Power spectrum of magnetic relaxation in spin ice: Anomalous diffusion in a Coulomb fluid

Magnetization noise measurements on the spin ice Dy2Ti2O7 have revealed a remarkable “pink noise” power spectrum S(f,T) below 4 K, including evidence of magnetic monopole excitations diffusing in a fractal landscape. However, at higher temperatures, the reported values of the anomalous exponent b(T) describing the high-frequency tail of S(f,T) are not easy to reconcile with other results in the literature, which generally suggest significantly smaller deviations from the Brownian motion value of b=2, that become negligible above T=20 K. We accurately estimate b(T) at temperatures between 2 and 20 K, using ac susceptibility measurements that, crucially, stretch up to the relatively high frequency of f=106 Hz. We show that previous noise measurements underestimate b(T) and we suggest reasons for this. Our results establish deviations in b(T) from b=2 up to about 20 K. However, we confirm that b(T) is sample dependent: The details of this dependence agree in part, though not completely, with previous studies of the effect of crystal defects on monopole population and diffusion. Our results establish the form of b(T) which characterizes the subtle, and evolving, nature of monopole diffusion in the dense Coulomb fluid, a highly correlated state, where several dynamical processes combine.