Hybrid improper ferroelectricity and magnetoelectric coupling in a two-dimensional perovskite oxide

Ying Zhou, Zefeng Chen, Zongshuo Wu, Xiaofan Shen, Jianli Wang, Junting Zhang, and Hui Sun

Phys. Rev. B 103, 224409 – Published 8 June 2021

Abstract

Unlike van der Waals materials, perovskite oxide may undergo structural reconstruction when reduced to two dimensions. Therefore, it is still an open question whether some of the ferroelectric mechanisms and magnetoelectric coupling effects found in perovskite bulks can be maintained at the two-dimensional (2D) limit. Here we demonstrate that the ferroelectricity induced by octahedral rotation and the magnetoelectric coupling mechanism dependent on Dzyaloshinskii-Moriya (DM) interaction exist in a proposed perovskite bilayer. Although the octahedral rotation distortion of the ${Ca}_{3} {Mn}_{2} O_{7}$ bilayer is reconstructed with respect to its layered bulk phase, tensile strain can induce the same octahedral rotation type as its bulk phase, resulting in the emergence of ferroelectric phase. We show that the modulation of the DM vector by octahedral rotation distortion provides a coupling between polarization and the induced magnetization. In ferroelectric switching, the polarization is reversed by a $180^{\circ}$ rotation of the tilt axis within the $a b$ plane, which also results in a reversal of magnetization. This work is expected to provide a guidance for realizing electric-field control of magnetization direction in 2D perovskite oxides.

Received 28 January 2021
Revised 12 April 2021
Accepted 28 May 2021

DOI:https://doi.org/10.1103/PhysRevB.103.224409

Physics Subject Headings (PhySH)

Ferroelectricity Magnetic anisotropy Magnetic order Spin-orbit coupling

2-dimensional systems Multiferroics

First-principles calculations

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ying Zhou¹, Zefeng Chen ¹, Zongshuo Wu¹, Xiaofan Shen¹, Jianli Wang¹, Junting Zhang ^1,*, and Hui Sun^2,†

¹School of Materials Science and Physics, China University of Mining and Technology, Xuzhou 221116, China
²College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China

^*juntingzhang@cumt.edu.cn
^†hsun@yzu.edu.cn

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Vol. 103, Iss. 22 — 1 June 2021

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Images

Figure 1
Crystal structure of the prototype phases for (a) the Ruddlesden-Popper layered perovskite and (b) the 2D perovskite bilayer. (c) Schematic diagram of the three octahedral rotation modes of a perovskite bilayer: in-phase rotation (IR), out-of-phase rotation (OR), and tilt. The curved arrow represents the direction of rotation of the octahedron.
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Figure 2
Change in energy as a function of the amplitude of each soft mode for the strain-free ${Ca}_{3} {Mn}_{2} O_{7}$ bilayer. The tilt mode is also included even though it is not a soft mode. The other modes except the three rotation modes represent the deformation of the octahedron.
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Figure 3
(a) Energy of each structural phase as a function of epitaxial strain for the strained ${Ca}_{3} {Mn}_{2} O_{7}$ bilayer. The strain value is defined as $ɛ = (d - d_{0}) / d_{0}$ , where $d$ and $d_{0}$ represent the in-plane lattice constants of the strained and unstrained (the prototype phase) bilayers, respectively. Note that for $0 %$ strain, the in-plane lattice constants of all structural phases are fixed to be the same as that of the prototype phase. (b) Change in the magnitude of the IR ( $M_{2}^{+}$ ), tilt ( $M_{5}^{-}$ ), and polar ( $Γ_{5}^{-}$ ) modes with epitaxial strain in the ground-state $P 2_{1} a m$ phase. (c) The energy of all AFM orders (relative to FM) of the ground-state structures as functions of epitaxial strain.
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Figure 4
Change in energy of the polar $P 2_{1} a m$ phase of the $0 %$ strained ${Ca}_{3} {Mn}_{2} O_{7}$ bilayer with the amplitudes of IR and tilt modes in the (a) absence and (b) presence of the polar ( $Q_{Γ_{5}^{-}}$ ) mode. For the nonpolar phase established by OR plus tilt distortion, the change in energy with the mode amplitudes in the (c) absence and (d) presence of the antipolar ( $Q_{Γ_{5}^{+}}$ ) mode.
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Figure 5
(a) Schematic of different ferroelectric switching paths of the bilayer through a one-step or a two-steps switching to reverse the IR or tilt mode. The curved arrows represent the directions of octahedral rotation in the IR and tilt modes. (b) Energy of the ${Ca}_{3} {Mn}_{2} O_{7}$ bilayer (at $0 %$ strain) as a function of switching coordinate along different ferroelectric switching paths.
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Figure 6
(a) The components of the DM vectors of the in-plane and out-of-plane NN bonds of the ${Ca}_{3} {Mn}_{2} O_{7}$ bilayer (at $0 %$ strain) as functions of the rotation angle of single IR or tilt mode. Changes in the components of (b) the DM vectors and (c) the induced magnetization M along the lowest-energy ferroelectric switching path. (d) Schematic of the directions of polarization (P), AFM vector (L), DM vector (D), and the induced magnetization (M) in the initial, intermediate, and final states of the lowest-energy ferroelectric switching.
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