The magnetocaloric effect (MCE) in a ferromagnetic material F is the heat emanated when a magnetic field H is applied. From a thermodynamic point of view, the application of H to F makes it unstable, creating the condition for its rearrangement that brings spin directions S closer to H. The only exception is when H is applied along S. If the rearrangement actually occurs, the corresponding energy gain is exothermic MCE. In certain cases the subsequent reduction in the H strength leads to endothermic MCE - the phenomenon allowing the creation of magnetic refrigeration technique.  

The actual physics of MCE was only superficially understood due to misinterpretation of solid-state phase transitions in general, ferromagnetic ones in particular.  Thus, endothermic MCE was explained in terms of "critical phenomena" notions as a result of "randomization of domains similar to the randomization at Curie temperature". At the same time, hysteresis (nucleation lags in a nucleation-and-growth process) was cited as one of the problems in the magnetic refrigeration technique.  

The basics of MCE are interpreted here in terms of the crystal growth concept. Let us analyze the application of H to a single ferromagnetic domain or a polydomain "single crystal". The material becomes simply an unstable crystal phase. Its rearrangement in magnetic field H is considered in Sec. 4.7 to 4.13. We deal with a structural rearrangement by nucleation-and-growth rather than rotation S toward H. The resultant crystal has the same crystal structure, but a new spatial orientation in which S directions are closer to H. The difference between the two free energies manifests itself as exothermic MCE. Subsequent H decrease to zero does not produce MCE.

No meaningful endothermic MCE is possible until phase transition FERROMAGNETIC-PARAMAGNETIC (F-P) comes into play. Even though it is called a "magnetic" phase transition, it proceeds by nucleation-and-growth. Paramagnetic phase is an orientation-disordered crystal phase (ODC) where arbitrary orientation of spins results from thermal rotation of their carriers. Application of H at temperatures not too far from T_o shifts T_o upward in favor of F with S preferably directed along H, triggering P ® F nucleation-and-growth phase transition. Its exothermic latent heat is MCE, Turning H off makes the paramagnetic phase preferable again, giving rise to the reverse phase transition F ® P; its latent heat manifests itself as the endothermic MCE.  

Conclusion: Every ferromagnetic can exhibit magnetocaloric effect. If a large effect is observed, it most probably involves the latent heat of the ferromagnetic-paramagnetic phase transition.