Spin-Charge Coupling and High-Energy Magnetodielectric Effect in Hexagonal HoMnO3
Multiferroic Oxide Research Highlight
The hexagonal HoMnO3 is multiferroic[cite: 8]. Ferroelectric and antiferromagnetic orders coexist below the Néel temperature (TN ~ 75 K), where Mn3+ spins order antiferromagnetically[cite: 8]. The ferroelectric transition temperature is TFE ~ 900 K and the ferroelectric moment along the c-axis in this phase is due to Ho-O displacements[cite: 8]. This material displays a rich H-T phase diagram, with at least six different magnetic phases reported[cite: 8]. We investigated the optical and magneto-optical properties of HoMnO3 to elucidate the spin-charge coupling and high-energy magnetodielectric effect[cite: 8].
Optical Conductivity and Spin-Charge Coupling Analysis
(a) Optical conductivity of HoMnO3 at 300 K and 6 K extracted from measured reflectance via Kramers-Kronig analysis[cite: 8]. The inset shows a typical peak fit of Mn d to d excitations at 6 K[cite: 8]. We find that Mn d to d excitations are sensitive to the cascade of low-temperature magnetic transitions involving the Mn3+ moment—direct evidence for spin-charge coupling[cite: 8]. An applied magnetic field also modifies the Mn d to d on-site excitations[cite: 8].
(b) Peak positions of model oscillators, fitted to the on-site excitations, as a function of temperature[cite: 8]. The shaded vertical lines designate the well-known TN (antiferromagnetic) and TSR (spin reorientation) transitions[cite: 8].
Dielectric Contrast Summary:
The high-energy dielectric contrast in HoMnO3 is associated with dispersive changes in Mn d to d on-site excitations around TN and TSR[cite: 8]. The highest-energy dxz → d(3z2-r2) and dyz → d(3z2-r2) excitations are most strongly affected, demonstrating that spin-charge coupling depends directly on symmetry[cite: 8].
Magneto-Optical & High-Field Response (H || c)
(a) The 6 K normalized magneto-optical response of HoMnO3, R(H) / R(H=0 T), in an applied magnetic field from 0 to 20 T (H || c), shown in 4 T steps[cite: 8].
(b) Optical conductivity spectra for H = 0 T (solid line) and 20 T (dashed line) measured at 6 K[cite: 8].
(c) Dielectric response for H = 0 T (Blue) and 20 T (Red) (H || c) at 6 K[cite: 8]. The inset highlights high-energy dielectric contrast, which reaches its maximum near the Mn d to d excitations[cite: 8].
Conclusion
We find that Mn d to d excitations are sensitive to the cascade of low-temperature magnetic transitions involving the Mn3+ moment because the local environment around the Mn3+ center is subtly modified at TN and TSR[cite: 8]. This sensitivity provides direct evidence of spin-charge coupling[cite: 8]. Field-induced optical property modifications are observed in the region of Mn d to d excitations[cite: 8]. The high-energy magnetodielectric contrast in HoMnO3 is ~8% at 20 T near 1.8 eV, comparable in magnitude to the frustrated Kagomé lattice compound Ni3V2O8 (16% at 30 T near 1.3 eV) and mixed-valent magnetic oxide K2V3O8 (5% at 30 T near 1.2 eV)[cite: 8]. The magnetodielectric contrast in HoMnO3 can be positive or negative depending on energy, deriving from substantial spin-charge mixing in this multiferroic system[cite: 8].
Reference: R. C. Rai et al., Phys. Rev. B 75, 184414 (2007)[cite: 8].
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