Ferroelectricity Driven by Y $d^0$-ness with Rehybridization in ${\mathrm{YMnO}}_3$

We investigated electronic structure of hexagonal multiferroic ${\mathrm{YMnO}}_3$ using the polarization dependent x-ray absorption spectroscopy (XAS) at O $K$ and Mn $L_{2,3}$ edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn $3d$ orbital occupation. Moreover, the O $K$ edge spectra show that Y $4d$ states are strongly hybridized with O $2p$ ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y $d^0$-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.

Figure 1

(a) O $K$ edge polarization dependent XAS spectra of hexagonal ${\mathrm{YMnO}}_3$ collected in FY mode in comparison with the CI model calculations and the band calculations taken from Ref. 13 for O $2p$ PDOS. The crystal structure is shown in the inset. The solid bars represent the intensities of the delta functions. (b) Schematic crystal field splittings for ${\mathrm{MnO}}_5$ ($D_{3h}$) and ${\mathrm{YO}}_8$ ($D_{3d}$).

Figure 2

Local structures of ${\mathrm{MnO}}_5$ and ${\mathrm{YO}}_8$ (${\mathrm{Y}}^1{\mathrm{O}}_8$ and ${\mathrm{Y}}^2{\mathrm{O}}_8$) cages in ferroelectric and the centrosymmetric phases.

Figure 3

Mn $L$ edge polarization dependent XAS spectra and the difference spectrum, $(E\parallel c)-(E\parallel ab)$, of ${\mathrm{YMnO}}_3$ in comparison with the CI model calculations.

Figure 4

Polarization dependent XAS spectra of hexagonal $R{\mathrm{MnO}}_3$ ($R=\mathrm{Y}$, Dy) and orthorhombic $R{\mathrm{MnO}}_3$ ($R=\mathrm{La}$, Dy) at (a) Mn $L_{2,3}$- and (b) O $K$ edge collected in TEY mode. The $E\parallel c$ spectra for ${\mathrm{DyMnO}}_3$ films were approximated by the spectra at $\angle (E,c)=20°$.