Mossbauer investigation of magnesium ferrite-aluminate

The magnesium ferrite-aluminate series MgAl_xFe _2-xO₄ (x=0.2, 0.4 and 0.7) has been synthesised, the lattice constants determined and the applicability of Vegard's law has been tested. The Mossbauer spectra have been recorded from 77K to a temperature well above the Neel point. The Neel temperature decreases linearly with the increase of aluminium concentration. The centre shifts are found to be independent of the aluminium concentration. The Mossbauer spectra have also been recorded at 4.2K in an external magnetic field of 4.2 T applied parallel to the direction of the gamma rays. For x=0.2 the canting of Fe spins is observed only on the A site and the B-site Fe spins have a collinear spin structure. This is the only spinel ferrite where the spin canting has been clearly observed on the A site alone when the B-site Fe spins have a collinear spin structure. For x=0.4 and x=0.7 both A- and B-site Fe ions have a noncollinear spin structure. The distribution of hyperfine fields at the A and B sites has been analysed in terms of supertransferred hyperfine fields from neighbouring ions.