Ultraslow calorimetric studies of the martensitic transformation of NiFeGa alloys: detection and analysis of avalanche phenomena

Abstract

We study the thermal properties of a bulk Ni55 Fe19 Ga26 Heusler alloy in a conduction calorimeter. At slow heating and cooling rates (∼ 1 K/h ), we compare as-cast and annealed samples. We report a smaller thermal hysteresis after the thermal treatment due to the stabilization of the 14M modulated structure in the martensite phase. In ultraslow experiments (40 mK/h), we detect and analyze the calorimetric avalanches associated with the direct and reverse martensitic transformation from cubic to 14M phase. This reveals a distribution of events characterized by a power law with exponential cutoff p(u) ∝ u−ε exp(−u/ξ) where ε ∼ 2 and damping energies ξ = 370 µJ (direct) and ξ = 27 µJ (reverse) that characterize the asymmetry of the transformation.