Yasuhiro Niimi

Giant Spin Hall Effect Induced by Skew Scattering from Bismuth Impurities inside Thin Film CuBi Alloys

We demonstrate that a giant spin Hall effect (SHE) can be induced by introducing a small amount of Bi impurities in Cu. Our analysis, based on a new three-dimensional finite element treatment of spin transport, shows that the sign of the SHE induced by the Bi impurities is negative and its spin Hall (SH) angle amounts to -0.24. Such a negative large SH angle in CuBi alloys can be explained by applying the resonant scattering model proposed by Fert and Levy [Phys. Rev. Lett. 106, 157208 (2011)] to 6p impurities.

STS observations of Landau levels at graphite surfaces.

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi-two-dimensional electrons and holes. A comparison with the calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with a finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum-Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.

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 (

Extrinsic spin Hall effects measured with lateral spin valve structures

\sigma_{SH}^{int}=1600\pm150\: \Omega^{-1}cm^{-1}

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

) 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.

Thickness dependence of spin torque ferromagnetic resonance in Co75Fe25/Pt bilayer films

The spin Hall effect (SHE), induced by spin-orbit interaction in nonmagnetic materials, is one of the promising phenomena for conversion between charge and spin currents in spintronic devices. The spin Hall (SH) angle is the characteristic parameter of this conversion. We have performed experiments of the conversion from spin into charge currents by the SHE in lateral spin valve structures. We present experimental results on the extrinsic SHEs induced by doping nonmagnetic metals, Cu or Ag, with impurities having a large spin-orbit coupling, Bi or Pb, as well as results on the intrinsic SHE of Au. The SH angle induced by Bi in Cu or Ag is negative and particularly large for Bi in Cu, 10 times larger than the intrinsic SH angle in Au. We also observed a large SH angle for CuPb but the SHE signal disappeared in a few days. Such an aging effect could be related to a fast mobility of Pb in Cu and has not been observed in CuBi alloys.

5d iridium oxide as a material for spin-current detection

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.

Quantum coherence at low temperatures in mesoscopic systems: Effect of disorder

The spin Hall angle of Pt in Co75Fe25/Pt bilayer films was experimentally investigated by means of the spin-torque ferromagnetic resonance and the modulation of damping measurements. By comparing the present results with the Ni80Fe20/Pt system, we found that the ferromagnetic layer underneath the Pt one greatly affects the estimation of the spin Hall angle. We also discuss the spin diffusion length of Pt and the ferromagnetic thickness dependence of the Gilbert damping coefficient.

The spin Hall effect as a probe of nonlinear spin fluctuations

Devices based on pure spin currents have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, electric detection of spin currents is essential. The 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, have been regarded as potential materials for a spin-current injector, owing to the large direct spin Hall effect. Their spin Hall resistivity ρSH, representing the performance as a detector, is not large enough, however, due mainly because of their low charge resistivity. Here we report that a binary 5d transition metal oxide, iridium oxide, overcomes the limitations encountered in noble metals and Cu-based alloys and shows a very large ρSH~38 μΩ cm at room temperature.

Influence of inverse spin Hall effect in spin-torque ferromagnetic resonance measurements

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.