Peculiarities of the Magnetocaloric Effect upon Metamagnetic Transition in Anisotropic Antiferromagnets

The magnetic entropy change upon transition from ferromagnetism to antiferromagnetism during isothermal magnetization is studied in terms of a two-sublattice Ising antiferromagnet model. Such a transition is shown to be both second- and first-order (metamagnetic) one, and the existence of sufficiently high intra-sublattice ferromagnetic exchange is necessary for arising the metamagnetic transition. The common peculiarity of the two variants of transition is found to be the increase in the magnetic entropy of antiferromagnetic phase of the system with increasing magnetic field (inverse or abnormal magnetocaloric effect). The cause for such an anomaly is shown to be the substantial prevalence of magnetic-order disordering effect in the magnetic sublattice under the action of magnetic field, the magnetization of which is directed against the magnetic field direction, over the ordering effect of the sublattice, the magnetization of which is directed along the field direction.