An optimized magneto-caloric material can be seen as an extremely efficient converter for energy from the spin sector (magnetization, magnetic field) to phonons (thermal energy) and vice versa. Due to the microscopic quantum nature of the spin system in a solid-state material, and its coupling to the lattice, this energy transfer possesses an inherently high efficiency.
Modern magnetic refrigerant-materials for room-temperature applications exhibit large magnetocaloric effects (MCE) in conjunction with a magnetic phase-transition of first order.
Hexagonal MnFe(P,X) (X = As, Ge, Si) compounds exhibits a novel form of magnetism: the coexistence of strong and weak magnetism in alternate atomic layers. In the Fe layers formation of covalent bonds competes with moment formation, resulting in disappearance of local magnetic moments at the Curie temperature. This is responsible for a large thermal effect while the strong magnetism in adjacent Mn-layers ensures Curie temperatures high enough to enable operation around room temperature.