Superparamagnetic Behavior of Zinc-Substituted Nickel Ferrite Nanoparticles and its Effect on Mossbauer and Magnetic Parameters

Zinc-substituted nickel ferrite (Ni 1− x Zn x Fe 2 O 4 with x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) nanoparticles were synthesized by solgel auto-combustion technique at low temperature and characterized by using X-ray diffraction, scanning electron microscopy, pulse field hysteresis loop technique, and Mossbauer spectroscopy. X-ray diffraction studies confirmed the formation of single-phase spinel structure of the prepared ferrite samples with average crystallite size of 30 nm, very close to that of the critical size for nanoparticles exhibiting superparamagnetism. Scanning electron micrographs of the ferrite samples showed uniform spherical morphology of nanograins with homogenous microstructure. Further investigations on magnetic properties by pulse field hysteresis loop technique and Mossbauer spectroscopy indicated the presence of superparamagnetic phases in the ferrite samples attributed to occupation of octahedral [B] sites by zinc ions in these Ni–Zn samples and also to the nanometer sizes of the ferrite particles. Magnetic behavior of the Ni–Zn ferrite system is in agreement, initially, with Neel’s two-sublattice collinear model and then with the Yafet–Kittel model for samples with higher zinc content (x ≥ 0.4). Value of hyperfine splitting is found to decrease with increase in zinc content and is attributed to the reduction in particle size giving rise to superparamagnetism. Other Mossbauer parameters like quadrupole splitting and the isomer shift are within the reported range for those of ferrites with spinel structure.