Strain-Induced Relaxor Multiferroicity at Room Temperature in Hexaferrite BaFe12O19 Thin Films

Abstract

Multiferroic materials that combine magnetic and electric order at room temperature are rare. Here, we demonstrate strain-induced room-temperature polar order in the ferrimagnetic hexaferrite BaFe12O19. First-principles calculations reveal a strain-tunable energy landscape with multiple competing dipolar configurations and predict that compressive strain favors polar distortions. Using an isostructural Sr1.03Ga10.81Mg0.58Zr0.58O19 substrate, we grow coherently strained BaFe12O19 films with 1.1% in-plane biaxial compression. Second-harmonic generation measurements demonstrate inversion-symmetry breaking and establish a strain-stabilized polar phase that persists to at least 1000 K. Multislice electron ptychography directly reveals enhanced off-centering of Fe3+ ions within the trigonal-bipyramidal sites of the strained films and spatially varying local polarization, demonstrating the formation of polar nanoregions. Path-integral Monte Carlo simulations further show that compressive strain suppresses quantum fluctuations and stabilizes these local polar distortions. Together, these results establish strain-engineered BaFe12O19 as a room-temperature relaxor multiferroic, in which robust ferrimagnetism coexists with nanoscale polar order. Our work demonstrates a route for transforming an incipient ferroelectric ferrimagnetic into a polar magnetic material through epitaxial strain.