Kohei Hamaya 교수

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 nons

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

We study room-temperature generation and detection of pure spin currents using lateral spin-valve devices with electrodes formed from the Heusler compounds Co${}_{2}$FeSi (CFS) or Fe${}_{3}$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 monotoni

Using a laterally fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N=3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B=1.2T. This means that, in the Kondo r

The anisotropic magnetotransport properties of a (Ga,Mn)As epilayer and the magnetization switching are studied as a function of temperature. The magnetization switching field shows asymmetry for crystallographically equivalent [110] and [1̄10] directions at 4 K, and the asymmetry is more significant at 40 K. The magnetization switching features clearly show that cubic magnetocrystalline anisotropy along 〈100〉, which is biased by a small uniaxial anisotropy along the [110] easy axis, is dominant

We show nonlocal spin transport in n-Ge based lateral spin-valve devices with highly ordered Co_2FeSi/n^+-Ge Schottky tunnel contacts. Clear spin-valve signals and Hanle-effect curves are demonstrated at low temperatures, indicating generation, manipulation, and detection of pure spin currents in n-Ge. The obtained spin generation efficiency of ~ 0.12 is about two orders of magnitude larger than that for a device with Fe/MgO tunnel-barrier contacts reported previously. Taking the spin related be

We experimentally study the tunneling magnetoresistance (TMR) effect as a function of bias voltage $({V}_{\mathrm{SD}})$ in lateral $\mathrm{Ni}∕\mathrm{In}\mathrm{As}∕\mathrm{Ni}$ quantum-dot (QD) spin valves showing Coulomb blockade characteristics. With varying ${V}_{\mathrm{SD}}$, the TMR value oscillates and the oscillation period corresponds to conductance changes observed in the current-voltage $(I\text{\ensuremath{-}}{V}_{\mathrm{SD}})$ characteristics. We also find an inverse TMR effect

For exploring group-IV semiconductor spintronics with ferromagnetic Heusler compounds, we study the local structural ordering of the stoichiometric Fe${}_{3}$Si and off-stoichiometric Fe${}_{3.2}$Si${}_{0.8}$ films epitaxially grown on Ge(111) at a very low temperature of 130 ${}^{ˆ}$C. Analyzing their $^{57}\mathrm{Fe}$ M\"ossbauer spectra, we can discuss the site occupation of Fe atoms in the films grown directly on a semiconductor substrate, where the influence of the interfacial reactions be

Two different ferromagnetic-paramagnetic transitions are detected in (Ga,Mn)As/GaAs(001) epilayers from ac susceptibility measurements: transition at a higher temperature results from (Ga,Mn)As cluster phases with [110] uniaxial anisotropy and that at a lower temperature is associated with a ferromagnetic (Ga,Mn)As matrix with 100 cubic anisotropy. A change in the magnetic easy axis from [100] to [110] with increasing temperature can be explained by the reduced contribution of 100 cubic anisotro

We experimentally study the transport features of electrons in a spin-diode structure consisting of a single semiconductor quantum dot (QD) weakly coupled to one nonmagnetic and one ferromagnetic (FM) lead, in which the QD has an artificial atomic nature. A Coulomb stability diamond shows asymmetric features with respect to the polarity of the bias voltage. For the regime of two-electron tunneling, we find anomalous suppression of the current for both forward and reverse bias. We discuss possibl

Using low-temperature molecular-beam epitaxy techniques on the (111) plane of Si or Ge, we can realize an atomically controlled Fe 3 Si/Si or Fe 3 Si/Ge heterojunction and simultaneously obtain D O 3 -ordered crystal structures of Fe 3 Si films. First, high-quality Fe 3 Si/Si(111) Schottky tunnel contacts enable us to inject and detect spin-polarized electrons in Si conduction channels at ∼180 K, where Fe 3 Si is a ferromagnetic spin injector and detector. This may lead to the accelerated develo

We study electrical spin injection and detection in n-Ge across Fe3Si/n+-Ge Schottky tunnel barriers. Spin-accumulation signals detected electrically by the three-terminal Hanle-effect measurements have large temperature dependence, and the spin signals disappear at around 200 K. We find that the temperature variation in the spin signals is strongly related to that in the interface resistance of the Fe3Si/n+-Ge contacts. We also observe marked reduction in nonlocal spin-valve signals with increa

We study a possible mechanism of the switching of the magnetic easy axis as a function of hole concentration in $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ epilayers. In-plane uniaxial magnetic anisotropy along [110] is found to exceed intrinsic cubic magnetocrystalline anisotropy above a hole concentration of $p=1.5\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This anisotropy switching can be realiz

Abstract Ferromagnetic Co 2 -Heusler compounds showing high spin polarization have been utilized as spin injectors and detectors for III–V and Group-IV semiconductors. In this article, we first describe the progress in the crystal growth of Co 2 -Heusler films on GaAs(001) and Ge(111) by low-temperature molecular beam epitaxy. Next, some examples of electrical spin injection from Co 2 -Heusler contacts into GaAs and Ge through Schottky-tunnel barriers are introduced. Thanks to those efforts, it