Atsuo Ono

Ultrathin films of polycrystalline MnGa alloy with perpendicular magnetic anisotropy

Room temperature growth of textured polycrystalline films of MnGa alloys using a CoGa buffer layer on a thermally oxidized Si substrate is demonstrated. MnGa thin films with a thickness of 2 nm exhibit out-of-plane rectangular hysteresis loops. A small saturation magnetization of about 200 emu/cm3 and a large perpendicular magnetic anisotropy of up to 3–5 Merg/cm3 were achieved for 2- and 3-nm-thick MnGa ultrathin films; such values have never been reported before, and they provide a pathway for integration with conventional Si technology.

Fabrication of highly ordered Co2Fe0.4Mn0.6Si Heusler alloy films on Si substrates

The structural and magnetic properties of Si(100)/MgO/Co2Fe0.4Mn0.6Si (CFMS) Heusler alloy thin films were systematically investigated. Highly B2-ordered CFMS Heusler films with an ordering parameter of ca. 70–80% were obtained by both the insertion of a very thin Mg layer into the Si/MgO interfaces to prevent oxidation of the Si surface and the optimization of the annealing temperature for the CFMS films. The prepared CFMS films exhibited high magnetization close to that of the CFMS bulk. Such highly B2-ordered CFMS films are very useful for realizing high spin injection efficiency in Si because of the half-metallicity of the CFMS films.

Perpendicular magnetic tunnel junctions with Mn-modified ultrathin MnGa layer

Perpendicular magnetic tunnel junctions (p-MTJs) with a MgO barrier and a 1-nm-thick MnGa electrode were investigated by inserting several monolayers (MLs) of Mn. The tunnel magnetoresistance (TMR) ratio systematically increased when increasing the Mn layer thickness with a maximum of 18 (38.4)% at 300 (5) K for a Mn layer thickness of 0.6–0.8 nm. This ratio is five times higher compared to that without the Mn layer. The perpendicular magnetic anisotropy (PMA) field and the PMA constant of the ultrathin MnGa layer also increased up to 62–90 kOe and 6.2–11.3 Merg/cm3, respectively, with an increase in the Mn interlayer thickness, even for the ultrathin regime of the MnGa layer. For p-MTJs showing a high TMR and PMA, electron microscopy indicated the presence of 3–4 MLs of Mn at the MnGa/MgO interface; thus, the Mn modification enhanced the TMR as well as improved the PMA. This may be a promising finding to develop a Mn-based free layer for spin-transfer-torque devices for high-recording-density magnetoresi...

Highly (001)-textured Mn-Ga polycrystalline ultrathin films with a perpendicular magnetic anisotropy

Perpendicularly magnetized ultrathin films of magnetic metals are technologically important for various spintronic applications, such as the free layer in magnetic tunnel junctions (MTJs) for the spin-transfer-torque (STT) magnetoresisive random access memory (MRAM) and in current-perpendicular magnetoresisive devices for the spin-torque radio frequency (rf) oscillators and detectors.