Abstrakt
Magnetite as a typical representative of ferrimagnetic iron oxides is a material with mixed valence. Its cubic crystal lattice of the Fd-3m space group contains 8 formula units in the elementary cell. Its formal valence composition can be written as (Fe3+)A[Fe3+ Fe2+]BO4, in which A denotes the tetrahedral and B the octahedral sublattices of the spinel structure. Around the Verwey temperature TV ~ 125 K, a phase transition occurs, which was detected, e.g., as an anomaly in the temperature dependence of specific heat, a jump in the temperature dependence of magnetization and a jump of two orders of magnitude in the temperature dependence of electrical conductivity [Walz02]. When studying magnetite above TV using 57Fe NMR [Novák00], no difference was observed between Fe3+ and Fe2+ ions in the B-sites of the crystal lattice, which indicates a delocalized character of charge carriers and is in agreement with previous NMR experiments and Mössbauer spectroscopy [García04].
We collected 57 Fe NMR and Mössbauer spectra of a monocrystalline plate of magnetite with a diameter of ~3 mm and thickness of ~40 µm at liquid helium temperature. In the Mössbauer spectroscopy experiments, the sample was cooled at zero external magnetic fields and in the field of 6 T. After cooling without the applied field, the magnetization is perpendicular to the plane of the sample, i.e., parallel to the direction of the γ-rays, which results in the suppression of the intensity of the 2nd and 5th line of the sextets. The NMR spectra provide higher resolution and enable determination of the hyperfine fields of all inequivalent iron crystallographic sites of magnetite in the Cc structure below the Verwey transition (8 A and 16 B-sites) [Novák01]. These values were used in the analysis of the Mössbauer spectra as fixed parameters, which allowed us to determine the isomer shift and quadrupole splitting of the three sextets with the lowest hyperfine fields.
References
[Walz02] F.Walz, J. Phys.: Condens. Matter 14 (2002), R285
[García04] J.García and G.Subías, J. Phys.: Condens. Matter 16 (2004), R145
[Novák00] P. Novák, H. Štěpánková, J. Englich, J. Kohout , VAM Brabers, Phys. Rev. B 61 (2000) 1256-1260
[Novák01] P. Novák, H. Štěpánková, J. Englich, J. Kohout, VAM Brabers, Proceedings of International Conference on Ferrites, Kyoto 2000, 131-134