Mössbauer and magnetic properties of annealed NixCo1−xFe2O4 nanoparticles

•NiCo ferrites have been synthesized by glycol-thermal method.•Thermal annealing shows significant effects on the magnetic properties.•Coercivity and hyperfine magnetic fields are sensitive to particle size.•Bulk properties emerge at the highest annealing temperatures.

The magnetic properties of NixCo1−xFe2O4 (x = 0.3 and 0.5) fine powders, prepared by glycol-thermal process and subjected to annealing temperatures, were investigated using hysteresis measurements and Mössbauer spectroscopy. Energy dispersive X-ray (EDX) analysis confirms presence of Ni, Co and Fe atoms as per stoichiometry. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) data reveal particle sizes of about 13 nm which increase to about 110 nm after annealing at 1100 °C. Mössbauer spectra could be resolved into two sextets corresponding to tetrahedral (A) and octahedral (B) sites and one quadrupole doublet. The doublet is associated with a fraction of small superparamagnetic nanoparticles. Significant changes have been observed with increasing particle size. A general increase in hyperfine fields with increasing grain size is attributed to stronger superexchange interactions as a result of enhanced particle size. The evolution of coercive fields with increasing crystallite size indicates transition from single- to multi-domain structure at a critical particle size.