Multiferroic properties of oxygen-functionalized magnetic i-MXene

Two-dimensional multiferroics inherit prominent physical properties from both low-dimensional materials and magnetoelectric materials, and they surpass their three-dimensional counterparts for their unique structures. Here, based on density functional theory calculations, a MXene derivative, i.e., i-MXene $({\mathrm{Ta}}_{2/3}{\mathrm{Fe}}_{1/3}{)}_{2}{\mathrm{CO}}_{2}$, is predicted to be a type-I multiferroic material. Originating from the reliable $5{d}^{0}$ rule, its ferroelectricity is robust, with a moderate polarization up to $\ensuremath{\sim}12.33\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$ along the $a$-axis, which can be easily switched and may persist above room temperature. Its magnetic ground state is layered antiferromagnetism. Although it is a type-I multiferroic material, its N\'eel temperature can be significantly tuned by the paraelectric-ferroelectric transition, manifesting a kind of intrinsic magnetoelectric coupling. Such a magnetoelectric effect originates from the conventional magnetostriction, but it is unexpectedly magnified by the exchange frustration. Our work not only reveals a nontrivial magnetoelectric mechanism, but it also provides a strategy to search for more multiferroics in the two-dimensional limit.