Magnetic properties of NiCr2O4 and MnCr2O4 based spinel chromites

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In the present thesis work we have studied the magnetic properties of Fe and Al substituted NiCr2O4 and MnCr2O4 series. Ni(Cr1-xMx)2O4 and Mn(Cr1-xMx)2O4 (M = Fe and Al) samples were prepared in single phase form by using solgel route. The parent compound NiCr2O4 is found to crystallize in tetragonal structure with I41/amd space group while all the Fe and Al doped samples exhibit cubic spinel structure with Fd 3m space group. Unlike the Ni(Cr1-xMx)2O4 series, all samples in Mn(Cr1-xMx)2O4 (M = Fe and Al) series including the parent compound (x = 0) exhibit cubic spinel structure. All samples in both the series undergo ferrimagnetic transition. The ferrimagnetic transition temperature is found to increase with increase in Fe concentration whereas it decreases with increase in Al concentration. Fe substituted NiCr2O4 samples show the most interesting magnetic properties. Fe substitution leads to the magnetization reversal behavior with enhanced magnetic compensation temperature such as 358 K, 366 K and 396 K respectively for x = 0.30, 0.40 and 0.50 samples. This temperature induced magnetization reversal is explained by considering different temperature dependence of the magnetic moments of the two sublattices. These samples exhibiting magnetization reversal also show tunable positive and negative exchange bias field in the vicinity of room temperature which is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.
Supervisor: S. Ravi