Magnetic, electric, mechanical, and optical properties of NiCrxFe2−xO4 ferrites

The Ni-Cr ferrites were prepared by the technology sol-gel with participation of auto-combustion (SGA). The effect of Cr3+ ions addition on the microstructure, chemical composition, mechanical, magnetic, optical, dielectric, and electric properties of NiCrxFe2−xO4 ferrites were systematically studied. The added of Cr3+ ions significantly affects the formation of pores and grain size of ferrites. The micro-hardness values decreases from 9.93 GPa to 5.76 GPa with increasing chromium content. The Hall coefficient, conductivity type, concentration of charge carriers, and specific conductivity were found. In Ni-Cr ferrites dc-conductivity increases with Cr-concentration and is within the limits from 2.86·10−5 Ω−1∙m−1 to 1.35·10−2 Ω−1∙m−1. It is shown that with the increasing temperature the conductivity of NiCrxFe2−xO4 ferrites also increases. It is found that at low temperatures the Verwey jump conductivity mechanism is dominant, and in high temperature range the conductivity is described by the band model. The substituted samples showed broad sextet, which is resolved into subsextets, corresponding to tetrahedrally and octahedrally coordinated Fe cations. Experimental Mössbauer spectra of nickel-chromium ferrite powders also contain doublets. Cation distribution calculated using XRD and Mössbauer data indicates a decrease in Fe3+[B]/Fe3+(A) ratio with increasing chromium concentration. The Curie temperature, initial permeability, and saturation magnetization decrease with increasing substitution of Cr3+ ions. The decreasing of saturation magnetization from 33.9 A·m·kg−1 to 7.7 A·m·kg−1 is due to the fact that the Cr3+ ions occupy exclusively B-sites in the spinel structure.