Ferromagnetic Semiconductor Heterostructures and MnAs/III-V Hybrid Nanostructures: Spin Dependent Tunneling, Magnetoresistance, and Electromotive Force

Masaaki Tanaka
The University of Tokyo

Wednesday, March 24, 2010 at 2:00pm
Engr IV Maxwell Room 57-124

Abstract
Introducing spin degrees of freedom into the present semiconductor electronics is a very important issue for realizing novel devices which will be needed in the future information technology. For fabricating such devices, it is necessary to exploit and fabricate semiconductor-based magnetic materials. III-V-based ferromagnetic semiconductors and MnAs/III-V hybrid nanostructures are hopeful candidates and model systems for future spintronic devices [1][2]. We show our recent studies on ferromagnetic semiconductor heterostructures, their spin transport, and tunneling devices [3-6]. We investigated the resonant tunneling effect and the increase of tunneling magnetoresistance (TMR) induced by it in GaMnAs quantum-well (QW) double-barrier heterostructures [3]. The resonant tunneling effect was observed when the GaMnAs QW thickness was from 3.8 to 20 nm, which indicates that highly coherent tunneling occurs in these heterostructures. It was also found that the Fermi level of the electrode injecting carriers is important to observe resonant tunneling in this system. We extended this study to three terminal devices, and characterization of the GaMnAs band structure and Fermi level position [5,6]. We successfully modulated the quantum levels of the GaMnAs QW and controlled the spin-dependent current by changing the voltage of the QW electrode (VQW). Also, TMR increase was observed at resonant levels by changing VQW. For nanostructures such as magnetic nanowires or spin valves, it is theoretically predicted that an electromotive force (e.m.f.) arises from a time-varying magnetization in a static magnetic field [7]. This reflects the conversion of magnetic energy to electrical energy. Here we show that such an e.m.f. can indeed be induced by a static magnetic field in magnetic tunnel junctions containing zinc-blende (ZB) MnAs quantum nano-magnets. The ZB MnAs nanomagnets are coupled to a hexagonal MnAs top electrode through an AlAs tunnel barrier, and to a GaAs:Be bottom electrode through a GaAs barrier. Under a static magnetic field, an e.m.f. of up to 21 meV was observed for a time scale of 102~103 sec. This e.m.f. is induced by a co-tunneling process of electrons and magnetization of ZB MnAs nanomagnets subject to a strong Coulomb blockade of 50 meV. Huge magnetoresistance of up to 100,000% was observed for certain bias voltages. Our results strongly suggest that Faraday's Law of induction must be generalized to account for purely spin effects in magnetic nanostructures [8].

Biography
Masaaki Tanaka received the B.S., M.S., and Ph.D. degrees in electronic engineering from the University of Tokyo, Japan, in 1984, 1986, and 1989, respectively. During the M.S. and Ph. D study, he was engaged in the research of epitaxial growth and electronic/optical properties of III-V semiconductor low-dimensional nanostructures. In 1989, he joined the Department of Electrical and Electronic Engineering at the University of Tokyo as a research associate and later in 1990 as a lecturer. There, he started to study heterostructures of dissimilar materials consisting of semiconductors, metals, and ferromagnets. In 1992, he joined Bell Communications Research (Bellcore) at Red Bank, New Jersey, in USA, as a visiting research scientist, where he studied molecular beam epitaxy and properties of ferromagnet/semiconductor heterostructures. In 1994, he retuned to the University of Tokyo as an associate professor, where he studied various materials, spin-related phenomena, and devices including magnetic semiconductors, ferromagnet/semiconductor heterostructures and nanostructures, magnetic tunnel junctions, and spin transistors. He is currently a professor of electrical and electronic engineering at the University of Tokyo. He has authored and coauthored over 200 scientific publications, and presented over 70 invited talks at international conferences and meetings. Dr. Tanaka is a member of the Japan Society of Applied Physics, the Physical Society of Japan, and the Magnetics Society of Japan.