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.
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Use of the Center for Nanoscale Materials and Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. Work at the University of Chicago was supported by the National Science Foundation, while that in London was funded by a Royal Society Wolfson Research Merit Award and the Basic Technologies programme of RCUK.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains Supplementary Methods with detailed description of experimental x-ray scattering geometry, sample preparation, data collection and data analysis and Supplementary Figures S1-S3 with Legends. (PDF 298 kb)
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Shpyrko, O., Isaacs, E., Logan, J. et al. Direct measurement of antiferromagnetic domain fluctuations. Nature 447, 68–71 (2007). https://doi.org/10.1038/nature05776
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