First-Order magnetic phase transitions

Abstract

The general properties of the first-order magnetic phase transitions between ordered structures have been investigated. The number of possible models has been limited by supposing the sensitivity of the exchange energy to the interatomic distance to be responsible for the phase transition. It is shown that the magnetic phase transition occurs as a result of a signchange in the proper combination of the exchange parameters. The transition may be influenced by the variation of the pressure and the magnetic field. The actual phase transition is, however, hindered in both directions by the elastic energy wall, separating the free energy minima of the different phases. TheT-P phase diagram has been calculated and the conditions for the existence of a triple point in theT-P plane have been investigated in the case of antiferromagnetic ⇆ ferromagnetic transitions. The temperature hysteresis, i.e. the difference between the upper (T sup) and lower (T inf) transition temperatures has been determined at different pressures and for both temperatures a linear dependence has been obtained. The character of the disappearance of the magnetic order has been also studied and it has been found that under certain conditions the order of the ferromagnetic-paramagnetic transition may be different from that of the antiferromagnetic-paramagnetic one. TheT-H coexistence curve between the ferromagnetic and the spin-flop antiferromagnetic phases has been determined. The upper and the lower phase transition temperature versus magnetic field curves are calculated and it is found that with increasing magnetic field and decreasing temperature the difference between these two curves (i. e. the hysteresis width) becomes increasingly small until at a critical field (H t) and temperature (T t) they end in a common point with the coexistence curve. BelowT t the variation of the magnetic field does not lead to a first-order antiferromagnetic ⇆ ferromagnetic transformation. The ferromagnetic phase is nothing, but the antiferromagnetic phase in a magnetic field strong enough to turn the moments parallel to the field direction. AboveH t but belowH c the variation of the temperature brings about also a second-order ferromagnetic ⇆ antiferromagnetic transition. The discontinuous change of the lattice parameter and that of the sublattice magnetization have been calculated at the coexistence temperature and at both the upper and lower transition temperatures. It has been shown that the entropy change of the transition has two contributions: one of them is associated with the volume change while the other with the magnetization change. Under certain conditions the order of magnitude of both contributions may be the same.