Direct measurement of antiferromagnetic domain fluctuations

TL;DR

This paper reports the first direct measurement of antiferromagnetic domain fluctuations at nanometer scales using X-ray Photon Correlation Spectroscopy, revealing temperature-dependent quantum and thermal domain wall dynamics in elemental Chromium.

Contribution

It introduces a novel measurement technique for antiferromagnetic domain dynamics and provides new insights into spin fluctuations at nanometer scales.

Findings

Thermally activated domain wall motion above 100K.

Saturation of domain wall motion rate below 40K, indicating quantum fluctuations.

First direct observation of antiferromagnetic domain fluctuations at nanometer scales.

Abstract

Measurements of magnetic noise emanating from ferromagnets due to domain motion were first carried out nearly 100 years ago and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise, but this must be sampled at spatial wavelengths of order several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present the first direct measurement of the fluctuations in the nanometre-scale spin- (charge-) density wave superstructure associated with antiferromagnetism in elemental Chromium. The technique used is X-ray Photon Correlation Spectroscopy, where coherent x-ray diffraction produces a speckle pattern that serves as a "fingerprint" of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micron distances. While the domain wall motion is thermally activated at temperatures above 100K, it is not so at lower temperatures, and indeed has a rate which saturates at a finite value - consistent with quantum fluctuations - on cooling below 40K. Our work is important because it provides an important new measurement tool for antiferromagnetic domain engineering as well as revealing a fundamental new fact about spin dynamics in the simplest antiferromagnet.