Altermagnetic signatures and topological textures in ultrathin multiferroic BiFeO₃

Magnetoelectric multiferroics promise low-power switching, non-volatile memory and signal transduction, yet practical deployment requires ultrathin films in which polar and magnetic order often degrade. Uncompensated bound charge drives depolarization in ferroelectrics, whereas off-stoichiometry, reduced anisotropy and charge transfer can produce magnetic dead layers. Here we realize a multiferroic phase of BiFeO₃ with no dead layer, revealing signatures of emergent altermagnetism in the ultrathin limit. First-principles calculations, symmetry analysis, atomic-resolution imaging and magnetic imaging show that short-circuit electrostatic boundary conditions and epitaxial strain drive a continuous, thickness-induced phase transition accompanied by multiferroic topological textures. At four unit cells, these boundary conditions stabilize a collinear monoclinic phase with d-wave altermagnetic order, supported by magnetic circular dichroism and spin-resolved photoemission measurements. These results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics.