Magnetic properties of the mixed spinel NiAl_xCr_xFe_2-2xO₄

Abstract

The crystal structure and magnetic properties of the Al and Cr co‐substituted disordered spinel series NiAl_xCr_xFe_2-2xO₄ $(x = 0.0 - 0.9)$ are investigated by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer 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 Mössbauer 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 $x \leqslant$ 0.5. Mössbauer results confirm a collinear ferrimagnetic structure for $x$ =0.1–0.5. The Curie temperature obtained from a.c. susceptibility decreases nearly linear with increase of Al–Cr concentration from $x$ =0.1 to 0.5.