Size variation in nanocrystalline Zn0.2Ni0.8Gd0.05Fe1.95O4 ferrites: Exchange bias effect and its correlation with disordered surface spins

The article reports the influence of particle size variation on microstructural, optical and magnetic properties of nanocrystalline Gd doped Ni-Zn ferrites with a generic formula Zn-0.2 Ni0.8Gd0.05Fe1.95O4 . The nanoparticles were synthesized using chemical co-precipitation technique and the average crystallite sizes were found in the range of 4 nm-29 nm as estimated from Scherrer's relation. Raman spectra confirmed the lowering of symmetry of the octahedral sites of the cubic spinel structure with rare earth doping. A blue shift was observed in the absorption profile of the nanoparticles due to the size effect. The 7 T field cooled M-H plots recorded at 5 K exhibited exchange bias for the sample with the smallest particle size which was attributed to the high anisotropy of the canted surface spins compared to the core spins.