arXiv subjects

Nobuaki Kikuchi

Publications and source records attributed to Nobuaki Kikuchi.

2 recordsLinked to original sources

arXiv2025

Magnetic Permeability Time-varying Metamaterials at Microwave Frequencies

We demonstrate magnetic permeability time-varying metamaterials at GHz frequencies using ferromagnetic permalloy (Ni80Fe20; Py). We observe frequency up and down conversion of 4 GHz microwaves through the metamaterials, which is caused by the temporal modulation of permeability in the Py layer. Moreover, the efficiency of the up-conversion to a higher frequency is much larger than that of the down conversion to a lower frequency. These experimental results are reproduced well via numerical calculation, verifying that the significant up-conversion efficiency is traced back to nonlinear magnetization dynamics in the metamaterials. The present study opens a door to microwave sources toward the 6th-generation mobile communication system, four-dimensional metamaterials with spatio-temporal modulation, and nonlinear spintronics.

physics.optics↗

arXiv2024

Direct observation of current-induced nonlinear spin torque in Pt-Py bilayers

We experimentally observe nonlinear spin torque in metallic bilayers of platinum and permalloy by means of spin-torque ferromagnetic-resonance (ST-FMR) under massive dc current injection. The observed nonlinear spin torque exerted to permalloy magnetization is attributed primarily to nonlinear spin polarization. Additional origin of the nonlinear spin torque is magnon generation (annihilation) followed by shrinkage (expansion) of effective magnetization, which is reveled by STFMR and unidirectional spin Hall magnetoresistance measurements. The present study paves a way to spin-Hall effect based nonlinear spintronic devices as well as time-varying nonlinear magnetic metamaterials with tailor-made permeability.

cond-mat.mes-hall↗