Modulated spin structure responsible for the magnetic-field-induced polarization switching in multiferroicTbMn2O5

Abstract

Orthorhombic ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$ ($o\text{-TMO}$) is a well-known multiferroic manganite with the remarkable property of polarization switching at 3 K under a bias magnetic (H) field along the $a$ axis of $Pb{2}_{1}m$. To theoretically account for this outstanding observation, we have proposed a modulated spin structure under the saturated bias $H$ field by considering the relative strength of the three relevant exchange parameters in $o\text{-TMO}$. The proposed modulated structure based on density-functional theory (DFT) calculations is described in terms of the spin angle $\ensuremath{\phi}$ between the neighboring $\mathrm{M}{\mathrm{n}}^{4+}\text{-M}{\mathrm{n}}^{3+}$ spin moments on the a-b plane. We have shown that the computed DFT polarization plotted as a function of $\ensuremath{\phi}$ satisfactorily accounts for the observed $H$-field-induced polarization switching. We have further theoretically shown that the square of the critical field strength (${H}_{c}$) needed for the polarization switching is inversely proportional to the degree of the extrinsic magnetoelectric coupling. The computed partial charge density demonstrates that the $H$-field-induced polarization switching also accompanies with the switching in the sign of the excess valence-electron density.