H-T phase diagram of rare-earth-transition-metal alloys in the vicinity of the compensation point

Anomalous hysteresis loops of ferrimagnetic amorphous alloys in high magnetic field in the vicinity of the compensation temperature have so far been explained by sample inhomogeneities. We obtain the H-T magnetic phase diagram for the ferrimagnetic GdFeCo alloy using a two-sublattice model in the paramagnetic rare-earth ion approximation and taking into account rare-earth (Gd) magnetic anisotropy. It is shown that if the magnetic anisotropy of the f sublattice is larger than that of the d sublattice, the tricritical point can be at higher temperature than the compensation point. The obtained phase diagram explains the observed anomalous hysteresis loops as a result of high-field magnetic phase transition, the order of which changes with temperature. It also implies that in the vicinity of the magnetic compensation point the shape of magnetic hysteresis loop strongly depends on temperature.