arXiv subjects

Kohei Hamaya

Publications and source records attributed to Kohei Hamaya.

7 recordsLinked to original sources

arXiv2025

Isotropic and anisotropic spin-dependent transport in epitaxial Fe$_3$Si

We investigate spin-dependent transport phenomena in epitaxially grown Fe$_3$Si films, focusing on the anisotropic magnetoresistance (AMR), planar Hall effect (PHE), anomalous Hall effect (AHE), and spin Hall effect (SHE). While the sign and magnitude of the AMR and PHE depend on the current orientation relative to the crystallographic axes, the AHE and SHE remain nearly independent of the current orientation. The anisotropic AMR and PHE are attributed to current and magnetization dependent local band properties, including band crossing/anticrossing at specific $k$ points. In contrast, the isotropic AHE and SHE arise from the Berry curvature integrated over the entire Brillouin zone, which cancels local variations. These findings highlight the interplay between symmetry, band structure, and magnetization in the spin-dependent transport phenomena.

cond-mat.mes-hall↗

arXiv2020

Structural insight using anomalous XRD into Mn2CoAl Heusler alloy films grown by magnetron sputtering, IBAS and MBE techniques

Inverse Heusler alloy Mn2CoAl thin films, known as a spin-gapless semiconductor (SGS), grown by three different methods: ultra-high vacuum magnetron spattering, Ar-ion beam assisted sputtering, and molecular beam epitaxy, are investigated by comparing their electric transport properties, microstructures and atomic-level structures. Of the samples, the Mn2CoAl thin film grown by MBE consists of Mn- and Co-rich phases, the structures of which are determined to be the L21B-type and disordered L21-type, respectively, according to anomalous XRD analysis. None of them forms the XA-type structure expected for SGS Heusler alloy, although they all show SGS characteristics. We suggest, to validate SGS characteristics, it is necessary to extract not only magnetic and electric transport properties but also information about microstructures and atomic-scale structures of the films including defects such as atomic swap.

cond-mat.mtrl-sci↗

arXiv2017

Room-temperature spin transport in n-Ge probed by four-terminal nonlocal measurements

We demonsrtate electrical spin injection and detection in $n$-type Ge ($n$-Ge) at room temperature using four-terminal nonlocal spin-valve and Hanle-effect measurements in lateral spin-valve (LSV) devices with Heusler-alloy Schottky tunnel contacts. The spin diffusion length ($λ$$_{\rm Ge}$) of the Ge layer used ($n \sim$ 1 $\times$ 10$^{19}$ cm$^{-3}$) at 296 K is estimated to be $\sim$ 0.44 $\pm$ 0.02 $μ$m. Room-temperature spin signals can be observed reproducibly at the low bias voltage range ($\le$ 0.7 V) for LSVs with relatively low resistance-area product ($RA$) values ($\le$ 1 k$Ω$$μ$m$^{2}$). This means that the Schottky tunnel contacts used here are more suitable than ferromagnet/MgO tunnel contacts ($RA \ge$ 100 k$Ω$$μ$m$^{2}$) for developing Ge spintronic applications.

cond-mat.mtrl-sci↗

arXiv2014

Exchange-driven magnetoresistance in silicon facilitated by electrical spin injection

We use electrical spin injection to probe exchange interactions in phosphorus doped silicon (Si:P). The detection is enabled by a magnetoresistance effect that demonstrates the efficiency of exchange in imprinting spin information from the magnetic lead onto the localized moments in the Si:P region. A unique Lorentzian-shaped signal existing only at low temperatures ($\lesssim 25 K$) is observed experimentally and analyzed theoretically in electrical Hanle effect measurement. It stems from spin-dependent scattering of electrons by neutral impurities in Si:P. The shape of this signal is not directly related to spin relaxation but to exchange interaction between spin-polarized electrons that are localized on adjacent impurities.

cond-mat.mtrl-sci↗

arXiv2013

Dynamical Spin Injection into p-type Germanium at Room Temperature

We demonstrate dynamical spin injection into p-type germanium (Ge) at room temperature (RT) using spin pumping. The generated pure spin current is converted to a charge current by the inverse spin-Hall effect (ISHE) arising in the p-type Ge sample. A clear electromotive force due to the ISHE is detected at RT. The spin-Hall angle for p-type Ge is estimated to be θSHE = 2.6x10-3 at RT, which is much larger than that for p-type Si.

cond-mat.mtrl-sci↗

arXiv2009

Dynamic nuclear polarization induced by breakdown of fractional quantum Hall effect

We study dynamic nuclear polarization (DNP) induced by breakdown of the fractional quantum Hall (FQH) effect. We find that voltage-current characteristics depend on current sweep rates at the quantum Hall states of Landau level filling factors $ν$ = 1, 2/3, and 1/3. The sweep rate dependence is attributed to DNP occurring in the breakdown regime of FQH states. Results of a pump and probe experiment show that the polarities of the DNP induced in the breakdown regimes of the FQH states is opposite to that of the DNP induced in the breakdown regimes of odd-integer quantum Hall states.

cond-mat.mes-hall↗

arXiv2007

Dynamic Nuclear Polarization in a Quantum Hall Corbino Disk

Electrical polarization of nuclear spins is studied in a Corbino disk under a breakdown regime of the quantum Hall effect (QHE). Since the edge channels are completely absent in the Corbino disk, we conclude that the electric current flowing in the bulk channel of a quantum Hall conductor is relevant to dynamic nuclear polarization (DNP). A pump and probe measurement demonstrates that DNP emerges near the critical voltage of the QHE breakdown. The agreement of the onset voltage of DNP with that of the QHE breakdown indicates that the underlying origin of DNP is closely related to that of the QHE breakdown.

cond-mat.mes-hall↗