Faraday rotation measurements of time dependent magnetic phenomena in insulating spin glasses (invited)

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Abstract
We report here on a detailed study of the relaxation of the magnetization in an external field and of the remanent magnetization for the insulating spin glass: Eu0.4Sr0.6S. The Faraday rotation has allowed us to extend previous experiments near and just above the spin glass temperature Tfo = 1.55 K on a large time scale (10−63 s) and to measure its susceptibility magneto‐optically. The temperature of the susceptibility cusp Tf(ν) varies as log ν for ν≳10 Hz and tends to reach the static value Tfo at lower frequencies. The time dependence of the thermoremanent magnetization (TRM) can generally be described by a power law TRM = At−a. The exponent a depends both on the temperature and on the initially applied field H, as recently found by Monte Carlo simulations for a 2d‐Ising spin glass. The unusual field dependence of the TRM, always found in SG systems, i.e., the maximum in TRM versus H, results from an increase of the demagnetization rate with H, which can compensate the large initial magnetization at high fields. The isothermal remanent magnetization is analyzed from our knowledge of the time and field dependence of the in‐field magnetization. The Tf(ν) variation and the TRM (H) behavior cannot be explained by a model of non interacting clusters and suggest a cooperative behavior, as for ferromagnets.