Magnetic properties of the mixed spinel NiAlxCrxFe2-2xO4

The crystal structure and magnetic properties of the Al and Cr co-substituted disordered spinel series NiAlxCrxFe2-2xO4 (z = 0.0-0.9) are investigated by means of X-ray diffraction, magnetization, a.c, susceptibility and Mossbauer effect measurements. The lattice constants are determined and the applicability of Vegard's law has been tested. The variation of the saturation magnetic moment per formula unit measured at 77 K and 300 K with Al-Cr content is satisfactorily explained on the basis of Neel's collinear spin ordering model for x = 0.1-0.5. The Mossbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe3+ at tetrahedral (A) and octahedral (B) sites for x less than or equal to 0.5. Mossbauer results confirm a collinear ferrimagnetic structure for a: = 0.1-0.5. The Curie temperature obtained from a.c. susceptibility decreases nearly linear with increase of AI-Cr concentration from x = 0.1 to 0.5.