Magnetocaloric

The magnetocaloric effect (MCE) is the change in temperature of a magnetic material when it is exposed to a changing magnetic field. In an adiabatic process, applying a magnetic field to such a material raises its temperature; removing the field lowers it. The effect is quantified by the adiabatic temperature change, ΔT_ad, and the isothermal entropy change, ΔS_m.

Mechanism: When a magnetic field aligns the magnetic moments in a material, magnetic entropy decreases. This change redistributes energy between magnetic and lattice degrees of freedom, and in an adiabatic process the temperature must adjust accordingly, while in an isothermal process heat is exchanged with a reservoir to maintain constant temperature.

Materials: Gadolinium shows a sizable MCE near room temperature and has historically been a key reference material. More pronounced or “giant” magnetocaloric effects are found in compounds that undergo first- or second-order magnetic transitions, such as Mn-Fe-P-As-C and La-Fe-Si-based systems. Performance is described by ΔS_m and ΔT_ad, and by the breadth of the temperature range over which the effect is large.

Applications: Magnetic refrigeration uses cycles of magnetization and demagnetization to transport heat, often with regenerators to improve efficiency. The approach can offer higher energy efficiency and eliminates the need for certain greenhouse-refrigerants, making it an area of active development for near-room-temperature cooling.

Challenges: Practical devices must address hysteresis losses, material fatigue under cyclic magnetic fields, the cost and supply of rare-earth elements, and the engineering of strong, efficient magnet assemblies. Scaling up to commercial systems remains an ongoing area of research.

Outlook: Ongoing research seeks new magnetocaloric compounds, composites, and system architectures, such as active magnetic regeneration, to improve performance, reduce costs, and enable broader adoption.