Ferromagnetic resonance lines of annealed Ni ferrites

This work reports a magnetic anisotropy study, using ferromagnetic resonance (FMR), of nickel ferrite magnetic nanoparticles produced by the sol-gel protein technique and annealed at temperatures (TA) of 600 °C, 900 °C, and 1100 °C. Theoretical fittings of the FMR lines were achieved by considering a four-fold symmetry model for randomly dispersed magnetic nanoparticles. The FMR results agree with Xray diffraction data and confirm that: (i) the crystalline structure of Ni ferrite particles increases proportionally with the annealing temperature; (ii) the intensity of the cubic anisotropy field (Hk = 2 K1/Ms) also changes evolving from positive (+680 Oe, for TA = 600 °C) to negative (−1600 Oe, for TA = 1100 °C) values, implying that the orientation of the easy axis of magnetization within the particles changes from the {100} to the {111} directions; and (iii) the decrease in the linewidth, from 1640 to 900 Oe, of the terms used in the FMR modeling, evidence the increase in crystallinity as a function of the heat treatment. This fact combined with the evolution of the values of the anisotropy fields to their negative bulk value, previously known from the literature, suggests a redistribution of the Ni/Fe ions, initially in metastable states, tending to the configuration of greater stability of the system.