Phase Transitions in Ordered and Disordered Ferrimagnets. II. Experimental Results

Abstract

Abstract The magnetic phase transition is investigated by a vibration magnetometer for the homogeneous ferrimagnet Fe 3 O 4 and the structurally disordered ferrites Zn x Ni 1− x Fe 2 O 4 with x = 0, 0.25, 0.5, and 0.75. For both ordered and disordered ferrimagnets the asymptotic critical exponents are compatible with the ones of a homogeneous 3d‐Heisenberg ferromagnet. Outside the asymptotic critical regime there is a strong effect of the structural inhomogeneities on the thermodynamic behaviour. In close analogy to the case of disordered ferromagnets, the effective Kouvel‐Fisher exponent γ( T ) first increases with increasing temperature T for disordered systems, as predicted by the correlated molecular field theory and by Monte Carlo computer simulations, whereas it initially decreases for a homogeneous ferrimagnet. Anomalies in the modified Arrott plots at low fields are discussed.