Direct measurement of antiferromagnetic domain fluctuations

Abstract

Ferromagnets are everywhere, but ferromagnetism itself is a rare property. The more subtle cousins, antiferromagnets, are more common, but have been recognized for less than 100 years and have only become technologically relevant in the past 20 years. One reason for this is the unavailability of the analogues of ferromagnetic domains — the bar magnets that a larger ferromagnet divides into. Using a new technique, X-ray photon correlation spectroscopy, it is now possible to examine the nanometre-scale superstructure of spin- and charge-density in the antiferromagnet chromium, and the results could lead to magnetic engineering techniques that bring antiferromagnets into wider use. The new technique shows that the antiferromagnetic domain walls are in fact never at rest, but are constantly advancing and retreating over micrometre distances. And though domain wall motion is thermally activated at temperatures above 100K, it is not so at lower temperatures, and on cooling below 40K the motion saturates at a finite value consistent with quantum fluctuations. A new X-ray-based technique, X-ray photon correlation spectroscopy, is developed to directly measure magnetic noise. Its use is demonstrated for elemental chromium, an antiferromagnet that displays a nanometre-scale superstructure of spin- and charge-density. Fingerprints of a particular magnetic domain configuration are obtained, and after the temporal evolution of the patterns, magnetic domain walls advancing and retreating over micron distances are observed. Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago1, and have underpinned much science and technology2,3. 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. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves 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 micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value—consistent with quantum fluctuations—on cooling below 40 K.