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Abstract: Among the exotic magnetic states which emerge from frustrated spin systems, spin ices have aroused strong interest. In particular, their excitations being described as magnetic charges, the so-called magnetic monopoles, govern the system dynamics. Understanding monopole dynamics in spin ices remains a central challenge of frustrated magnetism, and their low temperature, out-of-equilibrium properties are still poorly understood. Recently, we developed a new experimental setup which allows us to directly measure the fluctuation-dissipation relation (FDR), which provides an unambiguous test for out-of-equilibrium behavior in non-equilibrium statistical mechanics.
In this talk I will discuss our results on the dynamics of the spin ice compounds Dy2Ti2O7 and Ho2Ti2O7 down to 150 mK, and address both the equilibrium and out-of-equilibrium regimes. In the two systems, the FDR is satisfied down to 400 mK across the entire measured frequency range (from 0.001 Hz to 10^5 Hz), indicating that local equilibrium is achieved. Below 400 mK, we show that violations to the FDR develop as an excess of noise at frequencies below 0.1 Hz, while at higher frequencies, the FDR remains valid down to 150 mK. Such deviations indicate the presence of several timescales in the system, which may be the signature of the lifting of the degeneracy of the spin ice Pauling states due to dipolar interactions.