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

• Published in: Physical review. B 2017-03, Vol.95 (10), p.104422, Article 104422
• Main Authors: Morley, S. A., Alba Venero, D., Porro, J. M., Riley, S. T., Stein, A., Steadman, P., Stamps, R. L., Langridge, S., Marrows, C. H.
• Format: Article
• Language: English
• Subjects: Anisotropy; Center for Functional Nanomaterials; Correlation; Crossovers; Freezing; Ice formation; Magnetic islands; Magnetic measurement; NANOSCIENCE AND NANOTECHNOLOGY; Photon correlation spectroscopy; Soft x rays; Spectrum analysis; Spin ice; Variation; X ray spectra
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.95.104422

We report on the crossover from the thermal to the athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30 nm × 70 nm, is small enough that they are dynamic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy 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 of T0=178±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, TA=40±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 upon freezing, which is unexpected for a system with a well-defined ground state.