Simulating Spin Wave Noise in Synthetic Antiferromagnetic Textures

Abstract

Synthetic antiferromagnets (SAFs) comprise coupled ultrathin ferromagnetic films with a variety of ground states that can be stabilized at room temperature by tuning the magnetic parameters such as the effective anisotropy, interlayer exchange, and Dzyaloshinskii-Moriya interactions. As a result, nonuniform spin textures such as domain walls, spin spirals, and skyrmions can be stabilized under zero applied fields [1], which can be detected using quantum spin sensing [2]. Here, we present results of micromagnetics simulations in which we examined the spin wave dispersion associated with localized modes of domain walls, spin spirals, and skyrmions in such SAF structures. For domain walls and spin spirals, the combination of dipole-dipole and Dzyaloshinskii-Moriya interaction results in spectra with frequency-wave vector nonreciprocity (Fig. 1), which results from asymmetries in the micromagnetic ground state. Skyrmion breathing modes are found to be in the low GHz range, which differs from their counterparts in confined ferromagnetic systems. We also discuss a numerical spectroscopic technique to estimate the magnetic noise at the electron spin resonance frequency for nitrogen-vacancy center measurements, which relies on estimating the magnetic susceptibility by computing the transient response to driving fields that are harmonic in time but random in space. Good agreement is found with recent experimental results [2]. This work was supported by the Agence Nationale de la Recherche under contract no. ANR-17-CE24-0025 (TOPSKY).