Abstract
The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin–orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and are best explained as magnetic polarons.
Highlights
Numerous investigations have been carried out to verify the charge localization on the octahedrally (Oh) coordinated B sites, the charge-ordering pattern of magnetite is subtle and still elusive[19,20]
We use resonant inelastic X-ray scattering (RIXS) over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states
With the incident X-ray energy set to β, RIXS excitations arise mostly from Fe3+ ions of octahedral or tetrahedral symmetry
Summary
Numerous investigations have been carried out to verify the charge localization on the octahedrally (Oh) coordinated B sites, the charge-ordering pattern of magnetite is subtle and still elusive[19,20]. The tetragonal distortion of B-site Fe2+O6 octahedra removes the degeneracy of t2g orbitals, in going from Oh symmetry. To the best of our knowledge, the relation of the local tetragonal distortion field of Fe2+ ions with the magnetic excitations of magnetite has not been reported to date. We present measurements of resonant inelastic X-ray scattering (RIXS)[32] at the Fe L3-edge on magnetite to reveal the low-energy spin-orbital excitations of Fe2+ ions in both the monoclinic and cubic phases. In combination with crystal-field multiplet calculations, we show the the existence of magnetic polarons in magnetite which is driven by Jahn-Teller distortion
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