Few-layer perovskite ${\mathrm{YMnO}}_3$: A first-principles study

The successful preparation of freestanding oxide perovskites down to the few-layer limit inspires the development of two-dimensional multifunctional materials exhibiting both ferroelectric and ferromagnetic properties. In this study, we performed a comprehensive first-principles investigation on the few-layer perovskite ${\mathrm{YMnO}}_3$. Our findings reveal that its monolayer is an antiferromagnetic semiconductor, while other few layers are ferromagnetic half metals. All of them have an in-plane polarization with moderate energy barrier, although the “polarization” of metallic few layers may be not switchable. The distortion modes of few-layer ${\mathrm{YMnO}}_3$ are different from its orthorhombic bulk counterpart which is paraelectric. The symmetry mode analysis reveals that the monolayer ${\mathrm{YMnO}}_3$ displays proper ferroelectric behavior. We have investigated the polarization and magnetism of few-layer polar oxide perovskites and found that they strongly depend on the number of layers. These results will contribute to our current understanding of the properties of few-layer polar oxide perovskites.

Figure 1

The top and side views of the ground-state structures of (a) monolayer and (b) bilayer YMO. The dark green, purple, and red colors represent Y, Mn, O atoms, respectively. (c) Relative energy of the monolayer magnetic states at different $U$ values and (d) the monolayer angle-dependent magnetic anisotropy energies with magnetic axis varying in the $\mathit{XY}, \mathit{YZ}$, and $\mathit{ZX}$ planes.

Figure 2

Band structures and density of states of AFM monolayer (a), (c) and FM bilayer (b), (d) YMO. The top view of AFM and FM magnetic configurations of YMO using PBEsol+$U$ (e). The yellow and purple spheres represent opposite spins.

Figure 3

(a) Schematic diagram of the distortion modes of ${\mathrm{\Gamma }}_1$ (in phase), ${\mathrm{\Gamma }}_3$ (FE), ${\mathrm{Y}}_2$ (AFD), and ${\mathrm{Y}}_4$ (AFE) for the monolayer YMO. These modes are defined under the symmetry of Cmm2. (b) The group-subgroups relation of the monolayer. The numbers beside the arrows are corresponding to the energy decreases (meV/f.u.) between distortion modes. (c) Total energies as function of each individual symmetry mode, of ${\mathrm{\Gamma }}_3$ when combined with $Q_{\mathrm{Y}2}=1$ and $Q_{\mathrm{Y}4}=1$, and of all three symmetry distortion modes included simultaneously. $Q$ is the normalized dimensionless distortion amplitude.

Figure 4

(a) Schematic diagram of the distortion modes of ${\mathrm{\Gamma }}_1$ (in phase), ${\mathrm{\Gamma }}_3$ (FE), ${\mathrm{Y}}_2$ (AFD), and ${\mathrm{Y}}_4$ (AFE) for the bilayer YMO. (b) The group-subgroups relations of the modes are defined under the symmetry of Cmm2. (c) Total energies as function of each individual symmetry mode, of ${\mathrm{\Gamma }}_3$ when combined with $Q_{\mathrm{Y}2}=1$ and $Q_{\mathrm{Y}4}=1$, and of all three symmetry distortion modes included simultaneously.

Figure 5

(a) Total polarization and the contributions of each element and each atomic layer of the monolayer (shade of green) and bilayer (shade of yellow). (b) Ferroelectric switching energies calculated using nudge elastic band method. Lambda λ is a configuration coordinate, with λ = −1 for −$P$ and λ = 1 for $P$.