Altermagnetic signatures and topological textures in ultrathin multiferroic BiFeO(3).

At four unit cells of ultrathin BiFeO3, short-circuit electrostatic boundary conditions stabilize a collinear monoclinic phase with d-wave altermagnetic order.

• Why it matters: Practical multiferroic devices need ultrathin films where ferroelectric polarization and magnetic order do not degrade, but scaling often creates magnetic dead layers.
• What they did: The work combines first-principles calculations, symmetry analysis, atomic-resolution imaging, and magnetic imaging, then uses magnetic circular dichroism and spin-resolved photoemission to probe the four-unit-cell phase.
• The result: A thickness-induced phase transition, driven by boundary conditions and epitaxial strain, is accompanied by multiferroic topological textures, enabling unconventional multiferroicity at device-relevant thicknesses.

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