Kohei Hamaya

Electrical injection and detection of spin-polarized electrons in silicon through an Fe3Si/Si Schottky tunnel barrier

We demonstrate electrical injection and detection of spin-polarized electrons in silicon (Si) using epitaxially grown Fe3Si/Si Schottky-tunnel-barrier contacts. By an insertion of a δ-doped n+-Si layer (∼1019 cm−3) near the interface between a ferromagnetic Fe3Si contact and a Si channel (∼1015 cm−3), we achieve a marked enhancement in the tunnel conductance for reverse-bias characteristics of the Fe3Si/Si Schottky diodes. Using laterally fabricated four-probe geometries with the modified Fe3Si/Si contacts, we detect nonlocal output signals that originate from the spin accumulation in a Si channel at low temperatures.

Ferromagnetism and electronic structures of nonstoichiometric Heusler-alloy Fe3-xMnxSi Epilayers grown on Ge(111).

For the study of ferromagnetic materials which are compatible with group-IV semiconductor spintronics, we demonstrate control of the ferromagnetic properties of Heusler-alloy Fe3-xMnxSi epitaxially grown on Ge(111) by tuning the Mn composition x. Interestingly, we obtain L2(1)-ordered structures even for nonstoichiometric atomic compositions. The Curie temperature of the epilayers with x approximately 0.6 exceeds 300 K. Theoretical calculations indicate that the electronic structures of the nonstoichiometric Fe3-xMnxSi alloys become half-metallic for 0.75 < or = x < or = 1.5. We discuss the possibility of room-temperature ferromagnetic Fe(3-x)Mn(x)Si/Ge epilayers with high spin polarization.

Epitaxial ferromagnetic Fe3Si∕Si(111) structures with high-quality heterointerfaces

To develop silicon-based spintronic devices, we have explored high-quality ferromagnetic Fe3Si/silicon (Si) structures. Using low-temperature molecular beam epitaxy at 130°C, we realize the epitaxial growth of ferromagnetic Fe3Si layers on Si(111) with an abrupt interface, and the grown Fe3Si layer has the ordered DO3 phase. Measurements of magnetic and electrical properties for the Fe3Si∕Si(111) yield a magnetic moment of ∼3.16μB∕f.u. at room temperature and a rectifying Schottky-diode behavior with the ideality factor of ∼1.08, respectively.

Effect of atomically controlled interfaces on Fermi-level pinning at metal/Ge interfaces

We study electrical properties of metal/Ge contacts with an atomically controlled interface, and compare them with those with a disordered one, where atomically controlled interfaces can be demonstrated by using Fe3Si/Ge(111) contacts. We find that the Schottky barrier height of Fe3Si/n-Ge(111) contacts is unexpectedly lower than those induced by the strong Fermi-level pinning at other metal/n-Ge contacts. For Fe3Si/p-Ge(111) contacts, we identify clear rectifying behavior in I-V characteristics at low temperatures, which is also different from I-V features due to the strong Fermi-level pinning at other metal/p-Ge contacts. These results indicate that there is an extrinsic contribution such as dangling bonds to the Fermi-level pinning effect at the directly connected metal/Ge contacts.

Significant growth-temperature dependence of ferromagnetic properties for Co2FeSi/Si"111… prepared by low-temperature molecular beam epitaxy

We study ferromagnetic properties of Heusler-alloy Co2FeSi epilayers grown on silicon (Si). The magnetic moment and in-plane magnetic anisotropy of the Co2FeSi/Si(111) epilayers vary significantly with the growth temperature (TG) even in the low-temperature region (TG≤200 °C). These features are induced by reaction phases formed at the interface between Co2FeSi and Si. At TG=100 °C, however, we can obtain both highly ordered L21 structures on Si and high-quality Co2FeSi/Si heterointerfaces at the same time. This fact will open a road to realize a Co-based half-metallic spin injector and detector for Si-based spintronic devices.

Spin transport through a single self-assembled InAs quantum dot with ferromagnetic leads

The authors have fabricated a lateral double barrier magnetic tunnel junction (MTJ) which consists of a single self-assembled InAs quantum dot (QD) with ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance (TMR) effect, which is evidence for spin transport through a single semiconductor QD. The TMR ratio and the curve shapes are varied by changing the gate voltage.

Spin accumulation created electrically in an n-type germanium channel using Schottky tunnel contacts

Using high-quality Fe3Si/n+-Ge Schottky-tunnel-barrier contacts, we study spin accumulation in an n-type germanium (n-Ge) channel. In the three- or two-terminal voltage measurements with low bias current conditions at 50 K, Hanle-effect signals are clearly detected only at a forward-biased contact. These are reliable evidence for electrical detection of the spin accumulation created in the n-Ge channel. The estimated spin lifetime in n-Ge at 50 K is one order of magnitude shorter than those in n-Si reported recently. The magnitude of the spin signals cannot be explained by the commonly used spin diffusion model. We discuss a possible origin of the difference between experimental data and theoretical values.

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

Effect of the interface resistance of CoFe/MgO contacts on spin accumulation in silicon

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Electric-field control of spin accumulation signals in silicon at room temperature

FeSi (CFS) or Fe