Spin spiral order induced ferroelectricity in MnRe2O8 monolayer

In this work, on the basis of first-principles calculations we demonstrate that magnetic exchange frustration caused spin spiral can generate an electric polarization $(31.46\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/{\mathrm{m}}^{2})$ in two-dimensional (2D) $\mathrm{Mn}{\mathrm{Re}}_{2}{\mathrm{O}}_{8}$, which, therefore, can be identified as a very rare type-II multiferroic material with robust magnetoelectric coupling. It is of high interest that the ferroelectricity is out of plane, explained by the mechanism of $\mathbit{P}\ensuremath{\propto}{\mathbit{S}}_{\mathrm{a}}\ifmmode\times\else\texttimes\fi{}{\mathbit{S}}_{\mathrm{b}}$ in the generalized spin-current model. We propose that compressive strain can improve the N\'eel temperature (from 31 to 63 K) as well as the electric polarization in terms of enhancing the supersuperexchange interaction for this in-plane noncollinear ${120}^{\ensuremath{\circ}}$-ordered antiferromagnetic order. The spin spiral chirality can be flexibly switched by a vertical electric field, that is, the polarization-chirality locking effect. It can be confirmed conclusively that our findings deepen the understanding on the magnetoelectric physics, and open a different avenue for magnetoelectric and magneto-optical applications based on 2D type-II multiferroic materials.