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 ZnxCo1-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 TN. The magnetic phase below the lower-temperature peak at Tf 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 Tf<T<TN the ferrimagnetic state observed for x<<0.4 is unstable. For larger x, short-range order exists for T>>Tf. For Tf<T<2Tf the interacting groups of spins could be considered to be in a liquid-like state, from which they freeze into random directions at Tf. A suggestive phase diagram is also discussed.