多铁材料，由于具有丰富的物理现象和性质，是近二十年来的热点研究。多铁材料始于块体相多铁材料，然而铁电、铁磁的天然相互排斥，导致多铁材料存在种类少，磁电耦合能力弱等缺点。二维材料的界面接触面积大、可调控能力强等特点，给界面磁电耦合提供了新的研究思路，有望基于二维材料异质界面实现更优异的多铁性和磁电耦合性质。

Fig. 1 | Crystal and band structures of MnI3 and In2Se3 monolayers, and their stacking configurations.

来自四川大学力学系的田晓宝、梁英团队通过组合二维铁电In₂Se₃和铁磁MnI₃材料，提出了一种新的多铁异质结In₂Se₃/MnI₃结构。他们运用第一性原理计算探讨了该类异质结的结构性质和稳定性，分析了其接触特性和磁电耦合效应，获得了层间距离对接触类型和磁电耦合效应的影响规律。

Fig. 2 | Projected band structures.

作者发现，In₂Se₃/MnI₃异质结构具有失配率、形成能小的特点；通过切换In₂Se₃的极化方向可以实现界面的肖特基/欧姆接触转换，以及磁易轴和磁晶各项异性强度的变化。

Fig. 3 | Charge transfer. Schematic illustrations of the FETs designed based on the MnI3/In2Se3 vdWHs with the (a) upward and (b) downward polarized state of In2Se3. 

分析极化切换前后的能带结构、电荷转移情况，找到了接触类型变换的主要原因是，源于极化翻转对功函数的巨大影响，极化方向的切换改变了两层材料的功函数差的正负，进而导致对齐费米能级时出现不同的接触类型。

Fig. 4 | Charge density differences.

通过对极化切换对磁性性质的影响分析，发现MnI₃具有稳定4μB面内铁磁性，极化翻转可以改变磁晶各项异性强度和磁易轴方向，呈现对MnI₃的磁矩具有很好的调控作用。

Fig. 5 | AFM configurations and total energy.

作者进一步预测了不同堆垛类型和层间间距等因素，对In₂Se₃/MnI₃异质结结构的稳定可调接触性和可调磁矩方向等行为规律，为实现“电写磁读”存储器的多铁异质结构设计和开发提供新的可能。

相关论文近期发布于npj Computational Materials10, 238 (2024). 手机阅读原文，请点击本文底部左下角“阅读原文”，进入后亦可下载全文PDF文件。

Fig. 7 | The effect of interlayer distance.

Editorial Summary

Multiferroic materials have become a popular research topic over the past two decades due to their rich physical phenomena and properties. Initially, research focused on bulkphase multiferroic materials, but the inherent repulsion between ferroelectricity and ferromagnetism limits their variety and weakens their magnetoelectric coupling capabilities. The rapid development of two-dimensional (2D) materials offers new research avenues, leveraging the large atomic contact area and strong tunability of 2D materials to achieve superior multiferroic and magnetoelectric properties.

The team led by Xiaobao Tian from the Department of Mechanics at Sichuan University has proposed a new half-metallic multiferroic heterostructure, MnI₃/In₂Se₃, by combining the 2D ferroelectric In₂Se₃ and the ferromagnetic MnI₃ materials. Using first-principles calculations, they explored the structural properties and stability of this heterostructure, analyzing its contact characteristics and magnetoelectric coupling effects. Additionally, they investigated the impact of interlayer distance on contact types and magnetoelectric coupling effects.

Their findings indicate that this heterostructure features low mismatch rates and formation energies. By switching the polarization direction of In₂Se₃, they could achieve Schottky/Ohmic contact and observe changes in magnetic easy axis and magnetic anisotropy strength. Band structure and charge transfer analysis before and after polarization switching revealed that the variation in contact type primarily results from the significant impact of polarization reversal on the work function. The change in polarization direction alters the sign of the work function difference between the two materials, leading to different contact types when aligning the Fermi levels.

