X射线光子相关谱研究热涨落人工自旋冰的Vogel-Fulcher-Tammann冻结
Vogel-Fulcher-Tammann Freezing of a Thermally Fluctuating Artificial Spin Ice Probed by X-ray Photon Correlation Spectroscopy
摘要：
本文报道了由正方形排列的磁岛形成的人工自旋冰从热态到非热态的转变，磁岛的横向尺寸为30 nm~ $\times$ ~70 nm，它们足够小，在室温下是超顺磁性的。我们用共振磁软X射线光子相关谱（XPCS）观测了自旋冰在冷却过程中磁位形的时-时相关性，发现当接近冻结温度$T_0 = 178 \pm 5$ ~K时，自旋冰的磁位形的涨落突然变慢。这种减慢被Vogel-Fulcher-Tammann定律很好地描述，这意味着冷冻状态是玻璃状的，冷冻温度与阵列中静磁相互作用能的强度相称。的活化温度，$T_\mathrm{A} = 40 \pm 10$ ~K，是远远低于预期的Stoner-Wohlfarth相干旋转模型。零场冷却/场冷却磁强计揭示了一个释放的波动状态在这个温度以上的岛屿，在氧化边缘的局部各向异性轴的变化所造成的。这种Vogel-Fulcher-Tammann行为意味着该系统在冻结时进入玻璃态，这对于具有明确定义的基态的系统是意想不到的。
英文摘要：
We report on the crossover from the thermal to athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30~nm~$\times$~70~nm, is small enough that they are superparamagnetic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy (XPCS) as a method to observe the time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are found to slow abruptly as a freezing temperature $T_0 = 178 \pm 5$~K is approached. This slowing is well-described by a Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the freezing temperature being commensurate with the strength of magnetostatic interaction energies in the array. The activation temperature, $T_\mathrm{A} = 40 \pm 10$~K, is much less than that expected from a Stoner-Wohlfarth coherent rotation model. Zero-field-cooled/field-cooled magnetometry reveals a freeing up of fluctuations of states within islands above this temperature, caused by variation in the local anisotropy axes at the oxidised edges. This Vogel-Fulcher-Tammann behavior implies that the system enters a glassy state on freezing, which is unexpected for a system with a well-defined ground state.
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