arXiv subjects

K. Hamaya

Publications and source records attributed to K. Hamaya.

At least 19 recordsLinked to original sources

arXiv2019

Nonmonotonic bias dependence of local spin accumulation signals in ferromagnet/semiconductor lateral spin-valve devices

We find extraordinary behavior of the local two-terminal spin accumulation signals in ferromagnet (FM)/semiconductor (SC) lateral spin-valve devices. With respect to the bias voltage applied between two FM/SC Schottky tunnel contacts, the local spin-accumulation signal can show nonmonotonic variations, including a sign inversion. A part of the nonmonotonic features can be understood qualitatively by considering the rapid reduction in the spin polarization of the FM/SC interfaces with increasing bias voltage. In addition to the sign inversion of the FM/SC interface spin polarization, the influence of the spin-drift effect in the SC layer and the nonlinear electrical spin conversion at a biased FM/SC contact are discussed.

cond-mat.mes-hall↗

arXiv2018

Nonlinear Electrical Spin Conversion in a Biased Ferromagnetic Tunnel Contact

The conversion of spin information into electrical signals is indispensable for spintronic technologies. Spin-to-charge conversion in ferromagnetic tunnel contacts is well-described using linear (spin-)transport equations, provided that there is no applied bias, as in nonlocal spin detection. It is shown here that in a biased ferromagnetic tunnel contact, spin detection is strongly nonlinear. As a result, the spin-detection efficiency is not equal to the tunnel spin polarization. In silicon-based 4-terminal spin-transport devices, even a small bias (tens of mV) across the Fe/MgO detector contact enhances the spin-detection efficiency to values up to 140 \% (spin extraction bias) or, for spin injection bias, reduces it to almost zero, while, parenthetically, the charge current remains highly spin polarized. Calculations reveal that the nonlinearity originates from the energy dispersion of the tunnel transmission and the resulting nonuniform energy distribution of the tunnel current, offering a route to engineer spin conversion. Taking nonlinear spin detection into account is also shown to explain a multitude of peculiar and puzzling spin signals in structures with a biased detector, including two- and three-terminal devices, and provides a unified, consistent and quantitative description of spin signals in devices with a biased and unbiased detector.

cond-mat.mes-hall↗

arXiv2018

Pure spin current transport in a SiGe alloy

Using four-terminal nonlocal magnetoresistance measurements in lateral spin-valve devices with Si$_{\rm 0.1}$Ge$_{\rm 0.9}$, we study pure spin current transport in a degenerate SiGe alloy ($n \sim$ 5.0 $\times$ 10$^{18}$ cm$^{-3}$). Clear nonlocal spin-valve signals and Hanle-effect curves, indicating generation, manipulation, and detection of pure spin currents, are observed. The spin diffusion length and spin lifetime of the Si$_{\rm 0.1}$Ge$_{\rm 0.9}$ layer at low temperatures are reliably estimated to be $\sim$ 0.5 $μ$m and $\sim$ 0.2 ns, respectively. This study demonstrates the possibility of exploring physics and developing spintronic applications using SiGe alloys.

cond-mat.mes-hall↗

arXiv2018

Efficient spin transport along Si $\langle$100$\rangle$ at room temperature

We find efficient spin transport in Si at room temperature in lateral spin valves (LSVs). When the crystal orientation of the spin-transport channel in LSVs is changed from $\langle$110$\rangle$, which is a conventional cleavage direction, to $\langle$100$\rangle$, the maximum magnitude of the spin signals is markedly enhanced. From the analyses based on the one-dimensional spin diffusion model, we can understand that the spin injection/detection efficiency in Si$\langle$100$\rangle$ LSVs is larger than that in Si$\langle$110$\rangle$ ones. We infer that, in Si-based LSVs, the spin detection efficiency of the pure spin current is related to the crystallographic orientation of the valley structures of the conduction band in Si.

cond-mat.mes-hall↗

arXiv2018

Anomalous Hall conductivity and electronic structures of Si-substituted Mn$_{2}$CoAl epitaxial films

We study anomalous Hall conductivity ($σ$$_{\rm AHC}$) and electronic band structures of Si-substituted Mn$_{2}$CoAl (Mn$_{2}$CoAl$_{1-x}$Si$_{x}$). First-principles calculations reveal that the electronic band structure is like a spin-gapless system even after substituting a quaternary element of Si for Al up to $x = $0.2 in Mn$_{2}$CoAl$_{1-x}$Si$_{x}$. This means that the Si substitution enables the Fermi level shift without largely changing the electronic structures in Mn$_{2}$CoAl. By using molecular beam epitaxy (MBE) techniques, Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ epitaxial films can be grown, leading to the systematic control of $x$ (0 $\le$ $x$ $\le$ 0.3). In addition to the electrical conductivity, the values of $σ$$_{\rm AHC}$ for the Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ films are similar to those in Mn$_{2}$CoAl films shown in previous reports. We note that a very small $σ$$_{\rm AHC}$ of $\sim$ 1.1 S/cm is obtained for $x =$ 0.225 and the sign of $σ$$_{\rm AHC}$ is changed from positive to negative at around $x =$ 0.25. We discuss the origin of the sign reversal of $σ$$_{\rm AHC}$ as a consequence of the Fermi level shift in MCA. Considering the presence of the structural disorder in the Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ films, we can conclude that the small value and sign reversal of $σ$$_{\rm AHC}$ are not related to the characteristics of spin-gapless semiconductors.

