Soichiro Oki

Estimation of the spin polarization for Heusler-compound thin films by means of nonlocal spin-valve measurements: Comparison of Co 2 FeSi and Fe 3 Si

We study room-temperature generation and detection of pure spin currents using lateral spin-valve devices with electrodes formed from the Heusler compounds Co

Improvement of magnetic and structural stabilities in high-quality Co2FeSi1−xAlx/Si heterointerfaces

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Molecular Beam Epitaxy of Co2MnSi Films on Group-IV Semiconductors

FeSi (CFS) or Fe

Effect of Addition of Al to Single-Crystalline CoFe Electrodes on Nonlocal Spin Signals in Lateral Spin-Valve Devices

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Lateral spin valves with two-different Heusler-alloy electrodes on the same platform

Si (FS). The magnitude of the nonlocal spin-valve signals is strongly affected by resistivity variations observed particularly in low-temperature-grown Heusler compounds containing ordered structures. From an analysis based on a one-dimensional spin diffusion model, we find that the spin polarization monotonically increases with decreasing resistivity, which depends on the structural ordering, for both the CFS and FS electrodes, and show that CFS has a larger spin polarization than FS.

Spin injection into multilayer graphene from highly spin-polarized Co2FeSi Heusler alloy

We study high-quality Co2FeSi1−xAlx Heusler compound/Si (0 ≤ x ≤ 1) heterointerfaces for silicon (Si)-based spintronic applications. In thermal treatment conditions, the magnetic and structural stabilities of the Co2FeSi1−xAlx/Si heterointerfaces are improved with increasing x in Co2FeSi1−xAlx. Compared with L21-ordered Co2FeSi/Si, B2-ordered Co2FeAl/Si can suppress the diffusion of Si atoms into the Heusler-compound structure. This experimental study will provide an important knowledge for applications in Si-based spin transistors with metallic source/drain contacts.

Spin-related thermoelectric conversion in lateral spin-valve devices with single-crystalline Co2FeSi electrodes

We explore epitaxial growth of Co2MnSi films on Si(111) or Ge(111) by means of low-temperature molecular beam epitaxy. We find that as-grown Co2MnSi films consist of mixed phases with L21-ordered structures and microcrystalline ones. As a result, the magnetic moment, which is nearly half of the ideal value, can be obtained even at very low growth temperature. Post-growth annealing was effective to crystallize the microcrystalline phases observed in the as-grown layer, leading to a further enhancement in the magnetic moment. We discuss a difference in growth mechanism between Co2MnSi and other Heusler alloys examined in our previous works.

Room-Temperature Tunneling Magnetoresistance in Magnetic Tunnel Junctions with a D03-Fe3Si Electrode

Using single-crystalline spin injectors and detectors, we examine the effect of the addition of Al to CoFe electrodes on nonlocal spin signals in metallic lateral spin valves (LSVs). A molecular beam epitaxy technique enables us to obtain Heusler-alloy-like CoFeAl epitaxial layers with an ordered B2 structure. The LSVs with the CoFeAl electrodes show a remarkable enhancement of the nonlocal spin signals, six times larger than that of the LSV with the CoFe electrodes, at room temperature. The relatively low electrical resistivity of the CoFeAl electrodes means that the enhancement of the spin signals originates from an increase in the spin polarization due to the Al addition to CoFe.

Direct Evidence for Suppression of the Kondo Effect due to Pure Spin Current

Using room-temperature molecular beam epitaxy on Si(111), we demonstrate Heusler-alloy bilayers consisting of L21-Co2FeSi (CFS) and D03-Fe3Si (FS). By fabricating lateral spin valves with L21-CFS and D03-FS electrodes, we can see ideal spin signals even though we use one L21-CFS as a spin injector and another D03-FS as a spin detector. The difference in the spin absorption between L21-CFS and D03-FS can also be examined, and we find that the spin resistance of D03-FS is larger than that of L21-CFS. This work will be useful for understanding spin transport in lateral spin-valve devices with different Heusler-alloy electrodes.

Modulation of magnetization dynamics in an epitaxial Heusler ferromagnet due to pure spin current in a laterally configured structure

We demonstrate electrical spin injection into multilayer graphene (MLG) in a lateral spin valve device from a highly spin-polarized Co2FeSi (CFS) Huesler electrode. Exfoliated MLG was transferred onto prepatterned epitaxial CFS wires grown on an Si(111) substrate by a polymer-based transfer method. This method enabled us to fabricate multiple single-crystal CFS electrodes in contact with MLG. Electrical spin injection from CFS to MLG was detected through nonlocal magnetoresistance (MR) measurement. A nonlocal spin signal of 430 Ω was observed; this is the largest value among all reported nonlocal MR values in graphene-based devices.