MMMEHA

Magnetocaloric effect occurs when a materials undergoes significant change of its magnetic state as a function of temperature. For instance, the ferromagnetic – paramagnetic transition at Curie temperature (T_C) of a ferromagnetic material. Particularly, a significant change of magnetic entropy takes place since the system goes from a highly magnetic ordered phase (ferromagnetic state where all magnetic moments are parallel aligned) to a magnetically disordered phase where all magnetic moments are misaligned due the thermal agitation.   If simultaneously with magnetic transition, a structural transition occurs then the total change of entropy ΔS_T is even higher because the latent heat (structural component of entropy, ΔS_S) is added to the magnetic one (ΔS_M) [2]. When the structural transition is coupled to the magnetic state of the material, then it can be manipulated through an external magnetic field, for instance it can be shifted at higher or lower temperatures. Therefore, a magnetic thermodynamic cycle can be realized through adiabatic magnetization and demagnetization of a magnetocaloric material. The demagnetization-magnetization process is analogous to compression-expansion cycle of a conventional cooling machine (Figure 1).       

            Actually, when a magnetocaloric material is at thermal equilibrium with its surroundings, the increase in the external magnetic field in an adiabatic process results in an increase in the temperature of the material, because the magnetic component of the entropy (ΔS_M) decreases and it leads to an increase of lattice (structural) entropy (ΔS_S) to remain constant the total entropy (ΔS_T). Since the magnetocaloric material is at higher temperature, it can expel the extra heat with its surroundings (cold reservoir) through a heat-transfer fluid (non-adiabatic isofield process). After the heat transfer process, the external magnetic field is switched off (adiabatic demagnetization) leading to the drop of the temperature of the magnetocaloric material below the temperature of the surroundings (hot reservoir), it is this cooling effect that can be used for refrigeration purposes (2^nd non-adiabatic isofield process).

[2] Caloric materials near ferroic phase transitions Nature Materials v13, 439