A General Approach to First Order Phase Transitions and the Anomalous Behavior of Coexisting Phases in the Magnetic Case

Abstract

First order phase transitions for materials with exotic properties are usually believed to happen at fixed values of the intensive parameters (such as pressure, temperature, etc.) characterizing their properties. It is also considered that the extensive properties of the phases (such as entropy, volume, etc.) have discontinuities at the transition point, but that for each phase the intensive parameters remain constant during the transition. These features are a hallmark for systems described by two thermodynamic degrees of freedom. In this work it is shown that first order phase transitions must be understood in the broader framework of thermodynamic systems described by three or more degrees of freedom. This means that the transitions occur along intervals of the intensive parameters, that the properties of the phases coexisting during the transition may show peculiar behaviors characteristic of each system, and that a generalized Clausius–Clapeyron equation must be obeyed. These features for the magnetic case are confirmed, and it is shown that experimental calorimetric data agree well with the magnetic Clausius–Clapeyron equation for MnAs. An estimate for the point in the temperature‐field plane where the first order magnetic transition turns to a second order one is obtained (the critical parameters) for MnAs and Gd5Ge2Si2 compounds. Anomalous behavior of the volumes of the coexisting phases during the magnetic first order transition is measured, and it is shown that the anomalies for the individual phases are hidden in the behavior of the global properties as the volume.

Materials Challenges for High Performance Magnetocaloric Refrigeration Devices

“…Recently, Gama et al argued that the fi rst-order transition in magnetic materials such as MnAs or Gd 5 Ge 2 Si 2 should be considered as governed not by two thermodynamic degrees of freedom (temperature and fi eld) but by three; then, according to the Gibbs phase rule, instead of a single point there will be a fi nite region of the phase diagram where two phases can coexist. [ 66 ] Thus, the width of the transition should not be understood as merely due to non-ideal effects such as disorder but as due to a third degree of freedom in the thermodynamic system. Although this degree of freedom is not explicitly identifi ed by Gama et al, they show that some experimental features of the transition have a natural explanation within this framework.…”

Magnetic property and magnetocaloric effect in TmCoAl compound

“…However, most of them are associated with a first-order magnetic or structural phase transition. The first-order phase transition is usually accompanied by a considerable thermal and magnetic hysteresis, which can greatly reduce the actual RC [11,12,13,14]. RC is a crucial parameter for evaluating the technological significance in a refrigerant material.…”

The isothermal variation of the entropy (ΔST) may be miscalculated from magnetization isotherms in some cases: MnAs and Gd5Ge2Si2 compounds as examples

“…For irreversible (hysteretic) processes, these relations are approximations and the validity of these approximations depends on the free energy difference among metastable and stable states. For MnAs compound, for instance, the calculated free energy presents low relative differences among metastable and stable states [7,17]. In a few cases, S T obtained from M vs. H data may be a reasonable approximation, such as the results in Fig.…”

“…The polycrystalline sample of Gd5Ge2Si2 is the same that was used in the Ref. [17]. Magnetic measurements have been performed in a SQUID magnetometer (Quantum Design).…”