Two-Dimensional Hyperferroelectric Metals: a Different Route to Ferromagnetic-Ferroelectric Multiferroics

TL;DR

This paper introduces the concept of 2D hyperferroelectric metals, demonstrating their potential to realize ferromagnetic-ferroelectric multiferroics without the need for insulating materials, through first-principle calculations on CrN and CrB2.

Contribution

It proposes and validates the novel concept of 2D hyperferroelectric metals enabling FM-FE multiferroics in metallic systems, expanding the scope of multiferroic materials.

Findings

CrN is a hyperferroelectric metal with FM ground state.

CrB2 exhibits a metastable FM/FE state suitable for electric control of magnetism.

New mechanisms like spin-phonon coupling stabilize out-of-plane polarization.

Abstract

Recently, two-dimensional (2D) multiferroics have attracted numerous attention due to their fascinating properties and promising applications. Although the ferroelectric (FE)-ferroelastic and ferromagnetic (FM)-ferroelastic multiferroics have been observed/predicted in 2D systems, 2D ferromagnetic-ferroelectric (FM-FE) multiferroics remain to be discovered since FM insulators are very rare. Here, we proposed for the first time the concept of 2D hyperferroelectric metals, with which the insulating prerequisite for the FM-FE multiferroic is no longer required in 2D systems. We validate the concept of 2D hyperferroelectric metals and 2D metallic FM-FE multiferroics by performing first-principle calculations on 2D CrN and CrB2 systems. The 2D buckled monolayer CrN is found to be a hyperferroelectic metal with the FM ground state, i.e., a 2D FM-FE multiferroic. With the global optimization approach, we find the 2D CrB2 system has an antiferromagnetic (AFM)/planar ground state and a FM/FE metastable state, suggesting that it can be used to realize electric field control of magnetism. Our analysis demonstrates that the spin-phonon coupling and metal-metal interaction are two new mechanisms for stabilizing the out-of-plane electric polarization in 2D systems. Our work not only extends the concept of FE to metallic systems, but also paves a new way to search the long-sought high temperature FM-FE multiferroics.