cond-mat.mtrl-sci↗

arXiv2017

Giant spin accumulation in silicon nonlocal spin-transport devices

Although the electrical injection, transport and detection of spins in silicon have been achieved, the induced spin accumulation was much smaller than expected and desired, limiting the potential impact of Si-based spintronic devices. Here, using non-local spin-transport devices with an n-type Si channel and Fe/MgO magnetic tunnel contacts, we demonstrate that it is possible to create a giant spin accumulation in Si, with the spin splitting reaching 13 meV at 10 K and 3.5 meV at room temperature. The non-local spin signals are in good agreement with a numerical evaluation of spin injection and diffusion that explicitly takes the size of the injector contact into account. The giant spin accumulation originates from the large tunnel spin polarization of the Fe/MgO contacts (53 % at 10 K and 18 % at 300 K), and the spin density enhancement achieved by using a spin injector with a size comparable to the spin-diffusion length of the Si. The ability to induce a giant spin accumulation enables the development of Si spintronic devices with a large magnetic response.

cond-mat.mes-hall↗

arXiv2016

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

We study the effect of a pure spin current on the Kondo singlet in a diluted magnetic alloy using non-local lateral spin valve structures with highly spin polarized Co2FeSi electrodes. Temperature dependence of the non-local spin signals shows a sharp reduction with decreasing temperature, followed by a plateau corresponding to the low temperature Fermi liquid regime below the Kondo temperature (TK). The spin diffusion length of the Kondo alloy is found to increase with the evolution of spin accumulation. The results are in agreement with the intuitive description that the Kondo singlet cannot survive any more in sufficiently large spin accumulation even below TK.

cond-mat.mtrl-sci↗

arXiv2013

Large enhancement in the generation efficiency of pure spin currents in Ge using Heusler-compound Co_2FeSi electrodes

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 behavior with temperature evolution into account, we infer that it is necessary to simultaneously demonstrate the high spin generation efficiency and improve the quality of the transport channel for achieving Ge based spintronic devices.

cond-mat.mtrl-sci↗

arXiv2013

Room-temperature detection of spin accumulation in silicon across Schottky tunnel barriers using a MOSFET structure

Using a metal-oxide-semiconductor field effect transistor (MOSFET) structure with a high-quality CoFe/n^+Si contact, we systematically study spin injection and spin accumulation in a nondegenerated Si channel with a doping density of ~ 4.5*10^15cm^-3 at room temperature. By applying the gate voltage (V_G) to the channel, we obtain sufficient bias currents (I_Bias) for creating spin accumulation in the channel and observe clear spin-accumulation signals even at room temperature. Whereas the magnitude of the spin signals is enhanced by increasing I_Bias, it is reduced by increasing V_G interestingly. These features can be understood within the framework of the conventional spin diffusion model. As a result, a room-temperature spin injection technique for the nondegenerated Si channel without using insulating tunnel barriers is established, which indicates a technological progress for Si-based spintronic applications with gate electrodes.

cond-mat.mtrl-sci↗

arXiv2013

Giant enhancement of spin pumping efficiency using Fe3Si ferromagnet

Spincurrentronics, which involves the generation, propagation and control of spin currents, has attracted a great deal of attention because of the possibility of realizing dissipation-free information propagation. Whereas electrical generation of spin currents originally made the field of spincurrentronics possible, and significant advances in spin-current devices has been made, novel spin-current-generation approaches such as dynamical methods have also been vigorously investigated. However, the low spin-current generation efficiency associated with dynamical methods has impeded further progress towards practical spin devices. Here we show that by introducing a Heusler-type ferromagnetic material, Fe3Si, pure spin currents can be generated about twenty times more efficiently using a dynamical method. This achievement paves the way to the development of novel spin-based devices.

cond-mat.mtrl-sci↗

arXiv2013

Effect of Co-Fe substitutions on the room-temperature spin polarization in Co_3-xFe_xSi Heusler-compound films

