What is the Magnetocaloric Effect (Magnetic Refrigeration)?

An unmagnetized material (that is, its magnetic dipoles are randomly distributed) has a temperature \(T\). It is now adiabatically placed in an external magnetic field \(H\) (\(H = \class{violet}{B}/\mu_0\)). Adiabatic means that there is no heat exchange with the environment and the total entropy of the material therefore remains constant. The external magnetic field aligns (orders) the magnetic dipoles of the material, so the total entropy should decrease, but it cannot because the system is thermally insulated, so the proportion of entropy attributable to the magnetic dipoles decreases and the proportion of thermal entropy associated with the temperature increases. Therefore, the temperature of the material increases: \(T + \Delta T\). This process is called adiabatic magnetization (analogous to adiabatic compression, where pressure is used instead of the magnetic field).

The material is now brought into contact with the environment, which has the temperature \(T\). The material then releases the additional heat \(Q\) to the environment and the material cools down again to the ambient temperature \(T\). In order to achieve cooling, the magnetic field is switched off (without heat exchange with the environment). This causes the dipoles to become disordered again and the magnetic entropy increases. In order for the total entropy to remain constant, the thermal entropy must decrease. This leads to a reduction in the material temperature to \(T - \Delta T\). In this way, magnetic refrigeration of the material is achieved. The process can be repeated when the material is brought back into contact with the ambient temperature \(T\) and then absorbs the heat \(Q\).

The phenomenon itself, in which the magnetic field has an influence on the temperature of the material, is called the magnetocaloric effect.