Millisecond Dynamics of the Magnetocaloric Effect in a First‐ and Second‐Order Phase Transition Material

Abstract

AbstractThe millisecond‐dynamics of the magnetocaloric effect in Gd and La‐Fe‐Si‐Mn, which exhibit first‐ and second‐order phase‐transitions, respectively, are investigated. Direct measurements of the adiabatic temperature change ΔT are obtained from modulation infrared thermometry with field‐cycling frequencies exceeding 1 kHz at amplitudes of up to 45 mT. The peak amplitude of ΔT(T) shows a dependence on sample thickness and decreases with increasing modulation frequency for both materials despite a frequency independent susceptibility of Gd. The adiabatic ΔT depends quadratically on the external field for Gd while La−Fe−Si−Mn shows a peculiar bucket‐shaped curve for temperatures below the peak maximum. A comparative study of non‐caloric samples shows that dissipative heating by eddy currents or magnetic hysteresis does not explain the observed behavior. The transient ΔT(t) instead suggests a mechanism involving strong temperature gradients at the ferromagnetic–paramagnetic boundaries and underlines the importance of further dynamical studies for a fundamental understanding of the magnetocaloric effect in first‐order materials.