Kaoru Takeda

Interlayer coupling in ferromagnetic epitaxial Fe3Si∕FeSi2 superlattices

Ferromagnetic epitaxial B2-type Fe3Si∕FeSi2 superlattices were prepared on Si(111) at room temperature by facing target direct-current sputtering. The bilinear and biquadratic coupling constants J1 and J2 of the antiferromagnetically coupled superlattice were comparable to those of the similar superlattices using Fe layers although the saturation magnetization of Fe3Si is approximately half as large as that of Fe. The authors believe that this is due to the formation of a well-ordered quantum well in the spacers, which is mainly caused by the regular accumulation of highly oriented Fe3Si layers.

Epitaxy in Fe3Si/FeSi2 superlattices prepared by facing target direct-current sputtering at room tempertaure

Fe3Si/FeSi2 superlattices were prepared on Si(111) at two deposition rates by facing target direct-current sputtering. For the deposition rates of 2.0 nm/min for Fe3Si and 1.3 nm/min for FeSi2, the Fe3Si layers were nonoriented. On the other hand, for half-deposition rates, the Fe3Si layers were epitaxially grown not only on Si(111) but also up to the top layer across the FeSi2 layers. The antiferromagnetic interlayer coupling between the Fe3Si layers was induced in the epitaxial superlattices, whereas it disappeared in the nonepitaxial superlattices. The regular accumulation of highly oriented Fe3Si layers is crucial for the interlayer coupling induction.

Enhanced Interlayer Coupling and Magnetoresistance Ratio in Fe3Si/FeSi2 Superlattices

[Fe3Si/FeSi2]20 superlattices were prepared on Si(111) at an elevated substrate temperature of 300 °C, and the magnetoresistance ratio and interlayer coupling strengths were enhanced by approximately 100 and 34%, respectively, as compared to those of superlattices deposited at room temperature. While the elevated substrate temperature degraded the interface sharpness, the crystalline orientation and the crystallinity of the Fe3Si layers were apparently enhanced. The latters strongly influence on the interlayer coupling and the magnetoresistance ratio. This implies that quantum well states are tightly formed under the well-ordered crystalline planes, and the spin diffusion lengths are improved due to the enhanced crystallinity.

Temperature-Dependent Current-Induced Magnetization Switching in Fe3Si/FeSi2/Fe3Si Trilayered Films

Fe3Si/FeSi2/Fe3Si trilayered films were grown on Si(111) substrates at a substrate temperature of 300 °C by facing-targets direct-current sputtering, and current-induced magnetization switching in current-perpendicular-to-plane geometry was studied for the films wherein an antiferromagnetic interlayer coupling perpendicular to the plane was probably formed at room temperature. The appearance of a hysteresis loop in the electrical resistance–injection current curve well coincided with that of a hysteresis loop in the magnetization curve perpendicular to the plane. In addition, the hysteresis loop in the electrical resistance–injection current curve disappeared under large magnetic fields. The origin of the change in the electrical resistance for the injection current might be the change in the interlayer coupling.

Fabrication of Spin Valve Junctions Based on Fe3Si/FeSi2/Fe3Si Trilayered films

Yuki Asai, Ken-ichiro Sakai*, Kazuya Ishibashi, Kaoru Takeda, and Tsuyoshi Yoshitake** 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Control and Information Systems Engineering, Kurume National College of Technology, Kurume, Fukuoka 830-8555, Japan 3Department of Electrical Engineering, Fukuoka Institute of Technology, Fukuoka 811-0295, Japan

INTERFACIAL STRUCTURE OF Fe3Si/FeSi2 LAYERED FILMS DEPOSITED ON Si(111) AT ELEVATED SUBSTRATE-TEMPERATURES

Influence of substrate-temperature on the interfacial structure of Fe3Si/FeSi2 layered films deposited on a Si(111) substrate were studied. Fe3Si/FeSi2 films with sharp interfaces were grown at room substrate-temperature. At a substrate-temperature of 300 °C, interfaces between the Fe3Si and FeSi2 layers were obviously unsharpened, while the crystallinity of Fe3Si was enhanced. The compositional periodic structure was barely unsharpened and it was nearly the same as that of the films deposited at room substrate-temperature. Epitaxial growth of Fe3Si layers across FeSi2 layers was carried out. This substrate-temperature is the upper limit at which the heterostructure formation takes place. At 400 °C, e-FeSi was formed due to activated interdiffusion, and the structure of Fe3Si changed partially from B2-type to DO3-type.

Spin Valve behavior in Current-Perpendicular-to-Plane Crossover Structural Fe3Si/FeSi2/Fe3Si Trilayered junctions

Yuki Asai, Ken-ichiro Sakai*, Kazuya Ishibashi, Kaoru Takeda, and Tsuyoshi Yoshitake** 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Control and Information Systems Engineering, Kurume National College of Technology, Kurume, Fukuoka 830-8555, Japan 3Department of Electrical Engineering, Fukuoka Institute of Technology, Fukuoka 8110295, Japan

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe3Si/FeSi2 artificial lattices with various cross-sectional areas

Ken-ichiro Sakai*, Yūki Asai, Yūta Noda, Takeshi Daio, Aki Tominaga, Kaoru Takeda, and Tsuyoshi Yoshitake* 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Control and Information Systems Engineering, Kurume National College of Technology, Kurume, Fukuoka 830-8555, Japan 3International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 8190395, Japan 4Department of Electrical Engineering, Fukuoka Institute of Technology, Fukuoka 8110295, Japan

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe3Si/FeSi2 artificial lattices

Current-perpendicular-to-plane (CPP) junctions of Fe3Si/FeSi2 were fabricated from Fe3Si/FeSi2 artificial lattice films, which were prepared by facing-target direct-current sputtering, by employing a focused ion beam (FIB) technique. CPP structurization was confirmed by scanning electron microscopy. The CPP junctions, in which antiferromagnetic interlayer coupling is induced between the Fe3Si layers, exhibited a clear hysteresis loop in the electrical resistance for current injection, which is probably due to current-induced magnetization switching. The critical current density for it is approximately 3.3 × 101 A/cm2, which is at least four orders smaller than the values that have ever been reported.