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| Home > Publications database > Huge intra-atomic dipole moment of the iron spin-density in the low temperature phase of magnetite |
| Poster | FZJ-2016-01338 |
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2015
Abstract: Magnetite nanoparticles (NPs) with 6 nm diameter and a 200 nm thick magnetite reference film were studied with a combination of experimental and theoretical methods, i.e. x-ray absorption near edge spectroscopy (XANES), x-ray magnetic circular dichroism (XMCD), vibrating sample magnetometry (VSM) and electronic structure calculations based on density functional theory. For the NPs, increases of the white line intensity and the XMCD signal with increasing temperature were observed between 50 K and 100 K (Fig. 1a, b). A similar increase of the XMCD signal was also observed for the film between 125 K and 175 K, which is above its Verwey transition temperature of 120 K in agreement with the unusual temperature dependence of the magneto-crystalline anisotropy. A sum rule analysis of the XMCD spectra revealed that the transitions observed with XMCD are due to changing effective Fe spin moments. Since the transitions were not observed with VSM, we attribute them to changes of the intra-atomic dipole moment of the Fe 3d spin-density distribution, i.e. the Tz term in the XMCD sum rule for the effective spin moment. This conclusion was verified and explained theoretically. The sizable negative intra-atomic dipole moment in the monoclinic low-temperature phase of magnetite is due to the contribution of Fe2+ ions on B4 sites (-1.44 μB per atom) which is partly compensated by Fe2+ ions on B1 sites (0.72 μB per atom). It is a local indicator for the Verwey transition in small magnetite nanoparticles which is usually screened by blocking effects in classical magnetometry.
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