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.