arXiv subjects

Yasuhiro Niimi

Publications and source records attributed to Yasuhiro Niimi.

29 records · Page 2Linked to original sources

arXiv2020

Generation of Multipeak Spectrum of Spin Torque Oscillator in Non-linear Regime

We investigate the spectral characteristics of spin torque oscillator (STO) excited by the spin Hall-induced spin current. We observe that the modest spin current injection triggers the conventional single peak oscillating behavior of STO. As the spin current is further increased to enter the non-linear regime, we find the transition of the spectrum from a single peak to multipeak structure whose frequency spacing is constant. This behavior can be primarily explained by the extremely broadened peak of the STO, which is accompanied by the frequency-dependent filtering by the transmission line. To explain the observation more quantitatively, we also discuss that the multipeak may reflect the characteristics of the intrinsic dynamics of STO in the non-linear regime.

cond-mat.mes-hall↗

arXiv2019

Relation between spin Hall effect and anomalous Hall effect in 3$d$ ferromagnetic metals

We study the mechanisms of the spin Hall effect (SHE) and anomalous Hall effect (AHE) in 3$d$ ferromagnetic metals (Fe, Co, permalloy (Ni$_{81}$Fe$_{19}$; Py), and Ni) by varying their resistivities and temperature. At low temperatures where the phonon scattering is negligible, the skew scattering coefficients of the SHE and AHE in Py are related to its spin polarization. However, this simple relation breaks down for Py at higher temperatures as well as for the other ferromagnetic metals at any temperature. We find that, in general, the relation between the SHE and AHE is more complex, with the temperature dependence of the SHE being much stronger than that of AHE.

cond-mat.mes-hall↗

arXiv2017

Large room temperature spin-to-charge conversion signals in a few-layer graphene/Pt lateral heterostructure

Electrical generation and detection of pure spin currents without the need of magnetic materials are key elements for the realization of full electrically controlled spintronic devices. In this framework, achieving a large spin-to-charge conversion signal is crucial, since considerable outputs are needed for plausible applications. Unfortunately, the values obtained so far have been rather low. Here we exploit the spin Hall effect by using Pt, a non-magnetic metal with strong spin-orbit coupling, to generate and detect pure spin currents in a few-layer graphene channel. Furthermore, the outstanding properties of graphene, with long distance spin transport and higher electrical resistivity than metals, allows us to achieve in our graphene/Pt lateral heterostructures the largest spin-to-charge voltage signal at room temperature reported so far in the literature. Our approach opens up exciting opportunities towards the implementation of spin-orbit-based logic circuits and all electrical control of spin information without magnetic field.

cond-mat.mes-hall↗

arXiv2017

Dynamics of Pure Spin Current in High-frequency Quantum Regime

Pure spin current is a powerful tool for manipulating spintronic devices, and its dynamical behavior is an important issue. By using mesoscopic transport theory for electron tunneling induced by spin accumulation, we investigate the dynamics of the spin current in the high-frequency quantum regime, where the frequency is much larger than temperature and bias voltage. Besides the thermal noise, frequency-dependent finite noise emerges, signaling the spin current across the tunneling barrier. We also find that the autocorrelation of the spin current exhibits sinusoidal oscillation in time as a consequence of the Pauli exclusion principle even without any net charge current.

cond-mat.mes-hall↗

arXiv2016

Tuning the spin Hall effect of Pt from the moderately dirty to the superclean regime

We systematically measure and analyze the spin diffusion length and the spin Hall effect in Pt with a wide range of conductivities using the spin absorption method in lateral spin valve devices. We observe a linear relation between the spin diffusion length and the conductivity, evidencing that the spin relaxation in Pt is governed by the Elliott-Yafet mechanism. We find a single intrinsic spin Hall conductivity ($σ_{SH}^{int}=1600\pm150\: Ω^{-1}cm^{-1}$) for Pt in the full range studied which is in good agreement with theory. For the first time we have obtained the crossover between the moderately dirty and the superclean scaling regimes of the spin Hall effect by tuning the conductivity. This is equivalent to that obtained for the anomalous Hall effect. Our results explain the spread of the spin Hall angle values in the literature and find a route to maximize this important parameter.

cond-mat.mes-hall↗

arXiv2015

Reciprocal spin Hall effects in conductors with strong spin-orbit coupling: a review

