Co基Heusler合金薄膜的生长及表/界面磁性研究

In recent years, Co based Heusler alloys have attracted great attention in spin-related devices, due to their high Curie temperature, magnetism, and theoretical predicted high spin polarization. However, these excellent properties strongly rely on structure ordering, working temperature, and atomic states of surface and interface, so it is difficult to verify them experimentally and apply them very well. Moreover, the present studies concentrate mainly on the simple attempt using of the heterojunction and performance optimization in the relative devices, but for the heterojunction itself little research has been reported. This proposal will focus on preparing high quality and stable Co-based Heusler alloy/MgO(GaAs) heterostructures by combined system of magnetron sputtering and pulsed laser deposition and several important problems will be precisely investigated: (1) We will explore the growth parameters of the heterostructures and their influence on the crystal quality , and the structure ordering. (2) The constitution of surface and interface between Heusler alloy and MgO(GaAs) layers will be investigated and the epitaxial relationship is expected to be acquired. And also, we will investigate magnetic states, strength, and interaction types on the surface and interface of our samples, and the influences on them. (3) We will investigate the relationship of magnetic properties and polarization with structure ordering, lattice distortion, barrier, doping , atomic states of surface and interface, and so on, and explore their related physical mechanisms.

Co基Heusler合金由于其较高的居里温度、磁性和理论上预言的高自旋极化率，而成为自旋相关器件的理想候选材料之一，近年来在国际上得到广泛关注。然而，由于理论上预言的独特性质强烈依赖于样品的结构有序度、温度和表/界面原子态，使其很难在实验上得到证实和很好的应用；而且，目前的研究主要集中于该异质结构在相关器件中的性能优化尝试，对于异质结构本身的科学问题研究甚少。本项目拟采用磁控溅射-脉冲激光沉积复合系统，通过控制生长条件和探索工艺参数，致力于在MgO和GaAs上获得高晶体质量、结构稳定的Co基Heusler合金薄膜以及揭示影响薄膜晶体质量和有序度的关键因素；并且揭示异质结构表/界面处的原子排列方式，探明其表/界面磁性状态、强弱、作用类型及其影响因素；明确结构无序、晶格畸变、结势垒、调制掺杂、表/界面原子态等对样品磁性和自旋极化的影响，并阐明其影响机理。

Co基Heusler合金由于其理论上预言的高自旋极化率而被认为是自旋相关器件的理想候选材料之一，然而，由于理论上预言的独特性质强烈依赖于样品的结构有序度和表/界面原子态，使其很难在实验上得到证实和很好的应用。另外，对于MgO和GaAs衬底上制备Co基Heusler合金薄膜的表/界面状态往往会影响其磁性和磁各向异性。因此，获得高晶体质量、结构稳定Co基Heusler合金薄膜的制备方法以及揭示结构、表/界面状态对其磁性和磁各向异性的影响至关重要。在本项目中，项目组成员利用脉冲激光沉积-磁控溅射复合系统在MgO和GaAs衬底上探索了制备高质量、结构有序Co2FeAl和Co2MnAl薄膜的最佳实验条件，并对其结构和磁性进行了详细研究。（1）在MgO衬底上不同生长温度下（100-700℃）制备了系列Co2FeAl薄膜，样品为高有序的L21结构，表面出现明显的磁畴形貌，结晶质量、原子有序度和界面应力随生长温度增加而提高，从而导致样品矫顽力稍有增大，且界面应力诱导了附加的单轴磁各向异性。（2）在砷化镓（GaAs）衬底上300 ℃生长和600 ℃快速热退火温度下外延得到不同厚度（5 nm, 7 nm, 15 nm, 30 nm, 50 nm）的Co2MnAl薄膜样品，样品为L21有序的晶体结构，通过对立方各向异性常数和单轴各向异性常数的分析和总结对不同厚度样品在不同测量温度下出现的复杂磁化转变现象成功进行了解释。（3）对MgO衬底上外延制备的Co2Mn1.30Si0.84薄膜进行了线性和二次磁光克尔效应研究。测量结果表明样品表现为立方磁各向异性，各向异性常数为KC=6.7×10^4 erg/cm^3。其二次磁光克尔信号较强，且表现为四重各向异性特性，最大克尔旋转角度为3 mdeg。该项目的实施对于Co 基 Heusler合金在自旋电子功能器件上的潜在应用提供了重要理论和实践依据。