Paramagnetic-ferrimagnetic transition in strong coupling paramagnetic systems: effect of cubic anisotropy interaction

The purpose of the present work is a quantitative investigation of the biqu adratic exchange interaction effects on the paramagnetic-ferrimagnetic tran sition arising from two strongly coupled paramagnetic (1-spin) sublattices, of respective moments in and M. The free energy describing the physics of the system is of Landau type. In addition to the quadratic and quartic term s, in both in and if, this free energy involves two mixing interaction term s. The first is a lowest order coupling -CmM, where C < 0 stands for the co upling constant measuring the interaction between the two sublattices. Whil e the second, which is relevant for 1-spin systems and which traduces the d ipole-dipole (or biquadratic) interaction, is of type wm(2)M(2), where w > 0 is the new coupling constant. These two interactions enter in competition , and then, they induce drastic changes of the magnetic behavior of the mat erial. The main change is that, the presence of this high order coupling te nds to destroy the ferrimagnetic order of the system. We first show that th e introduction of this biquadratic interaction does not affect the values o f critical exponents. Also, we find that the compensation temperature (when it exists) and the compensation magnetic field are shifted to their lowest values, in comparison with the w = 0 case. The Arrott-phase-diagram shape is also investigated quantitatively. We show the existence of three regimes depending on the values of w. When the latter is small, we find that the r egion of competition between the coupling C and the applied magnetic field H becomes more narrow under the effect of it, (by competition, we mean the passage from the antiparallel state to the parallel one). While for higher values of w, this competition disappears completely, and then, the system l oses its ferrimagnetic character. Kinetics of the phase transition is also examined, when the temperature is lowered from an initial value T-i to a fi nal one T-f very close to the critical temperature T-c. As in the it, = 0 c ase, we find that kinetics is controlled by two kinds of relaxation times t au (1) and tau (2). The former is the relevant time, and is associated to l ong-wavelength fluctuations driving the system to undergo a phase transitio n. The second is a short time, which controls local dynamics. Near T-c, we show that, in particular, the longest relaxation time tau (1) becomes less important in comparison with that relative to the it, = 0 case. Finally, we note that the existence of two relaxation times is consistent with the pre dictions of a recent experiment, which was concerned with the 1/2-spin comp ounds LixNi2-xO2, where the composition x is close to 1. (C) 2001 Elsevier Science B.V. All rights reserved.