Magnetic monopole relaxation effects probed by modulation calorimetry in small spin-ice samples

Abstract

We use modulation calorimetry to study the heat capacity of small samples (30 ng − 10 µg) of the classical spin-ice compounds Dy2Ti2O7 and Ho2Ti2O7 at low temperature (0.5 − 3 K). Using modulation frequencies of 0.1-200 Hz we find a strong frequency dependence in the measured heat capacity and are able to study thermal relaxation effects on the corresponding timescales. Performing dynamic Monte Carlo simulations we verify that the specific heat frequency response has its origin in the slow magnetic monopole dynamics indigenous to spin ice. We find a timescale of 20 ms per Monte Carlo step at 4 K in contrast to 2.5 ms mentioned in previous studies by other techniques 1 . Our study establishes modulation calorimetry as a versatile experimental probe of dynamic processes in frustrated magnetic materials.

Dynamical fractal and anomalous noise in a clean magnetic crystal

“…Increasing the energy in the Arrhenius law to D t > D m is largely precluded by basic statistical mechanics, whereas estimates suggest that a D t in excess of twice D m is actually required to fit the experimental growth of the relaxation time (25). Previous theories of the steep rise of the relaxation time upon cooling invoked extrinsic contributions caused by open boundary effects, disorder, and an autonomously temperaturedependent microscopic time scale (21,22,(28)(29)(30)(31); the identification of an intrinsic mechanism leading to a parametrically faster growth of the relaxation time compared with 1/r has been lacking.…”

Dynamical fractal and anomalous noise in a clean magnetic crystal

“…In-creasing the energy in the Arrhenius law to ∆ τ > ∆ m is largely precluded by basic statistical mechanics, whereas estimates suggest that a ∆ τ in excess of twice ∆ m is actually required to fit the experimental growth of the relaxation time [29]. Previous theories of the steep rise of the relaxation time upon cooling invoked extrinsic contributions due to open boundary effects, disorder, and an autonomously temperature-dependent microscopic time scale [26,27,[32][33][34][35]; the identification of an intrinsic mechanism leading to a parametrically faster growth of Frequency (Hz) the relaxation time than 1/ρ has been lacking.…”