Magnetic Hysteresis and Curie Temperature in Nickel-Chromium Ferrites Synthesized by Sol-Gel Auto-Combustion Method
Keywords:ferrite, nanoparticle, coercivity, specific saturation magnetization, Curie temperature
Nanopowders of nickel-chromium ferrites with the general formula NiCrxFe2−xO4 are synthesized, by using the sol-gel auto-combustion method. Specimens of all ferrite compositions had a cubic spinel structure. The average size of powder crystallites varied from 23 to 43 nm. After the powders were pressed and sintered at a temperature of 1573 K, the average size of crystallites grew to 65–83 nm. The shape of a hysteresis loop (the low coercivity Hc) showed that the
specimens were soft magnetic materials. The residual magnetic field induction Br decreased with the growing concentration of Cr3+ ions due to the weakening of the interaction between the spinel sublattices, whereas Hc increased at that. The increase of the Cr3+ ion content in the ferrite specimens diminished the value of specific saturation magnetization qs from 33.9 to 7.7 A m2/kg, which testifies that less magnetic Cr3+ ions substitute Fe3+ ones in the ferrite octahedral sublattice. The magnetic moments calculated for the proposed cation distribution in the framework of the N´eel two-sublattice model turned out some lower than the experimentally measured values, which can be explained as a manifestation of the tilted spin effect and the influence of a surface layer 0.22–2.01 nm in thickness with a violated magnetic structure. As the temperature was lowered to 77 K, the magnetic parameters of the ferrite cores increased owing to their superparamagnetic properties. The Curie temperature decreased from 831 to 685 K with the growth of Cr content.
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