Ferromagnetic monolayer with interfacial Dzyaloshinskii-Moriya interaction: magnon spectrum and effect of quenched disorder

TL;DR

This paper theoretically investigates a ferromagnetic monolayer with interfacial Dzyaloshinskii-Moriya interaction, revealing anisotropic magnon spectra, the impact of quenched disorder, and the destruction of long-range order by defects.

Contribution

It introduces a detailed analysis of magnon spectra and disorder effects in ferromagnetic monolayers with iDMI, highlighting the quasi-1D spectrum and defect-induced short-range order.

Findings

Magnon spectrum is anisotropic with linear and quadratic dispersion.

Quenched disorder destroys long-range magnetic order, leading to short-range correlations.

Defects induce a power-law dependence of correlation length on defect concentration.

Abstract

We discuss theoretically a ferromagnetic monolayer with an interfacial Dzyaloshinskii-Moriya interaction (iDMI) and a small axial anisotropy. It is shown that the system has a long-period cycloid magnetic order slightly distorted by the anisotropy whose modulation vector $\bf k$ can have several orientations. We find that due to iDMI-induced umklapp terms in the Hamiltonian, the spectrum of long-wavelength magnons is essentially anisotropic: it is linear and quadratic for momenta directed along and perpendicular to $\bf k$, respectively. Due to such a quasi-1D spectrum, the temperature correction to the mean spin value has a power-law singularity hampering the magnetic ordering at $T\ne0$. We demonstrate that the umklapps lead to a peculiar band structure of the magnon spectrum similar to electronic bands in solids. We discuss also the effect of vacancies and defect bonds on system properties at $T=0$. Owing to the quasi-1D character of the spectrum, the distortion of the cycloid structure by a single defect bond is described by the field of 1D electric dipole. As a result, even infinitesimal concentrations of defects $c$ destroy the long-range order and establish a short-range order whose correlation length shows a power-law dependence on $1/c$. Our findings should also be applicable to ultra-thin films with strong enough iDMI.