Mechanically and electrically switchable triferroic altermagnet in a pentagonal FeO2 monolayer

TL;DR

This paper reports the discovery of a pentagonal FeO₂ monolayer that intrinsically exhibits coupled ferroelectric, ferroelastic, and altermagnetic orders, enabling electric and mechanical control of its magnetic properties for spintronics.

Contribution

It introduces a novel pentagonal monolayer FeO₂ as an intrinsic triferroic altermagnet with coupled ferroic orders, expanding the scope of 2D multiferroics and demonstrating control pathways.

Findings

FeO₂ monolayer exhibits coexisting FE, FA, and AM orders.

Electric fields and strain can switch polarization and magnetic states.

The material has Néel temperatures over 200 K.

Abstract

Two-dimensional multiferroics promise low-power, multifunctional devices, yet the intrinsic coexistence and mutual control of three coupled ferroic orders in a single layer remains elusive. Here, we identify pentagonal monolayer FeO$_2$ as an intrinsic triferroic altermagnet where ferroelectric (FE), ferroelastic (FA), and altermagnetic (AM) orders coexist and are tightly coupled, accompanied by a competing antiferroelectric (AFE) phase using first-principles calculations. The sole presence of glide mirror $M_x$ symmetry in a FeO$_2$ sublayer, with the breaking of four-fold rotation $C_{4z}$ symmetry, induces in-plane vector ferroelectricity and twin-related ferroelastic strains. Both FE and AFE phases break combined parity - time symmetry and display sizable altermagnetic spin splitting with N\'eel temperatures over 200~K. Electric-field-induced rotation of the FE polarization reverses the sign of the spin splitting, while in-plane uniaxial strain triggers ferroelastic switching that simultaneously rotates the FE polarization vector by $90^\circ$ and reverses the AM state. These electric-field- and strain-mediated pathways interlink six distinct polarization states that can be selected purely by electric fields and/or mechanical strain. This work extends intrinsic triferroicity to pentagonal monolayers and outlines a symmetry-based route toward mechanically and electrically configurable altermagnetic spintronics.