Further analysis of the influence of polarization switching on magnetic properties showed that MnI₃ exhibits stable 4μB in-plane ferromagnetism, with its magnetic moment direction controllable by the polarization direction, demonstrating magnetoelectric coupling properties. This effect arises from the change in magnetic anisotropy strength and magnetic easy axis direction due to polarization switching. The underlying mechanism involves the regulation of spin-orbit coupling (SOC) strength in Mn-I by polarization. Additionally, research on different stacking types and interlayer distances further revealed that the In₂Se₃/MnI₃ heterostructure possesses stable and adjustable contact characteristics and magnetic moment directions. This article was recently published in npj Computational Materials 10:238 (2024).

原文Abstract及其翻译

Tunable Schottky barriers and magnetoelectric coupling driven by ferroelectric polarization reversal of MnI₃/In₂Se₃ multiferroic heterostructures

Tao Zhang,Hao Guo, Jiao Shen, Ying Liang, Haidong Fan, Wentao Jiang, Qingyuan Wang & Xiaobao Tian 

Abstract Two-dimensional (2D) multiferroic materials are recognized as promising candidates for next-generation nanodevices due to their tunable magnetoelectric coupling and distinctive physical phenomena. In this study, we proposed a novel 2D multiferroic van der Waals heterostructure (vdWH) by stacking atomic layers of ferroelectric In₂Se₃ and ferromagnetic MnI₃. Using first-principles calculations, we found that the MnI₃/In₂Se₃ vdWH exhibit robust metallic conductivity across various spin and polarization states, preserving the distinctive band characteristics of isolated In₂Se₃ and MnI₃. However, the alignment of Fermi levels causes the conduction band minimum (CBM) and valence band maximum (VBM) of In₂Se₃ and MnI₃ to shift relative to their original band structures. Remarkably, the MnI₃/In₂Se₃ with the upward polarization state of In₂Se₃ exhibits an Ohmic contact. Switching the polarization direction of In₂Se₃ from upward to downward can transform the MnI₃/In₂Se₃ vdWH from an Ohmic contact to a p-type Schottky contact, while also modifying its dipole moment, magnetic strength and direction. Based on these properties of MnI₃/In₂Se₃ vdWH, we designed the field-effect transistors (FETs) with high on/off rates and nonvolatile data storage device. Furthermore, the Schottky barrier heights (SBHs), magnetic moment, and dipole moment of MnI₃/In₂Se₃ vdWH can also be effectively regulated by reducing the interlayer distance. With the continuous reduction of the interlayer distance of MnI₃/In₂Se₃ vdWH, its easy magnetization axis is expected to shift from in-plane to out-of-plane. These findings offer new insights for the design and development of the next-generation spintronic and nonvolatile memory nanodevices.

摘要 二维多铁性材料由于其可调谐的磁电耦合和独特的物理现象而被认为是下一代纳米器件的潜在候选材料。本研究中，通过铁电In₂Se₃和铁磁MnI₃原子层的堆叠，我们提出了一种新的二维多铁范德华异质结构In₂Se₃/MnI₃（vdWH）。利用第一性原理计算，我们发现In₂Se₃/MnI₃ vdWH在各种自旋和极化状态下都表现出强大的金属导电性，并保留了独立In₂Se₃和MnI₃的独特能带特征。然而，费米能级的排列导致In₂Se₃和MnI₃的导带底（CBM）和价带顶（VBM）相对于其原始能带结构发生偏移。当铁电极化层In₂Se₃的极化状态背向MnI₃层时，In₂Se₃/MnI₃呈现欧姆接触。将In₂Se₃的极化方向由背向至面向MnI₃切换，可以使In₂Se₃/MnI₃ vdWH由欧姆接触转变为p型肖特基接触，同时改变异质结的电偶极矩，磁矩大小和方向。基于In₂Se₃/MnI₃ vdWH的接触特性，本文设计了具有高开关率的场效应晶体管（FETs）和非易失性数据存储器件。此外，通过简单调节层间距离可以有效调节In₂Se₃/MnI₃ vdWH的肖特基势垒高度（SBHs）、磁矩和电偶极矩。随着In₂Se₃/MnI₃ vdWH层间距离的不断减小，其易磁化轴还有望由面内向面外转变。这些特性的发现，为下一代自旋电子和非易失性存储纳米器件的设计和开发提供了新的认识。