the magnetic proximity effect by performing various transport measurements on the magnetic/nonmagnetic bilayer films and trilayer films. We will also study the unconventional AMR of nonmagnetic layers with induced magnetism, and try to clarify all factors that shape its behavior. We propose that the tiny induced magnetism of the nonmagnetic layer could be distinguished by transport measurements. Finally, we will study the AMR of the half metal based trilayer films, and search for a new perspective of this fascinating kind of materials.

磁性纳米薄膜的各向异性磁电阻(AMR)具有重要实用价值，并且物理内涵丰富。本项目围绕"铁磁/非磁"、"非磁/铁磁/非磁"人工结构，以NiFe、半金属合金为磁性层，以表现磁近邻效应的4d、5d元素以及它们之间的二元合金为非磁层，全面系统研究非磁层的磁近邻效应、自旋-轨道耦合系数、非磁合金层原子无序散射、以及磁性层自旋极化率和相应电子结构等因素对三明治结构AMR的影响与规律，显著提高现有NiFe纳米薄膜的AMR，推进相关材料的实用化进程；同时，掌握4d、5d元素发生诱导磁性的基本条件及磁近邻效应的变化特点，深刻理解磁近邻效应的微观机理，阐明磁近邻效应界面层的非常规AMR对合金无序散射、自旋-轨道耦合系数的依赖规律，发展全金属多层膜结构中判断非磁层是否具有磁近邻效应的输运表征手段；研究 "半金属磁体/非磁金属"磁近邻效应与相关三明治结构AMR的新特征，并探索具有应用前景的半金属三明治AMR薄膜。

磁性异质结构自旋相关输运的研究引起了凝聚态物理的深刻发展，改变了信息技术的发展进程。起源于磁性材料极化电子自旋-轨道散射的各向异性磁电阻（AMR），是磁性金属的本质特征之一。不同异质结构自旋电子材料被普遍认为具有不同的磁电阻角度依赖关系，是其内在独特自旋相关输运特性的体现。我们在SiO2/Fe/SiO2与MgO/Fe/MgO三明治结构中发现四种不同角度依赖关系的AMR，包括与自旋霍尔磁电阻（SMR）完全相同的角度关系，扩展了磁性金属薄膜AMR的角度关系及其内在物理，证明使用一种特定的角度依赖磁电阻关系作为AMR来源或者自旋霍尔磁电阻的判断是不合适的，同时，通过氧化物/金属的界面效应显著增强了Fe纳米薄膜的AMR；在YIG/Au-Cu异质结构中发现反常霍尔效应恒为零，证实由于Au、Cu的d电子远离费米面，不存在磁近邻效应，发现Au-Cu无序合金的强烈无序原子散射显著增强YIG/Au-Cu结构的自旋霍尔磁电阻、自旋赛贝克效应和Au-Cu的自旋霍尔角，为自旋霍尔效应的新一类外禀增强机制；通过FeMn薄膜相邻铁磁层的AMR效应表征异质结构中FeMn的反铁磁性，发现反铁磁FeMn和非磁态具有大小不同、符号相反的自旋霍尔效应，证明反铁磁序对材料內禀自旋霍尔效应的强烈调控。此外，通过Co-Pt的大矫顽力和反铁磁IrMn对铁磁金属的交换偏置，在相关YIG异质结构中实现多种热-磁-电效应的独立调控，证明了磁性金属交换偏置结构是一种有效的自旋流探测器，同时，磁性金属与非磁重金属的合金增强其自旋-轨道耦合，影响相关的逆自旋霍尔效应、反常霍尔效应等多种物理效应；通过5d重金属Ta为分隔层获得垂直磁化人工反铁磁MgO/CoFeB/Ta/CoFeB/MgO，并以Ta自旋霍尔效应为主的自旋-轨道力矩实现了反平行排列磁矩的同步翻转；另外，还发现过量B增强Ta/CoFeB/ MgO薄膜垂直各向异性和温度稳定性。