Vogel-Fulcher-Tammann freezing of a thermally fluctuating artificial spin ice probed by x-ray photon correlation spectroscopy

摘要

We report on the crossover from the thermal to the athermal regime of an artificial spin ice formed from asquare array of magnetic islands whose lateral size, 30 nm × 70 nm, is small enough that they are dynamic atroom temperature.We used resonant magnetic soft x-ray photon correlation spectroscopy as a method to observethe time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are foundto slow abruptly as a freezing temperature of T_0 = 178 ± 5 K is approached. This slowing is well describedby a Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the freezing temperature beingcommensurate with the strength of magnetostatic interaction energies in the array. The activation temperature,T_A = 40 ± 10 K, is much less than that expected from a Stoner-Wohlfarth coherent rotation model. Zerofield-cooled/field-cooled magnetometry reveals a freeing up of fluctuations of states within islands above thistemperature, caused by variation in the local anisotropy axes at the oxidised edges. This Vogel-Fulcher-Tammannbehavior implies that the system enters a glassy state upon freezing, which is unexpected for a system with awell-defined ground state.