Fig. 5-7 Photograph of magnetite [Natural mineral] Magnetite "Fe3O4" is the first magnetic material discovered by humans. It is one of the most important industrial spinel type ferrites. Fe3O4 exhibits metal-insulator transition at low temperature (ca. 120K). From a fundamental solid-state physics viewpoint, it is interesting because of its typical charge ordered phase transition (Verwey transition).

Fig. 5-8 Crystal structure of magnetite "Fe3O4" The Fe atoms are located in two crystallographically nonequivalent positions in the unit cell. At room temperature, one-third of the Fe ions (Fe3+) occupy the A sites and the two-thirds of the Fe ions (Fe3+ and Fe2+) occupy the B sites. At a temperature below 120K, metal-insulator transition takes place. This is the result of a charge ordered transition whereby the distribution of Fe3+ and Fe2+ ions in B sites changes from dynamic disorder to long-range order.

Fig. 5-9 Phonon DOS of Fe in Fe3O4 The phonon DOS of all Fe is shown by closed black diamonds. When the time spectra are measured at each energy point of the nuclear resonance inelastic scattering spectrum, the patterns of quantum beat that reflect the different electronic states of Fe at the A and B sites are observed (see the upper right hand side of the figure). Here, the phonon DOS of Fe located in the A and B sites can be measured selectively by analyzing the measured time spectra. Downward pointing blue and upward pointing red triangles are of the A and B sites, respectively.

Magnetite "Fe3O4" exhibits an abrupt decrease in electrical conductivity at low temperature (metal-insulator transition). It is the well-known Verwey transition (Fig. 5-7). Nevertheless, the physical origin of this phenomenon is not yet fully understood in spite of many studies. To explore its origin, it is necessary to examine the temperature dependence of the electronic and lattice vibration (phonon) state of the iron (Fe) atom located in the B site of the inverse spinel structure of Fe3O4 (Fig. 5-8). Inelastic nuclear resonant scattering is a unique technique that can investigate the phonon state of only the atom contributing to nuclear resonant excitation. Spectra are observed by scanning the energy of the synchrotron radiation X-ray energy that enters a sample near the nuclear resonance excitation energy of a specific element. However, it was very difficult to selectively measure the phonon density of states (DOS) of each Fe atom located in the two crystallographically different sites in Fe3O4. To overcome this problem, we observed for the first time the quantum beat patterns of time spectra. These patterns reflect the difference of the electronic states of Fe atoms located in the A and B sites at each energy of the inelastic nuclear resonant scattering spectrum. The phonon DOS of the Fe atom in the B site of Fe3O4 was discerned using the difference in electronic states of the Fe atoms in the A and B sites at room temperature (Fig. 5-9). Our site-specific measurements are a powerful tool for the study of the local phonon states of the Verwey transition of Fe3O4 in the low temperature phase and in other complex materials.

Reference M. Seto et al., Site-Specific Phonon Density of States Discerned Using Electronic States, Phys. Rev. Lett., 91(18), 185505 (2003).

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