Antiferroelectric-to-ferroelectric phase transition in hexagonal rare-earth iron oxides

Ferroic oxides often exhibit exotic behavior, accompanied by phase transitions. Hexagonal rare-earth iron oxides (h-RFeO₃), a promising multiferroic system, have been reported to exhibit ferroelectricity (FE) when the lattice parameter ratio (c/a) exceeds 1.93 and antiferroelectricity (AFE) when c/a is equal to 1.89. Although the AFE–FE phase boundary in the h-RFeO₃ systems is assumed to exist at c/a ≈ 1.9, the phase transition has not been observed so far due to the lack of samples with such a c/a ratio. In this study, we show the AFE–FE phase transition in h-RFeO₃ films, where R = Dy. We fabricated h-DyFeO₃ films with c/a ratios of 1.90–1.92 by controlling the film thicknesses. The h-DyFeO₃ films with a c/a ratio of 1.91 exhibited AFE at temperatures below 200 K and FE at temperatures up to 300 K. The phase transition temperature (T_p) was modulated by the c/a ratio. The films also underwent an AFE–FE phase transition upon adjusting the frequency of the voltage applied at the T_p. We discuss the possible origin of the AFE–FE phase transition from the viewpoint of the migration length of the FE domain wall motion.