基于磁性半金属Heusler合金的自旋电子器件中材料和界面的调控及相关机制

CPP-spin valve and magnetic tunnel junction are two kinds of spintroic devices which are very promising for the applications in information storage.Before they can be applied in magnetic memories, the magnetoresistant ratio (MR) and the signal to noise ratio (SNR)of the devices should be improved. Half metallic Heusler alloies which have very high spin polarization are very promising for great enhancement of MR and SNR if being used in the devices. In this project, the focus will be put on the study of related materials and interfaces in Heusler based MR devices (CPP-SVs and MTJs). In the proposed project, both experimental investigation and theoretical study will be performed. Experimentally, we will fabricate MTJ and SV devices based on different materials, and try to improve MR ratio of the devices.by optimizations (composition, structure of Heuslers and fabrication process), selection of spaced layer ,and control of the interface. In theoretical study, we will figure out how composition and chemical order affect the electronic structure and band structure; Find out the band stucuture and magnetc structure at the interface of devices composed of different materials, which may help us clarify the mechanism for interface and materials affecting the MR properties of the device, and explore methods for optimization of interface and materials. Besides,we will also do some research on supperlattice Heusler films, the goal is to understand how composition and structure of the superlattice controls the magnetic anisotropy and to find a proper way to obtain novel Heusler alloys which have low magnetization and perpendicular magnetic anisotropy as well.

磁性半金属Heusler合金具有高的自旋极化率，如果用于自旋电子器件中，有望大大提高器件的磁电阻性能。本项目将以基于磁性半金属Heusler合金的CPP-SV和MTJ器件作为研究对象，研究材料以及界面对磁电阻性能的影响以及调控机制。在实验上，选取不同的材料，研究成分、结构以及工艺等因素对材料磁性能、输运性能的影响；研究基于不同材料的器件的界面特性以及调控方法；探索通过材料成分的优化、工艺的控制、界面的控制与修饰等手段提高器件的磁电阻性能的方法与途径；在理论上，结合实验结果，研究不同的材料组合的界面的能带结构和磁结构，阐明不同界面影响磁电阻的机制，为更有效提高器件的磁电阻性能提供方法和依据。此外，还将对不同成分、不同超晶格结构的Heusler合金进行研究，弄清成分与结构对磁各向异性的影响及相关机制，探索具有低磁化强度与垂直各向异性的新材料。

本项目对基于磁性半金属Heusler合金自旋电子器件中材料和界面进行研究，探索材料以及界面特性对磁电阻性能的影响以及调控机制，为进一步研发高性能自旋电子器件提供依据。研究获得的主要进展如下：1）通过工艺优化和成分调控，在多种基片(MgO, MgAl2O4以及GaAs) 上外延制备出多种成分的高质量的薄膜；2）在MgO(100)基片上采用磁控溅射方法成功制备出了基于Co2FeAl的自旋阀三明治结构，最高室温MR值在5%以上；3）采用CoFe对自旋阀的界面进行修饰，同时采用磁场热处理改善界面磁排列，成功将MR值提高到8%左右；4）对MgAl2O4/Co2FeAl和MgO/Co2FeAl界面中不同界面稳定性进行了研究，发现Co/O型和FeAl/O型界面最为稳定，通过界面类型和电场可调控不同轨道的耦合强度，从而调控垂直各向异性;5) 研究了Co2FeAl/MgO2/Co2FeAl隧道结结构中不同界面构型对电子透射率的影响，发现Co/O界面比FeAl/O界面更为有效抑制了自旋向下电子的隧穿，从而可获得更高的MR；6）研究了多种成分Heusler合金半金属稳定性，获得多种具有应用前景的Heusler四元合金成分；7)通过退火工艺的优化和成分的调控，制备出多种成分具有L21结构的合金。以上研究结果已达到预定的研究目标，后续更为深入的研究将会继续进行。