Kouta Kondou

Fermi-level-dependent charge-to-spin current conversion by Dirac surface states of topological insulators

The spin–momentum locking of Dirac surface states offers intriguing possibilities for converting between charge and spin currents. Experiments show that fine tuning of the Fermi level is critical for maximizing the efficiency of such conversions.

Evaluation of Spin Hall Angle and Spin Diffusion Length by Using Spin Current-Induced Ferromagnetic Resonance

We experimentally demonstrate the thickness dependence of the spin current-induced ferromagnetic resonance in Ni80Fe20/nonmagnetic (Pt,Pd) bilayer thin films. The spectrum shape depends on the Ni80Fe20 layer thickness, due to extrinsic excitation in addition to the spin Hall effect. Detailed analysis of the thickness dependence of the spectrum, both for Ni80Fe20 and nonmagnetic layers, provides the spin Hall angle (Pt: 0.022±0.004, Pd: 0.008±0.002) and spin diffusion length (Pt: 1.2±0.06 nm, Pd: 2.0±0.09 nm).

Current-Driven Domain Wall Motion in CoCrPt Wires with Perpendicular Magnetic Anisotropy

We report the direct observation of current-driven domain wall (DW) motion in a CoCrPt wire with perpendicular magnetic anisotropy. Magnetic force microscopy showed that a single DW introduced in the wire is displaced back and forth by positive and negative pulsed current. This is the first demonstration of the current-driven DW motion in a metallic magnetic wire with perpendicular magnetic anisotropy in the absence of a magnetic field.

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

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.

Spin-transfer switching in full-Heusler Co2FeAl-based magnetic tunnel junctions

We demonstrated spin-transfer magnetization switching using magnetic tunnel junctions (MTJs) with a full-Heusler alloy Co2FeAl (CFA). We prepared CFA (1.5 nm)/MgO/CoFe (4 nm) (“CFA-free”) and CFA (30 nm)/MgO/CoFeB (2 nm) (“CFA-reference”) MTJs on a Cr(001) layer. The intrinsic critical current density (Jc0) of the CFA-free (CFA-reference) MTJ was 29 MA/cm2 (7.1 MA/cm2). The larger Jc0 of the CFA-free MTJ is attributed to the significant enhancement of the Gilbert damping factor (∼0.04) of the CFA due to the Cr layer. The Jc0 of the CFA-reference is as small as that reported for typical CoFeB/MgO/CoFeB MTJs.

Experimental observation of spin-to-charge current conversion at non-magnetic metal/Bi2O3 interfaces

We here demonstrate the interfacial spin to charge current conversion by means of spin pumping from a ferromagnetic Permalloy (Py: Ni80Fe20) to a Cu/Bi2O3 interface. A clear signature of the spin to charge current conversion was observed in voltage spectrum of a Py/Cu/Bi2O3 trilayer film whereas no signature in a Py/Cu and Py/Bi2O3 bilayer films. We also found that the conversion coefficient strongly depended on Cu thickness, reflecting the thickness dependent momentum relaxation time in Cu layer.

Three-Terminal Device Based on the Current-Induced Magnetic Vortex Dynamics with the Magnetic Tunnel Junction

Current-induced resonant motion of the vortex core was probed by a three-terminal device with the magnetic tunnel junction. The large output signal due to the large magnetoresistance ratio of the magnetic tunnel junction allows us to detect the vortex dynamics clearly. The output signal can be tuned by the dc bias voltage, indicating that the device acts as a resonant transistor.

Current-Nonlinear Hall Effect and Spin-Orbit Torque Magnetization Switching in a Magnetic Topological Insulator

The current-nonlinear Hall effect or second harmonic Hall voltage is widely used as one of the methods for estimating charge-spin conversion efficiency, which is attributed to the magnetization oscillation by spin-orbit torque (SOT). Here, we argue the second harmonic Hall voltage under a large in-plane magnetic field with an in-plane magnetization configuration in magnetic-nonmagnetic topological insulator (TI) heterostructures, Cr_{x}(Bi_{1-y}Sb_{y})_{2-x}Te_{3}/(Bi_{1-y}Sb_{y})_{2}Te_{3}, where it is clearly shown that the large second harmonic voltage is governed not by SOT but mainly by asymmetric magnon scattering without macroscopic magnetization oscillation. Thus, this method does not allow an accurate estimation of charge-spin conversion efficiency in TI. Instead, the SOT contribution is exemplified by current pulse induced nonvolatile magnetization switching, which is realized with a current density of 2.5×10^{10}  A m^{-2}, showing its potential as a spintronic material.

Modulation of effective damping constant using spin Hall effect

We have investigated modulation of the effective damping constant αeff via spin currents through the spin Hall effect for Permalloy/Pt bilayer films with various thicknesses. The observed linear and sinusoidal dependences of current density and field direction on αeff are in agreement with the analytical model. By comparing the thickness dependence of spin Hall angle obtained from the damping modulation with that previously obtained by spin-torque-induced ferromagnetic resonance, we show that there is no clear extrinsic contribution in the present method. We also show the large modulation of the effective damping constant (down to ∼20%) in the high-current-density region.

Magnetochiral nonreciprocity of volume spin wave propagation in chiral-lattice ferromagnets

Spin current, i.e. the flow of spin angular momentum or magnetic moment, has recently attracted much attention as the promising alternative for charge current with better energy efficiency. Genuine spin current is generally carried by the spin wave (propagating spin precession) in insulating ferromagnets, and should hold the chiral symmetry when it propagates along the spin direction. Here, we experimentally demonstrate that such a spin wave spin current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice ferromagnet. This phenomenon originates from the interference of chirality between the SWSC and crystal-lattice, which is mediated by the relativistic spin-orbit interaction. The present finding enables the design of perfect spin current diode, and highlights the importance of the chiral aspect in SWSC.