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Doping induced itinerant ferromagnetism and enhanced ferroelectricity in BL-InSe

Junlan Shi, Li Chen, Jiani Zhang, Botao Fu

TL;DR

Addressing the challenge of coexisting ferroelectricity and ferromagnetism in 2D materials, the paper demonstrates that stacking nonpolar monolayers and tuning carrier density can induce both orders in bilayer InSe. Using first-principles DFT with vdW corrections and NEB switching analysis, it shows AB-stacked BL-InSe develops a switchable out-of-plane polarization and a low-energy switching path, while hole doping triggers itinerant ferromagnetism via a Mexican-hat valence band, with a polarization that can be enhanced by doping. The interlayer charge and spin redistributions couple the ferroelectric and magnetic orders, yielding a linear relationship between interlayer spin density and doping and enabling magnetoelectric coupling when switching AB/BA. Overall, BL-InSe provides a viable, voltage-tunable multiferroic platform in a 2D nonpolar semiconductor, illustrating a general stacking- and doping-driven route to multiferroicity in layered materials.

Abstract

The microscopic coexistence of ferroelectricity and ferromagnetism in solids remains a fundamental challenge in condensed matter physics, with far-reaching implications for multifunctional materials and next-generation electronic devices. Using first-principles calculations, we predict emergent sliding ferroelectricity and doping-mediated ferromagnetism in bilayer (BL) InSe. The energetically favored AB stacked BL-InSe spontaneously breaks the out-of-plane mirror symmetry, resulting in a switchable polarization with a saturated component of 0.089 pC/m and a low transition barrier of 28.8 meV per unit cell. Strikingly, low-concentration electrostatic doping enhances rather than suppresses the ferroelectric polarization due to the abnormal layer-dependent electronic occupation in BL-InSe, in contrast to the conventional screening paradigm. In addition, the characteristic Mexican-hat-shaped valence band enables doping-induced itinerant half-metallic ferromagnetism, where the interlayer spin density difference scales linearly with doping concentration and can be reversed by switching the polarization direction. These results demonstrate the coexistence of ferroelectric and ferromagnetic orders in BL-InSe and establish a viable platform for realizing voltage-tunable multiferroicity through stacking and carrier doping in otherwise nonpolar and nonmagnetic semiconductors.

Doping induced itinerant ferromagnetism and enhanced ferroelectricity in BL-InSe

TL;DR

Addressing the challenge of coexisting ferroelectricity and ferromagnetism in 2D materials, the paper demonstrates that stacking nonpolar monolayers and tuning carrier density can induce both orders in bilayer InSe. Using first-principles DFT with vdW corrections and NEB switching analysis, it shows AB-stacked BL-InSe develops a switchable out-of-plane polarization and a low-energy switching path, while hole doping triggers itinerant ferromagnetism via a Mexican-hat valence band, with a polarization that can be enhanced by doping. The interlayer charge and spin redistributions couple the ferroelectric and magnetic orders, yielding a linear relationship between interlayer spin density and doping and enabling magnetoelectric coupling when switching AB/BA. Overall, BL-InSe provides a viable, voltage-tunable multiferroic platform in a 2D nonpolar semiconductor, illustrating a general stacking- and doping-driven route to multiferroicity in layered materials.

Abstract

The microscopic coexistence of ferroelectricity and ferromagnetism in solids remains a fundamental challenge in condensed matter physics, with far-reaching implications for multifunctional materials and next-generation electronic devices. Using first-principles calculations, we predict emergent sliding ferroelectricity and doping-mediated ferromagnetism in bilayer (BL) InSe. The energetically favored AB stacked BL-InSe spontaneously breaks the out-of-plane mirror symmetry, resulting in a switchable polarization with a saturated component of 0.089 pC/m and a low transition barrier of 28.8 meV per unit cell. Strikingly, low-concentration electrostatic doping enhances rather than suppresses the ferroelectric polarization due to the abnormal layer-dependent electronic occupation in BL-InSe, in contrast to the conventional screening paradigm. In addition, the characteristic Mexican-hat-shaped valence band enables doping-induced itinerant half-metallic ferromagnetism, where the interlayer spin density difference scales linearly with doping concentration and can be reversed by switching the polarization direction. These results demonstrate the coexistence of ferroelectric and ferromagnetic orders in BL-InSe and establish a viable platform for realizing voltage-tunable multiferroicity through stacking and carrier doping in otherwise nonpolar and nonmagnetic semiconductors.
Paper Structure (8 sections, 6 equations, 5 figures)

This paper contains 8 sections, 6 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Computational methods
  3. RESULTS AND DISCUSSION
  4. Stacking Landscape of BL-InSe
  5. Sliding Ferroelectricity in BL-InSe
  6. Tunable Ferroelectricity via Electrostatic Doping
  7. Doping-Induced Itinerant Ferromagnetism
  8. summary

Figures (5)

  • Figure 1: (a) Side and top views of BL-InSe in AB, AA, and BA stackings with arrows indicating the interlayer sliding vectors. (b)-(c) Interlayer binding energy and interlayer distance versus sliding displacement. (d) Top view of AD stacked BL-InSe.
  • Figure 2: (a)-(b) Phonon spectra of the AA and AB stackings. (c) Energy profile of ferroelectric switching as a function of the number of CI-NEB steps. (d)-(f) Differential charge density and planar-averaged charge density maps for the AB, AD, and BA phases, respectively. The yellow and cyan regions denote charge accumulation and depletion.
  • Figure 3: (a) Differential charge density between doped and undoped BL-InSe at $n$=7.04$\times$10$^{13}$ cm$^{-2}$. (b) The planar-averaged charge density along $z$ for various doping levels. (c) The relative polarization versus hole concentration with an inset highlighting the differential charge near $Z_0/2$ at the two marked points ($n_c = 17.39 \times 10^{13},\text{cm}^{-2}$). (d) Schematic diagram illustrating the effect of hole doping on OOP polarization.
  • Figure 4: (a) and (b) show the band structure and DOS of the AB-stacking BL-InSe using the PBE functional and the HSE06 method, respectively. (c) and (d) present the band projections and PDOS for In and Se atoms in the of BL-InSe, respectively. (e) and (f) depict the three-dimensional Mexican hat-shaped band structures corresponding to VB1 amd VB2 in (a).
  • Figure 5: (a) Spin magnetic moment and spin-polarization energy of AB-stacked BL-InSe as functions of hole doping concentration. (b) Spin-polarized band structure at $n_0=1.83\times 10^{13}\text{cm}^{-2}$, with the corresponding spin charge density distribution shown in (c). (d) Interlayer spin charge difference as a function of hole doping concentration for AB and BA stackings