On the magnetic ordering in the insulating spinel Ga_0.8Fe_0.2NiCrO₄ with the A sublattice diluted to below the percolation threshold

Abstract

The results of a.c. susceptibility, d.c. magnetization and Mössbauer measurements on the insulating spinel system Ga_0.8Fe_0.2NiCrO₄ are reported. This system has the A sublattice diluted to below the percolation threshold and hence cannot retain any long-range ferrimagnetic order. The a.c. susceptibility shows a peak at T_f ≈ 50K. For T#62; 1.5T_f the magnetization is highly non-linear and tend to saturate above H #62; 2kOe. The Mössbauer spectra show broad doublets at 77 and 300 K. These observations lead to the picture of gradually evolving spin clusters ferrimagnetically coupled within, as the temperature is lowered, till the intercluster interaction brings about a cluster spin-glass ordering with randomly frozen cluster moments at T_f. The absence of the Zeeman splitting in the Fe⁵⁷ Mössbauer spectra could be understood in terms of the dynamical effects arising from the thermal excitation of spin clusters.