Analysis of Cationic Impurity Impact on Hyperfine Interactions in Magnetite

The Al, Ga, Ti and Zn impurities were studied in high quality single crystal samples of magnetite (Fe₃O₄). The cases when the valence of the substitution ion differs from that of the original iron ion are of a particular interest. The measurement of nuclear magnetic resonance (NMR) of ⁵⁷Fe was chosen as the experimental method for its sensitivity to Fe magnetic and electronic structure and to local ordering in the neighborhood of resonating nuclei. Local electronic structure and hence also the hyperfine interactions are affected by the presence of the substitution and thus resonance frequencies of nuclei in the vicinity of the substitution are shifted due to the modified hyperfine field, therefore satellite signals are observed in the NMR spectra. Temperature dependences of spectra above the Verwey transition were measured in a zero external magnetic field. Subsequently, the dependences of frequencies of main lines and satellite signals in the spectra on temperature were fitted with spin-wave model dependences and compared for different types of the substitution. Furthermore, a mean-field model was applied on the substituted magnetite system and calculated temperature dependences were confronted with the experimental data. The agreement of mean field model with the experimental temperature dependences of satellite resonances is better for those induced by the cationic substitutions at octahedral sites.