arXiv subjects

Koichi Mizushima

Publications and source records attributed to Koichi Mizushima.

5 recordsLinked to original sources

arXiv2019

Layer-selective detection of magnetization directions from two layers of antiferromagnetically-coupled magnetizations by ferromagnetic resonance using a spin-torque oscillator

We use micromagnetic simulation to demonstrate layer-selective detection of magnetization directions from magnetic dots having two recording layers by using a spin-torque oscillator (STO) as a read device. This method is based on ferromagnetic resonance (FMR) excitation of recording-layer magnetizations by the microwave field from the STO. The FMR excitation affects the oscillation of the STO, which is utilized to sense the magnetization states in a recording layer. The recording layers are designed to have different FMR frequencies so that the FMR excitation is selectively induced by tuning the oscillation frequency of the STO. Since all magnetic layers interact with each other through dipolar fields, unnecessary interlayer interferences can occur, which are suppressed by designing magnetic properties of the layers. We move the STO over the magnetic dots, which models a read head moving over recording media, and show that changes in the STO oscillation occur on the one-nanosecond timescale.

physics.app-ph↗

arXiv2019

Reservoir Computing on Spin-Torque Oscillator Array

We numerically study reservoir computing on a spin-torque oscillator (STO) array, describing the magnetization dynamics of the STO array by a nonlinear oscillator model. The STOs exhibit synchronized oscillation due to coupling by magnetic dipolar fields. We show that reservoir computing can be performed using the synchronized oscillation state. The performance can be improved by increasing the number of STOs. The performance becomes highest at the boundary between the synchronized and disordered states. Using an STO array, we can achieve higher performance than that of an echo-state network with similar number of units. This result indicates that STO arrays are promising for hardware implementation of reservoir computing.

cond-mat.mes-hall↗

arXiv2009

Measurement of nonlinear frequency shift coefficient in spin-torque oscillators based on MgO tunnel junctions

The nonlinear frequency shift coefficient, which represents the strength of the transformation of amplitude fluctuations into phase fluctuations of an oscillator, is measured for MgO-based spin-torque oscillators by analyzing the current dependence of the power spectrum. We have observed that linewidth against inverse normalized power plots show linear behavior below and above the oscillation threshold as predicted by the analytical theories for spin-torque oscillators. The magnitude of the coefficient is determined from the ratio of the linear slopes. Small magnitude of the coefficient has been obtained for the device exhibiting narrow linewidth at high bias current.

cond-mat.mtrl-sci↗

arXiv2008

Amplitude-Phase Coupling in a Spin-Torque Nano-Oscillator

The spin-torque nano-oscillator in the presence of thermal fluctuation is described by the normal form of the Hopf bifurcation with an additive white noise. By the application of the reduction method, the amplitude-phase coupling factor, which has a significant effect on the power spectrum of the spin-torque nano-oscillator, is calculated from the Landau-Lifshitz-Gilbert-Slonczewski equation with the nonlinear Gilbert damping. The amplitude-phase coupling factor exhibits a large variation depending on in-plane anisotropy under the practical external fields.

cond-mat.mtrl-sci↗

arXiv2005

Synchronized Magnetization Oscillations in F/N/F Nanopillars

Current-induced magnetization dynamics in a trilayer structure composed of two ferromagnetic free layers and a nonmagnetic spacer is examined. Both free layers are treated as a monodomain magnetic body with an uniform agnetization. The dynamics of the two magnetizations is modeled by modified Landau-Lifshitz-Gilbert equations with spin-transfer torque terms. By solving the equations simultaneously, we discuss their various solutions in detail. We show that there exists the synchronous motion of two magnetizations among the various solutions; the magnetizations are resonantly coupled via spin-transfer torques and perform precessional motions with the same period. The condition to excite the synchronous motion depends on the difference between the intrinsic frequencies of the two ferromagnetic free layers as well as the magnitude of current.

cond-mat.mtrl-sci↗