Phase diagram and high-temperature behaviour in dilute system FexMg1-xCl2

Abstract

Presents an experimental study by low-field DC susceptibility measurements of the phase diagram of the dilute system FexMg1-xCl2. At high Fe2+ concentrations, antiferromagnetic long-range order appears. The initial slope of the concentration dependence of the Neel temperature shows the quasi-bidimensional character of this system. Throughout a large concentration range, 0.3<or=x<or=0.6, FexMg1-xCl2 exhibits a behaviour characteristic of a spin-glass-like state. The appearance of this state is probably due to the competition between the inplane nearest-neighbour ferromagnetic interaction and the next-nearest-neighbour antiferromagnetic interaction. At high temperature, the variation of the susceptibility plotted against the concentration-scaled temperature follows a Curie-Weiss law with an effective Bohr magneton number and an asymptotic Curie temperature which are nearly independent of concentration. Decreasing the temperature, the authors observe large deviations from this Curie law. However, for x<or=0.5, at the approach of the paramagnetic to spin-glass phase transition, the variation of the susceptibility against the concentration-scaled temperature obeys a Curie-Weiss law whose effective Bohr magneton number and asymptotic temperature depend on the concentration. This feature is attributed to the superparamagnetic behaviour of ferromagnetic clusters.