Electronic States and Magnetic Coupling in Fe/Fe 3 O 4 Junctions
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Electronic States and Magnetic Coupling in Fe/Fe3O4 Junctions J. Inoue1, T. Kida1, S. Honda2, H. Itoh3, H. Yanagihara4, E. Kita4, and K. Mibu5 1
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan
2
ORDIST, Kansai University, Suita 564-8680, Japan
3
Department of Pure and Applied Physics, Kansai University, Suita 564-8680, Japan
4
Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
5
Graduate School of Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
ABSTRACT Exchange coupling observed recently in Fe/ Fe3O4 (001) junctions shows comparable intensity to that in Co/Ru/Co trilayers, and has potential applicability to spintronics devices. To clarify the mechanism of the exchange coupling, electronic and magnetic states of Fe/ Fe3O4 junctions are calculated in the first principles method by assuming four junction structures of bcc Fe and Fe3O4 layers. It is shown that the local moments of bcc Fe atoms at the interface increase, but those of Fe ions at the interface of Fe3O4 layer decrease. The total energy of the junctions is plotted as a function of distance between Fe and Fe3O4 layers. Calculated results of the coupling energy between Fe and Fe3O4 layers, however, are larger than experimental ones by two orders of magnitude, and they correspond to inter-atomic exchange interactions at the interface. In order to explain the experimental results, we propose a mechanism of exchange coupling mediated by impurity-like states of interfacial Fe atoms which possess reversed magnetic moments in bcc Fe layer. Frustration effects in exchange coupling between Fe and Fe3O4 layers are also discussed. INTRODUCTION Magnetic junctions made of more than two thin magnetic layers have provided interesting physics and applications in the field of spintronics. Magnetic multilayers, which consist of alternate stacking of ferromagnetic and nonmagnetic metals, give rise to the giant magnetoresistance (GMR) and interlayer exchange coupling (IEC). Ferromagnetic tunnel junctions made of two ferromagnetic layers separated by a thin insulator bring about tunnel magnetoresistance (TMR). Direct contacts of a ferromagnet and an antiferromagnet produce the so-called exchange bias (EB) in the magnetization process. These concepts, GMR, IEC, TMR and EB are essential for spintronics applications. Search for novel ferromagnetic materials has also provided fruitful results to produce high quality half-metallic ferromagnets, in which one of two spin states is metallic and the other is insulating. The magnetite Fe3O4 known as the first ferromagnet ever discovered was predicted to be half-metallic in the first principles band calculations [1, 2]. Several attempts were performed to utilize Fe3O4 in tunnel junctions for high TMR ratio, but TMR ratio observed was not high
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enough for practical applications [3, 4]. Nevertheless, Fe3O4 films still attract much interests to explore spin dependent properties. Quite recently, we have observed exchange coupling (EC) in Fe/MgO/Fe3O4 [5] and
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