Shpyrko O G, Isaacs E D, Logan J M, Feng Yejun, Aeppli G, Jaramillo R, Kim H C, Rosenbaum T F, Zschack P, Sprung M, Narayanan S, Sandy A R
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA.
Nature. 2007 May 3;447(7140):68-71. doi: 10.1038/nature05776.
Measurements of magnetic noise emanating from ferromagnets owing 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. 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.
近100年前，人们首次对铁磁体因磁畴运动而产生的磁噪声进行了测量，这些测量为许多科学技术奠定了基础。反铁磁体虽然不携带净外部磁偶极矩，但具有在宏观距离上延伸的电子自旋的周期性排列，也应该会表现出磁噪声。然而，这种噪声必须在几个原子间距量级的空间波长上进行采样，而不是像铁磁体那样在宏观尺度上采样。在这里，我们展示了对与元素铬中的反铁磁性相关的自旋和电荷密度波的纳米级超结构波动的直接测量。所使用的技术是X射线光子相关光谱学，其中相干X射线衍射产生一个散斑图案，作为特定磁畴构型的“指纹”。图案的时间演化对应于磁畴壁在微米距离上的前进和后退。这项工作展示了一种用于反铁磁畴壁工程的有用测量工具，但也揭示了关于最简单反铁磁体中自旋动力学的一个基本发现：尽管磁畴壁运动在100 K以上的温度下是热激活的，但在较低温度下并非如此，实际上在冷却至40 K以下时，其速率会饱和到一个与量子涨落一致的有限值。
