Spin-charge-orbital coupling in multiferroic LuFe2O4 thin films
Multiferroic Thin Film Research Highlight
LuFe2O4 (LFO) Fe ions have mixed valence states with an equal number of Fe2+ and Fe3+ ions in the hexagonal lattice. Such spin and charge frustrations in the system lead to its multiferroic property. Further, LFO is unique because its ferroelectricity is induced by the Fe2+-Fe3+ charge ordering mechanism, not by the spin structures, and the magnetism is induced by the spin ordering of the Fe2+ and Fe3+ ions, thus leading to a direct magnetoelectric coupling.
Absorption Spectra & Temperature Dependence
(a) Absorption spectra of the LFO film on sapphire at 78 K and 300 K, extracted from transmittance data. The arrows indicate observed electronic transitions: two spin-allowed (Fe2+) dxy → (dxz, dyz) and dxy → dz2 on-site transitions at 1.60 eV and 2.40 eV, and the O 2p → Fe 3d charge-transfer transition at 3.30 eV. The inset shows the Fe2+ (d6) crystal-field splitting scheme.
(b) The absorption peaks at 2.40 eV (Fe2+ d to d on-site) and 3.30 eV (O 2p → Fe 3d charge transfer) transitions as a function of temperature. The decrease in oscillator strengths of the two absorption peaks at the magnetic transition temperature provides direct evidence of the spin-charge coupling effect in the LFO thin film. The shaded vertical lines designate TN and TCO.
Energy Bandgap & Resistivity Analysis
Temperature dependence of the energy band gap of the LFO thin film, indicating two transitions designated by two shaded vertical lines: a 3D CO transition at 350 K and a ferrimagnetic transition at 240 K, respectively.
Resistivity of the LFO thin film on sapphire as a function of temperature. The inset highlights a close-up view of the discontinuity around the two transition temperatures, as indicated by the arrows.
Conclusion
The absorption spectra of the LFO thin film, deposited on (001) sapphire and (111) YSZ by electron beam deposition, show Fe2+ d to d on-site transitions at 1.60 and 2.40 eV, and O 2p to Fe 3d and O 2p to Lu 6s and 5d charge-transfer transitions at 3.30 eV. Based on optical absorption data, the LFO thin film has a direct energy band gap of ~2.18 eV at 300 K. Electronic transitions, energy band gap, and resistivity demonstrate sensitivity to a ferrimagnetic transition at 235 K and CO transition at 350 K, confirming strong coupling of spin, charge, and orbital degrees of freedom in the LFO thin film.
Reference: R. C. Rai et al., Appl. Phys. Lett. 100, 212904 (2012).
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