Using low-temperature molecular beam epitaxy, we study substitutions of Fe atoms for Co ones in Co_3-xFe_xSi Heusler-compound films grown on Si and Ge. Even for the low-temperature grown Heusler-compound films, the Co-Fe atomic substitution at A and C sites can be confirmed by the conversion electron M"ossbauer spectroscopy measurements. As a result, the magnetic moment and room-temperature spin polarization estimated by nonlocal spin-valve measurements are systematically changed with the Co-Fe substitutions. This study experimentally verified that the Co-Fe substitution in Co_3-xFe_xSi Heusler compounds can directly affect the room-temperature spin polarization.

cond-mat.mtrl-sci↗

arXiv2012

Atomically Controlled Epitaxial Growth of Single-Crystalline Germanium Films on a Metallic Silicide

We demonstrate high-quality epitaxial germanium (Ge) films on a metallic silicide, Fe3Si, grown directly on a Ge(111) substrate. Using molecular beam epitaxy techniques, we can obtain an artificially controlled arrangement of silicon (Si) or iron (Fe) atoms at the surface on Fe3Si(111). The Si-terminated Fe3Si(111) surface enables us to grow two-dimensional epitaxial Ge films, whereas the Fe-terminated one causes the three-dimensional epitaxial growth of Ge films. The high-quality Ge grown on the Si-terminated surface has almost no strain, meaning that the Ge films are not grown on the low-temperature-grown Si buffer layer but on the lattice matched metallic Fe3Si. This study will open a new way for vertical-type Ge-channel transistors with metallic source/drain contacts.

cond-mat.mtrl-sci↗

arXiv2012

Effect of the magnetic domain structure in the ferromagnetic contact on spin accumulation in silicon

We show a marked effect of the magnetic domain structure in an epitaxial CoFe contact on the spin accumulation signals in Si detected by three-terminal Hanle-effect measurements. Clear reduction in the spin accumulation signals can be seen by introducing the domain walls in the CoFe contact, caused by the lateral spin transport in the Si channel. The domain walls in the CoFe contact largely affect the spin lifetime and bias-current dependence of the spin signals. These results indicate that the estimation of the spin related properties without considering the domain structure in the contact causes non-negligible errors in the three-terminal Hanle-effect measurements.

cond-mat.mtrl-sci↗

arXiv2012

Evidence for the Presence of Spin Accumulation in Localized States at Ferromagnet-Silicon Interfaces

We experimentally show evidence for the presence of spin accumulation in localized states at ferromagnet-silicon interfaces, detected by electrical Hanle effect measurements in CoFe/$n^{+}$-Si/$n$-Si lateral devices. By controlling the measurement temperature, we can clearly observe marked changes in the spin-accumulation signals at low temperatures, at which the electron transport across the interface changes from the direct tunneling to the two-step one via the localized states. We discuss in detail the difference in the spin accumulation between in the Si channel and in the localized states.

cond-mat.mtrl-sci↗

arXiv2012

Temperature evolution of spin accumulation detected electrically in a nondegenerated silicon channel

We study temperature evolution of spin accumulation signals obtained by the three-terminal Hanle effect measurements in a nondegenerated silicon channel with a Schottky-tunnel-barrier contact. We find the clear difference in the temperature-dependent spin signals between spin-extraction and spin-injection conditions. In a spin-injection condition with a low bias current, the magnitude of spin signals can be enhanced despite the rise of temperature. For the interpretation of the temperature-dependent spin signals, it is important to consider the sensitivity of the spin detection at the Schottky-tunnel-barrier contact in addition to the spin diffusion in Si.

cond-mat.mes-hall↗

arXiv2012

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 Heusler-compound electrodes, Co$_{2}$FeSi (CFS) or Fe$_{3}$Si (FS). The magnitude of the nonlocal spin-valve (NLSV) signals is seriously affected by the dispersion of the resistivity peculiarly observed in the low-temperature grown Heusler compounds with ordered structures. From the analysis based on the one-dimensional spin diffusion model, we find that the spin polarization monotonically increases with decreasing the resistivity, which depends on the structural ordering, for both CFS and FS electrodes, and verify that CFS has relatively large spin polarization compared with FS.

cond-mat.mes-hall↗

arXiv2011

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

Using high-quality Fe$_{3}$Si/$n^{+}$-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.

cond-mat.mtrl-sci↗

arXiv2011

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

We demonstrate spin-accumulation signals controlled by the gate voltage in a metal-oxide-semiconductor field effect transistor structure with a Si channel and a CoFe/$n^{+}$-Si contact at room temperature. Under the application of a back-gate voltage, we clearly observe the three-terminal Hanle-effect signal, i.e., spin-accumulation signal. The magnitude of the spin-accumulation signals can be reduced with increasing the gate voltage. We consider that the gate controlled spin signals are attributed to the change in the carrier density in the Si channel beneath the CoFe/$n^{+}$-Si contact. This study is not only a technological jump for Si-based spintronic applications with gate structures but also reliable evidence for the spin injection into the semiconducting Si channel at room temperature.

cond-mat.mes-hall↗