Abstract: Spintronic devices that manipulate the spin degree of freedom of the conduction electron are being extensively studied, which could lead future electron devices featuring nonvolatility and reconfigurability. Half-metallic ferromagnets feature an energy gap for one spin direction at the Fermi level (EF), which provides complete spin polarization at EF. Because they exhibit complete spin polarisation at EF, half-metallic ferromagnets are one of the key materials for ferromagnetic electrodes in spintronic devices. Heusler alloys are amongst the most extensively studied potentially half-metallic electrode materials. This is because half metallicity is theoretically predicted for several of these alloys and because they have high Curie temperatures, which are well above room temperature (RT). We have recently proposed and developed fully epitaxial magnetic tunnel junctions (MTJs) with Co-based Heusler thin films and a MgO barrier, which are advantageous for fully utilizing high spin polarizations of potentially half-metallic Co-based Heusler-alloy electrodes in terms of (1) preparing structurally high-quality single-crystal Heusler-alloy electrodes, (2) forming atomically flat and abrupt interfaces, and (3) enabling an enhancement of tunnel magnetoresistance due to coherent tunneling of electrons in epitaxial MTJs with a MgO barrier. We also investigated the effect of defects possibly associated with nonstoichiometry in Heusler alloy Co2MnSi (CMS) thin films on spin-dependent tunneling characteristics and found that fully epitaxial CMS/MgO/CMS MTJs with Mn-rich CMS electrodes exhibited high tunnel magnetoresistance (TMR) ratios at both 4.2 K and room temperature, exceeding those of MTJs with CMS electrodes having an almost stoichiometric composition. The observed higher TMR ratio for MTJs with Mn-rich CMS electrodes was explained by suppressed harmful CoMn antisites, which caused a reduced density of minority-spin in-gap states around EF. Nonstoichiometry is inevitable, to various degrees, in Heusler alloy thin films, which are mostly prepared by magnetron sputtering. Our findings, however, suggest that detrimental CoMn antisites can be suppressed by preparing Co2MnSi films with a Mn-rich composition.Ref. M. Yamamoto et al., J. Phys.: Condens. Matter 22 (2010) 164212.

Biography: Masafumi Yamamoto received the B.S., M.S., and Ph. D degrees in Physics from Hokkaido University, Sapporo, Japan, in 1973, 1975, and 1978, respectively. He worked as a research scientist, supervisor for Electrical Communication Laboratories, Nippon Telegraph and Telephone Corporation (NTT) for 22 years from 1978 to 2000 in Tokyo and Kanagawa Prefecture and he was the Quantum Effect Devices Group Leader at NTT System Electronics Laboratories from 1993 to 1999. In NTT Research Laboratories, he was engaged in the research and development of superconducting Josephson-junction devices and circuits, quantum interference devices using compound-semiconductor quantum wires, and ultrahigh-speed electronic circuits using InP-based resonant tunneling diodes in combination with InP-based HEMT’s. In April 2000, he joined Hokkaido University as a Full Professor in the Division of Electronics for Informatics, Graduate School of Information Science and Technology. After he moved to Hokkaido University, he has been engaged in the research and development of spintronic devices in particular, epitaxial magnetic tunnel junctions with potentially half-metallic Heusler alloy thin films and a MgO barrier.