Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3

TL;DR

This study demonstrates that ultrathin BiFeO3 films can maintain multiferroic properties and exhibit emergent altermagnetic signatures at the atomic scale, driven by boundary conditions and strain, opening new pathways for oxide electronics.

Contribution

The paper reveals a method to stabilize multiferroic and altermagnetic phases in ultrathin BiFeO3 films through boundary conditions and strain, enabling device-relevant thicknesses.

Findings

Ultrathin BiFeO3 sustains multiferroicity at room temperature.

Emergent altermagnetic signatures observed in four-unit-cell films.

Thickness-driven phase transition enables topological textures.

Abstract

Magnetoelectric multiferroics, materials with intrinsically coupled electric polarization and magnetic order, promise ultralow-power switching, nonvolatile memory, and energy-efficient signal transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains a tremendous challenge, as uncompensated bound charge drives nanoscale depolarization in most ferroelectrics, while off-stoichiometry, reduced anisotropy, and charge transfer can produce magnetic dead layers in ultrathin oxides at substrate interfaces. Here, we realize a multiferroic phase of BiFeO3 that not only sustains both order parameters at room temperature with no dead layer but also exhibits signatures of emergent altermagnetism in the four-unit-cell, ultrathin limit. First-principles calculations, spin symmetry analysis, atomic-resolution imaging, and angle-resolved magnetic imaging reveal that short-circuit electrostatic boundary conditions, together with epitaxial strain, drive a continuous second-order, thickness-driven phase transition that enables the formation of multiferroic topological textures. Moreover, the imposed boundary conditions stabilize a d-wave altermagnetic time-reversal symmetry breaking, with corresponding signatures observed in magnetic circular dichroism. Collectively, these results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics.