Magnetocaloric Effect

a change in the temperature of a magnetic substance upon an adiabatic change in the intensity H of the magnetic field in which the substance is located.

Energy of magnetization δ = JdH (where J is the magnetization) is achieved as the field changes by dH. According to the first law of thermodynamics, δA = δQ — dU, where δQ is the amount of heat imparted to a magnetic substance (equal to zero under adiabatic conditions) and dU is the change in the internal energy of the substance. Thus, when δQ = 0, work is accomplished only as a result of change in the internal energy (δA = —dU), which leads to a change in the temperature of the magnetic substance if its internal energy is dependent on temperature T. In paramagnetic and ferromagnetic substances, the magnetization J increases with H, that is, there is an increase in the number of atomic magnetic moments (spin or orbital) parallel to H. As a result, there is a reduction in the energy of paramagnets and ferromagnets with respect to the field and in their internal energy of exchange reaction. On the other hand, the internal energy of paramagnets and ferromagnets increases with T. Therefore, according to the Le Chatelier-Brown principle, heating of paramagnets and ferromagnets should take place on magnetization. For ferromagnets, this effect is at a maximum near the Curie point; for paramagnets, the magnetocaloric effect in-creases with a decrease in temperature. Upon an adiabatic reduction of the field, there is partial or total destruction (the latter when the field is shut off) of the ordered orientation of the moments at the expense of the internal energy. This leads to the cooling of the magnetic substance.