On the magnetic ordering in the disordered spinels Zn_xCo_1−xFeCrO₄

Abstract

The AC susceptibility and the low-field magnetisation measurements on the disordered spinel system Zn_xCo_1−xFeCrO₄ (x=0.0, 0.2, 0.4, 0.5, 0.8) are reported. The X-ray analysis shows that the lattice parameter shows a minimum at x=0.5, indicating a redistribution of the cations in the A and B sites on introduction of Zn²⁺. The low-field AC susceptibility and DC magnesidation indicate two transitions for x=0.0 and 0.2 while only one for x > or=0.4. The high-temperature peak in chi corresponds to a ferrimagnetic transition at T_N. The magnetic phase below the lower-temperature peak at T_f could be considered as that of clusters of spins randomly frozen. This state is characterised by strong irreversibility in low DC fields, rapid decay of remanence on increasing the temperature and saturation in rather low applied magnetic fields. For T_f < T < T_N the ferrimagnetic state observed for x < < 0.4 is unstable. For larger x, short-range order exists for T > > T_f. For T_f < T < 2T_f the interacting groups of spins could be considered to be in a liquid-like state, from which they freeze into random directions at T_f. A suggestive phase diagram is also discussed.