Spin Hall effect and its inverse provide essential means to convert charge to spin currents and vice versa, which serve as a primary function for spintronic phenomena such as the spin-torque ferromagnetic resonance and the spin Seebeck effect. These effects can oscillate magnetization or detect a thermally generated spin splitting in the chemical potential. Importantly this conversion process occurs via the spin-orbit interaction, and requires neither magnetic materials nor external magnetic fields. However, the spin Hall angle, i.e., the conversion yield between the charge and spin currents, depends severely on the experimental methods. Here we discuss the spin Hall angle and the spin diffusion length for a variety of materials including pure metals such as Pt and Ta, alloys and oxides determined by the spin absorption method in a lateral spin valve structure.

cond-mat.mes-hall↗

arXiv2014

Spin Relaxation Mechanism in a Highly Doped Organic Polymer Film

We report the systematic studies of spin current transport and relaxation mechanism in highly doped organic polymer film. In this study, we have determined spin diffusion length (SDL), spin lifetime, and spin diffusion constant by using different experimental techniques. The spin lifetime estimated from the electron paramagnetic resonance experiment is much shorter than the previous expectation beyond the experimental ambiguity. This suggests that significantly large spin diffusion constant, which is reasonably explained by the hopping transport mechanism in degenerate semiconductors, exists in highly doped organic semiconductors. The calculated SDL using the spin lifetime and spin diffusion constant estimated from our experiment is comparable to the experimentally obtained SDL of the order of one hundred nanometers. Moreover, the present study revealed that the spin angular momentum is almost preserved in the hopping events. In other words, the spin relaxation mainly occurs due to the spin-orbit coupling at the nanoscale crystalline grains.

cond-mat.mtrl-sci↗

arXiv2013

5d transition metal oxide IrO2 as a material for spin current detection

Devices based on a pure spin current (a flow of spin angular momentum) have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, an electric detection of spin current is essential. Inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, owing to the large direct spin Hall effect, have been regarded as potential materials for a spin-current injector. Those materials, however, are not promising as a spin-current detector based on inverse spin Hall effect. Their spin Hall resistivity rho_SH, representing the performance as a detector, is not large enough mainly due to their low charge resistivity. Here we demonstrate that heavy transition metal oxides can overcome such limitations inherent to metal-based spintronics materials. A binary 5d transition metal oxide IrO2, owing to its large resistivity as well as a large spin-orbit coupling associated with 5d character of conduction electrons, was found to show a gigantic rho_SH ~ 38 microohm cm at room temperature, one order of magnitude larger than those of noble metals and Cu-based alloys and even comparable to those of atomic layer thin film of W and Ta.

cond-mat.mes-hall↗

arXiv2013

Experimental Verification of Comparability between Spin-Orbit and Spin-Diffusion Lengths

We experimentally confirmed that the spin-orbit lengths of noble metals obtained from weak anti-localization measurements are comparable to the spin diffusion lengths determined from lateral spin valve ones. Even for metals with strong spin-orbit interactions such as Pt, we verified that the two methods gave comparable values which were much larger than those obtained from recent spin torque ferromagnetic resonance measurements. To give a further evidence for the comparability between the two length scales, we measured the disorder dependence of the spin-orbit length of copper by changing the thickness of the wire. The obtained spin-orbit length nicely follows a linear law as a function of the diffusion coefficient, clearly indicating that the Elliott-Yafet mechanism is dominant as in the case of the spin diffusion length.

cond-mat.mes-hall↗

arXiv2010

Quantum Coherence at Low Temperatures in Mesoscopic Systems: Effect of Disorder

We study the disorder dependence of the phase coherence time of quasi one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semi-ballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi liquid theory, without any sign of low temperature saturation. Surprisingly, in the semi-ballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime.

cond-mat.mes-hall↗

arXiv2004

Photoemission study of poly(dA)-poly(dT) DNA : Experimental and theoretical approach to the electronic density of states

We present results of an ultraviolet photoemission spectroscopy study of artificially synthesized poly(dA)-poly(dT) DNA molecules on $p$-type Si substrates. For comparison, we also present the electronic density of states (DOS) calculated using an \emph{ab initio} tight-binding method based on density-functional theory (DFT). Good agreement was obtained between experiment and theory. The spectra of DNA networks on the Si substrate showed that the Fermi level of the substrate is located in the middle of the band gap of DNA. The spectra of thick ($\sim 70$ nm) DNA films showed a downward shift of $\sim 2$ eV compared to the network samples.

cond-mat.soft↗