A Multiferroic Morphotropic Phase Boundary

Bismuth ferrite (BiFeO 3 ) thin films possess large ferroelectric polarization and antiferromagnetic order, yet their magnetoelectric coupling is limited by weak intrinsic magnetization. Here, a multiferroic morphotropic phase boundary (MPB) is demonstrated wherein the crystal structure, polarization, and magnetic order simultaneously evolve across a chemically induced phase boundary in strain‐engineered (1− x )BiFeO 3 ‐( x )BaTiO 3 thin films. Between 0.1 < x < 0.2, the crystal structure evolves from a monoclinic phase to a newly stabilized tetragonal phase through an intermediate mixed‐phase region. This structural transition is accompanied by concurrent changes in magnetic order, resulting in dramatically enhanced functional responses as compared with those of BiFeO 3 . Specifically, films with x = 0.2 exhibit larger electromechanical strains (≈ 0.3%, about three‐times larger than BiFeO 3 ) and a significantly enhanced magnetoelectric‐coupling coefficient (α ME ≈ 416 mV cm −1 Oe −1 , nearly 1000‐ and 19‐times larger than bulk and thin‐film BiFeO 3 , respectively). These enhancements diminish beyond the MPB ( x > 0.2) and arise from polarization rotation and evolving spin configurations driven by the near degeneracy of competing ferroic states at the multiferroic MPB. These results establish a rare multiferroic MPB where both the polar and magnetic order evolve simultaneously, providing a promising strategy for designing materials with strongly coupled ferroic order parameters.