Magnetic properties of the mixed spinel NiAl 2 xCr xFe 2−3 xO 4

Abstract

The crystal and magnetic properties of the Al and Cr co-substituted disordered spinel series NiAl 2 x Cr x Fe 2−3 x O 4 ( x=0.1–0.6) are investigated by means of X-ray diffraction, magnetization, Mössbauer effect and AC susceptibility measurements. The lattice constants are determined and the applicability of Vegards law has been tested. The variation of the saturation magnetic moment per formula unit measured at 77 K with the Al–Cr content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.2. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe 3+ at tetrahedral (A) and octahedral (B) sites for x⩽0.4. Mössbauer results confirm a collinear ferrimagnetic structure for x=0.1–0.2. The n B values obtained from magnetization and Mössbauer data for x=0.3–0.6 display a discrepancy between them, thus exhibiting a significant canting on the B-site which is explained satisfactorily on the basis of the non-collinear spin ordering model of random canting.The Curie temperature obtained from AC susceptibility decreases nearly linear with x from x=0.1–0.4 and it shows a sudden change near x⩾0.4 indicating a change in magnetic structure.