Strong intrinsic multiferroism and magnetoelectric coupling in (1-x)BiFeO₃-(x)BaTiO₃ films

The coexistence of ferroelectric and antiferromagnetic order in BiFeO₃ makes it promising for next-generation magnetoelectric devices. But, single-phase multiferroics with robust room-temperature polarization and magnetization are rare. Here, enhanced, room-temperature ferroelectric polarization (≈ 120 µC cm^-2), saturation magnetization (≈ 40 emu cm^-3), and strong magnetoelectric coupling (≈ 400 mV cm^-1 Oe^-1) are observed in epitaxial (1-x)BiFeO₃-(x)BaTiO₃ thin films. These values of magnetization and magnetoelectric coupling are, respectively, one- and two-orders of magnitude larger than those same properties in the widely studied parent material BiFeO₃. This sought after combination of properties is found in a distinct tetragonal phase, which is different from rhombohedral and super-tetragonal variants of BiFeO₃, that emerges at x = 0.2 to 0.3 via combined chemical substitution and epitaxial strain. Structural and physical-property characterization, along with first-principles calculations, reveal a transition from monoclinic to tetragonal symmetry and suggest that short-range ordering of the titanium in the tetragonal phase results in ferrimagnetic spin ordering. This work demonstrates a unique single-phase multiferroic combining strong polarization, magnetization, and magnetoelectric coupling achieved through manipulation of the coupled chemical order and spin order; thereby addressing a major challenge in multiferroics research and providing a path toward practical room-temperature, efficient charge-to-spin and spin-to-charge conversion technologies.