Tuning Surface Spin Polarization of CoFeB by Boron Diffusion Detected by Spin Resolved Photoemission
Qi Liu1†, Xianyang Lu1,4†*, Chengrui Fu2, Jiarui Chen3, Zhe Zhang1, Yuting Gong1, Xinyue Wang1, Yu Yan1,4, Qinwu Gao1, Hui Li2*, Xuezhong Ruan1, Yao Li1, Jun Du3, Jing Wu4, Liang He1, Bo Liu5, Rong Zhang1, and Yongbing Xu1,4*
1Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China 2Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China 3Department of Physics, Nanjing University, Nanjing 210093, China 4York-Nanjing Joint Center (YNJC) for Spintronics and Nano-engineering, Department of Electronics and Physics, University of York, York YO10 5DD, UK 5Key Laboratory of Spintronics Materials, Devices and Systems of Zhejiang Province, Hangzhou 311300, China
Abstract：Research of spin polarization of magnetic CoFeB thin films is of practical importance in spintronic applications. Here, using a direct characterization technique of spin-resolved photoemission spectroscopy, we obtain the surface spin polarization of amorphous Co$_{40}$Fe$_{40}$B$_{20}$ thin films with different annealing temperatures from 100 ℃ to 500 ℃ prepared by magnetron sputtering. After high annealing temperature, a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion. While the global magnetization remains almost constant, the secondary electrons' spin polarization, average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing annealing temperature above 100 ℃. These distinct surface properties are attributed to the enhanced Fe–B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films, which should be important for development of high-performance magnetic random-access memories.
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2023, Vol. 40 
