Cation distribution and magnetic properties in ultrathin (Ni1–xCox)Fe2O4(x=0–1) layers on Si(111) studied by soft x-ray magnetic circular dichroism

作     者：Yuki K. Wakabayashi Yosuke Nonaka Yukiharu Takeda Shoya Sakamoto Keisuke Ikeda Zhendong Chi Goro Shibata Arata Tanaka Yuji Saitoh Hiroshi Yamagami Masaaki Tanaka Atsushi Fujimori Ryosho Nakane 

作者机构：Department of Electrical Engineering and Information Systems The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-8656 Japan Department of Physics The University of Tokyo Bunkyo-ku Tokyo 113-0033 Japan Materials Sciences Research Center Japan Energy Atomic Agency Sayo Hyogo 679-5148 Japan Department of Quantum Matters ADSM Hiroshima University Higashi-Hiroshima 739-8530 Japan Department of Physics Kyoto Sangyo University Motoyama Kamigamo Kita-Ku Kyoto 603-8555 Japan Center for Spintronics Research Network Graduate School of Engineering The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-8656 Japan Institute for Innovation in International Engineering Education The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-8656 Japan 

出 版 物：《Physical Review Materials》 (Physic. Rev. Mat.)

年 卷 期：2018年第2卷第10期

页      面：104416-104416页

核心收录：

基　　金：Spintronics Research Network of Japan Japan Society for the Promotion of Science, KAKEN, (15H02109, 16H02095) Japan Society for the Promotion of Science, KAKEN Ministry of Education, Culture, Sports, Science and Technology, MEXT Japan Science and Technology Agency, JST, (JPMJCR1777) Japan Science and Technology Agency, JST Japan Atomic Energy Agency, JAEA, (2016B-E20, A-16-AE-0030) Japan Atomic Energy Agency, JAEA Tohoku University 

主　　题：Electronic structure of atoms & molecules Ferrimagnetism Interfaces Magnetic insulators Oxides Cluster models Epitaxy X-ray magnetic circular dichroism 

摘      要：We study the electronic structure and magnetic properties of epitaxial (Ni1–xCox)Fe2O4(111) layers (x=0–1) with thicknesses d=1.7–5.2nm grown on Al2O3(111)/Si(111) structures, to achieve a high value of inversion parameter y which is the inverse-to-normal spinel-structure ratio, and hence to obtain good magnetic properties even when the thickness is thin enough for electron tunneling as a spin filter. We revealed the crystallographic (octahedral Oh or tetrahedral Td) sites and the valences of the Fe, Co, and Ni cations using experimental soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism spectra and configuration-interaction cluster-model calculation. In all the (Ni1–xCox)Fe2O4 layers with d∼4 nm, all Ni cations occupy the Ni2+(Oh) site, whereas Co cations occupy the three different Co2+(Oh),Co2+(Td), and Co3+(Oh) sites with constant occupancies. According to these features, the occupancy of the Fe3+(Oh) cations decreases and that of the Fe3+(Td) cations increases with decreasing x. Consequently, we obtained a systematic increase of y with decreasing x and achieved the highest y value of 0.91 for the NiFe2O4 layer with d=3.5nm. From the d dependences of y and magnetization in the d range of 1.7–5.2 nm, a magnetically dead layer is present near the NiFe2O4/Al2O3 interface, but its influence on the magnetization was significantly suppressed compared with the case of CoFe2O4 layers reported previously [Wakabayashi et al., Phys. Rev. B 96, 104410 (2017)], due to the high site selectivity of the Ni cations. Since our epitaxial NiFe2O4 layer with d=3.5nm has a high y value (0.91) and a reasonably large magnetization, it is expected to exhibit a strong spin-filter effect which can be used for efficient spin injection into Si.