Magnetic properties of nanocrystalline Al doped nickel ferrite synthesized by the sol-gel method

Nanocrystalline NiAlxFe2-xO4 spinels have been synthesized by the sol-gel method. The particle size decreased from 29 nm to 6 nm as the non-magnetic Al content increased. The crystal structure and magnetic properties of the Al co-substituted disordered spinel series of NiAlxFe2-xO4 (x = 0.0 - 1.0) have been investigated at 300 K by means of X-ray diffraction, magnetization, a.c. susceptibility, and Mossbauer effect measurements. The observed features for the sol-gel samples, such as the co-existence of a paramagnetic doublet and magnetic sextets in the Mossbauer spectra and lower saturation magnetization values, confirm nanoparticle ferrite behavior. The lattice constants were determined, and the applicability of Vegard's law has been tested. Magnetization results exhibit a non-collinear ferrimagnetic structure for x = 0.0 to 0.2 and Neel's collinear ferrimagnetic structure for x = 0.4 to 0.8, while the variation of the Curie temperature (T-C) obtained from the a.c. susceptibility with Al content x confirms the change of spin ordering from non-collinear to collinear at x = 0.4. Though Mossbauer spectra of x = 0.0 and 0.2 samples exhibit normal Zeeman split sextets, the spectra of the samples with 0.4 <= x <= 0.8 are characterized by the simultaneous presence of a central paramagnetic doublet and a magnetically split component.