A Multiferroic Morphotropic Phase Boundary.

Bismuth ferrite (BiFeO3) 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)BiFeO3-(x)BaTiO3 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 BiFeO3. Specifically, films with x = 0.2 exhibit larger electromechanical strains (≈ 0.3%, about three-times larger than BiFeO3) and a significantly enhanced magnetoelectric-coupling coefficient (αME ≈ 416 mV cm-1 Oe-1, nearly 1000- and 19-times larger than bulk and thin-film BiFeO3, 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.