Studies of the magnetic ordering in the spinel system Zn0.4Co0.6AlxFe2−xO4 by neutron diffraction

Magnetic properties of the spinel ferrite Zn0.4Co0.6AlxFe2−xO4 with x = 0.0, 0.25, 0.50, 0.75 and 1.0 have been investigated by neutron diffraction studies in the temperature range 800 K ≥ T ≥ 17 K. The distributions of the magnetic moments in the A and B sublattices and their ordering as a function of temperature and composition have been obtained. The B sublattice magnetization deviates from the usual Brillouin function. An increasing loss in B sublattice magnetization with increasing x causing gradual destabilization of the ferrimagnetic order has been revealed. A ferromagnetic transition is found to take place before reaching the paramagnetic state at higher x values. The net magnetization becomes zero at some temperatures for compositions x ≤ 0.75 indicating a state of spin compensation. The presence of a diffuse signal at low temperature causing significant broadening of the (1 1 1) Bragg peak indicates the formation of magnetic spin clusters in the system. The diffuse signal slightly fluctuates with temperature indicating freezing of the clusters at some temperatures. The progressive loss of B moment combined with the superposition of magnetic short-range clusters over the magnetic long-range order has induced a complicated magnetic structure in the system. The features observed in the neutron results suggest several transitions for the system: (i) a ferromagnetic transition for x ≥ 0.75 before the paramagnetic state is reached; (ii) a spin compensation state or anti-ferromagnetic state at some temperatures when the A and B sublattice moments become equal in magnitude for x ≤ 0.75; (iii) a semi-spin glass like state for x ≥ 0.75 as evident from the very high reduction in ordered B sublattice moment and freezing effect of short-range clusters.