Magnetostatic interactions in a natural magnetite-ulvospinel system

[ 1] Magnetostatic interactions have been investigated in an intergrown material consisting of similar to 200-nm magnetite blocks separated by similar to 30-nm-wide ulvospinel lamellae. First-order reversal curve (FORC) measurements provide a direct measure of the interaction fields, giving a value for the full width at half maximum (FWHM) of 30 mT ( at H-c = 20 mT). Hysteresis parameters are M-rs/M-s = 0.22, H-cr/H-c = 1.98 and transient energy dissipation (TED) = 0.18. Elimination of the intergrowth structure ( by heating in vacuo) causes the FORC contours to shrink down toward the origin, yielding FWHM = 14 mT ( at H-c = 6 mT) with corresponding changes in M-rs/M-s, H-cr/H-c and TED to 0.11, 2.73 and 0.28, respectively. All these characteristics reveal the strong influence of particle-to-particle magnetostatic interactions between the magnetite blocks in the starting material and demonstrate the change from single-domain/pseudosingle-domain to multidomain behavior due to the fundamental structural change brought about by the experimental homogenization. Micromagnetic calculations of particle assemblages representative for the intergrown structure confirm that the magnetite blocks will interact by assuming a supervortex magnetization structure.