Structure and cationic distribution dependent soft magnetic properties of single-domain Mg1−xNixFe2O4 (0 ≤ x ≤ 1.0) nanocrystals

Abstract

The effects of Ni2+ doping on the crystalline lattice and corresponding magnetic properties of Mg1−xNixFe2O4 nanocrystals, synthesized by a microwave combustion, are systematically studied. Highly crystalline nanoparticles with average size of 20–30 nm and stoichiometric chemical compositions are indicated by results of transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The structural properties including lattice parameter, cationic radii, cationic distribution, bond lengths and angles have been estimated based on Rietveld analysis of the x-ray diffraction (XRD) patterns. Mostly inverse spinel structure in MgFe2O4 with an observed nonmonotonic increase of the inversion factor by Ni substitution and almost complete inversion for × ≳ 0.5 indicate competitive site preferences of the cations. The synthesized nanocrystals exhibit soft single-domain magnetic properties within a critical domain size of about 30–40 nm and magnetic moment that is well theoretically generated based on the cationic distribution, consistently estimated from the XRD and Mössbauer spectroscopy analyses. A monotonic increase in the magnetization and drastic decrease in coercivity by Ni substitution result in low anisotropy constant of about 5500 erg/g for the mostly soft ferromagnet Mg0.5Ni0.5Fe2O4. We discuss the evolution of the magnetic properties in a correlation with the cationic re-distribution and corresponding structural changes in the interionic distances and angles by Ni doping.