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