Magnetic and Magnetocaloric Effects in Systems with Reverse First-Order Transitions

Reverse first-order magnetostructural phase transitions have been theoretically analyzed within the model of interacting parameters of magnetic and structural orders. A characteristic feature of these transitions is stepwise occurrence of the magnetic order upon cooling (as in the case of the first-order phase transition) and its smooth disappearance upon heating (as in the conventional second-order phase transition). These transitions are observed in some alloys of the Mn 1 – x Cr x NiGe magnetocaloric systems under pressure ( x = 0.11) and without it ( x = 0.18) and are accompanied by specific magnetic and magnetocaloric features. These specific features are phenomenologically described within the concept of soft mode for a structural subsystem undergoing the first-order structural phase transition ( P 6 3 / mmc – Pnma ) and the Heisenberg model for a spin subsystem. It is shown for systems with magnetostructural instability within the molecular-field approximation for a spin subsystem and the approximation of biased harmonic oscillator for a lattice subsystem that reverse phase transitions occur when the magnetic disordering temperature is in the range of temperature hysteresis of the P 6 3 / mmc – Pnma first-order structural phase transition. It is also shown that the two-peak form of isothermal entropy (characteristic of reverse transitions) is due to separation of contributions from the structural and magnetic entropies.