Magnetic field induced structural changes in magnetite observed by resonant x-ray diffraction and Mössbauer spectroscopy

Abstract

When a magnetic field is applied to a single crystal of magnetite at 124 K > T > 50 K, the monoclinic c M axis, which is the easy magnetization axis, switches to one of the 100 cubic directions, nearest to the direction of the magnetic field, and the phenomenon known as an axis switching (AS) occurs. A global symmetry probe, resonant x-ray scattering, and a local probe, Mössbauer spectroscopy, are used to better understand the mechanism of axis switching. The behavior of three subsystems ordered below the Verwey transition temperature T V , i.e., lattice distortion, an orbital, and charge orderings, was observed via resonant x-ray scattering as a function of an external magnetic field. This was preceded by calculation of selected peak intensities using the FDMNES code. The Mössbauer spectroscopy studies confirmed that the magnetic field triggers electronic rearrangements and atomic displacements. The structure observed after the process of axis switching is very similar to the one obtained after cooling below T V with the magnetic field applied along one of the initial 100 cubic directions and distinct from the cooling in the absence of a magnetic field. From all the experimental observations of the phenomenon done so far, it is clear that AS starts from the fluctuations between octahedral iron orbitals that ultimately lead to the Verwey transition, but also to the higher-temperature trimeron dynamics. Therefore, further observation of the axis switching may be a key point to the understanding of a majority of strongly correlated electronic behavior in magnetite as well as in other transition metal oxides.

Trimeron-phonon coupling in magnetite

“…The obtained results can be compared with other studies which probe the local charge distribution on Fe B ions. The NMR and Mössbauer measurements combined with DFT calculations revealed that all B-sites can be divided into three groups with the ratio 8:5:3 [18,19,77,78]. The first group consists of 8 Fe 3+ ions, the second group includes 5 Fe 2+ ions in which electrons occupy the d xz or d yz orbitals, and the third group comprises 3 Fe 2+ ions with occupied d xy orbitals.…”

Magnetism and the Trimeron Bond

“…24 , 25 Further Mössbauer and resonant X-ray experiments have suggested that trimeron direction changes around a fixed central Fe 2+ ion when the easy magnetization axis of the Cc phase is switching by an applied magnetic field. 27 Recent phonon calculations for the Cc structure have shown good agreement with inelastic neutron, X-ray, and nuclear scattering data, revealing strong trimeron–phonon coupling, especially for trimerons oriented parallel to the axes of the monoclinic Cc cell. 28 …”

“…The four signals are in an 8:8:5:3 ratio to account respectively for 8 A-site Fe 3+ ions, 8 B-site Fe 3+ ions, 5 B-site Fe 2+ ions where the extra electron occupies d xz or d yz orbitals, and 3 B-site Fe 2+ ions with the extra electron in the d xy orbital. The latter group is distinct as the d xy orbitals lie perpendicular to the magnetization in the Cc structure giving rise to lower effective magnetic fields and larger electric field gradients, and they also have distinctive NMR frequencies. , Further Mössbauer and resonant X-ray experiments have suggested that trimeron direction changes around a fixed central Fe 2+ ion when the easy magnetization axis of the Cc phase is switching by an applied magnetic field . Recent phonon calculations for the Cc structure have shown good agreement with inelastic neutron, X-ray, and nuclear scattering data, revealing strong trimeron–phonon coupling, especially for trimerons oriented parallel to the axes of the monoclinic Cc cell …”

“…Much of this has been enabled by technique developments. In particular, state-of-the art X-ray synchrotron beamlines have been used for microcrystal determinations of the Cc ground state crystal structure, 3 , 16 , 53 , 54 resonant and multiwave diffraction studies, 19 , 27 nuclear inelastic scattering, 50 ambient and high pressure powder diffraction studies, 32 , 45 diffuse and total scattering (PDF) experiments, 37 , 45 , 46 X-ray absorption spectroscopy and MCD, 36 and resonant and nonresonant IXS, 41 − 44 with the majority of these experiments performed at the ESRF. Coherent light sources have also been important for pump–probe studies of lattice dynamics.…”

“…Much of this has been enabled by technique developments. In particular, state-of-the art X-ray synchrotron beamlines have been used for microcrystal determinations of the Cc ground state crystal structure, ,,, resonant and multiwave diffraction studies, , nuclear inelastic scattering, ambient and high pressure powder diffraction studies, , diffuse and total scattering (PDF) experiments, ,, X-ray absorption spectroscopy and MCD, and resonant and nonresonant IXS, − with the majority of these experiments performed at the ESRF. Coherent light sources have also been important for pump–probe studies of lattice dynamics. , Improved DFT codes have supported many of these investigations, also enabling accurate NMR and Mössbauer spectra to be simulated. − , Developments in nanoparticle chemistry have enabled influences of particle size and oxygen content on the Verwey transiton to be determined in exquisite detail. , High pressure discoveries of new mixed-valent binary iron oxides have broadened insights into magnetite. ,− …”