Magnon Spectrum of the Amorphous Ferromagnet Co$_4$P from Atomistic Spin Dynamics

TL;DR

This study uses advanced simulation techniques to analyze the magnon spectrum of amorphous ferromagnet Co$_4$P, revealing two magnon valleys and suggesting residual order causes observed anomalies, with implications for spintronics.

Contribution

It introduces a combined atomistic simulation approach to re-investigate magnon excitations in amorphous magnets, providing new insights into their spectral features.

Findings

Identified two magnon valleys at the origin and finite wavenumber.

Found no magnon gap in the spectrum.

Attributed the second dip to Umklapp scattering from residual order.

Abstract

An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P, often referred to as a `roton-like' excitation, attracted much attention half a century ago. With the current interest in heat and spin currents in amorphous magnets, we apply modern simulation methods, combining reverse Monte Carlo to build the atomic structure and the stochastic Landau-Lifshitz equation for spin dynamics, to re-investigate the magnetic excitation spectrum. We find two magnon valleys, one at the origin and another at a finite wavenumber close to the observations, but without a magnon gap. We conclude that the second dip is due to Umklapp scattering caused by residual long-range order, which may be an alternative explanation of the putative roton excitation. Our study paves the way to study magnon transport in amorphous magnets and related spintronic applications.