Magnetic and dielectric properties of NiCrxFe2-xO4 nanoparticles

The magnetic and dielectric properties of sol-gel prepared NiCrxFe2-xO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 2) nanoparticles have been studied in detail. XRD revealed cubic spinel structure for all the samples and Rietveld analysis of XRD confirmed the single phase formation for all samples except x = 2 Cr concentration. Vegard's law linear relation of lattice parameter with Cr concentration confirmed the successive doping of Cr. Average crystallite size calculated by Scherrer formula was in the range 28 to 44 nm. TEM micrographs showed cubic and agglomerated nanoparticles. The M-H loops of these nanoparticles were non-saturation even at high field of +/- 5 T due to canted surface spins. The coercivity showed increasing trend with addition of Cr which was attributed to variation of magneto-crystalline anisotropy and presence of defects. The saturation magnetization revealed decreasing trend with increasing concentration of Cr due to small magnetic moment of Cr+3 as compared to Fe3+. Simulated ZFC curves of these nanoparticles revealed large value of effective anisotropy constant (K-eff) as compared to bulk due to frozen disorder surface spins. The value of K-eff was decreased by the addition of Cr. The frequency dependent dielectric constant revealed maximum value at lowest frequency which was described with Koop's theory. The value of dielectric constant at fixed frequency revealed also increasing trend with addition of Cr content which was due to less electrically conductive nature of Cr as compared to Fe.