Direct measurement of antiferromagnetic domain fluctuations
Direct measurement of antiferromagnetic domain fluctuations
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DOI：
10.1038/nature05776
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发表时间：
2007-05-03
期刊：
影响因子：
64.8
通讯作者：
Sandy, A. R.
中科院分区：
文献类型：
--
作者：
Shpyrko, O. G.;Isaacs, E. D.;Sandy, A. R.
近100年前，首次对铁磁体因磁畴运动而产生的磁噪声进行了测量（1），并为许多科学技术提供了基础（2，3）。反铁磁体，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，也应该显示磁噪声。然而，这必须在几个原子间间距的空间波长的顺序，而不是铁磁体的宏观尺度特征进行采样。在这里，我们提出了一个直接测量的自旋和电荷密度波与元素铬的反铁磁性的纳米级超结构的波动。所使用的技术是X射线光子相关光谱学，其中相干X射线衍射产生散斑图案，作为特定磁畴配置的“指纹”。图案的时间演变对应于畴壁在微米距离上的前进和后退。这项工作为反铁磁畴壁工程提供了一个有用的测量工具，但也揭示了最简单的反铁磁体中关于自旋动力学的基本发现：尽管畴壁运动在100 K以上的温度下被热激活，但在较低的温度下并非如此，并且确实具有在40 K以下冷却时在有限值处饱和的速率-与量子涨落一致。
Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago(1), and have underpinned much science and technology(2,3). 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.