Heterogeneous nucleation and heat flux avalanches in La(Fe,Si)$_{13}$ magnetocaloric compounds near the critical point

TL;DR

This study investigates the phase transformation kinetics of a magnetocaloric compound near its critical point, revealing heat flux avalanches, their smoothing near the critical point, and the influence of magnetic field and disorder.

Contribution

It provides detailed calorimetry and magneto-resistivity measurements of phase transition dynamics in La(Fe,Si) compounds, highlighting the role of heterogeneity and critical phenomena.

Findings

Multiple heat flux avalanches during phase transitions.

Disappearance of avalanches near the critical point.

Magnetic field influences avalanche behavior.

Abstract

The phase transformation kinetics of LaFe$_{11.41}$Mn$_{0.30}$Si$_{1.29}$-H$_{1.65}$ magnetocaloric compound is addressed by low rate calorimetry experiments. Scans at 1 mK/s show that its first order phase transitions are made by multiple heat fllux avalanches. Getting very close to the critical point, the step-like discontinuous behavior associated with avalanches is smoothed out and thermal hysteresis disappears. This result is confirmed by magneto-resistivity measurements and allows to measure accurate values of the zero field hysteresis ($\Delta T_{hyst}$ = 0.37 K) and of the critical field (H$_c$ = 1.19 T). The number and magnitude of heat flux avalanches change with magnetic field, showing the interplay between the intrinsic energy barrier between phases and the microstructural disorder of the sample.