Ferroelectric polarization switching with a remarkably high activation energy in orthorhombic GaFeO3 thin films

Abstract

Orthorhombic GaFeO3 (o-GFO) with the polar Pna21 space group is a prominent ferrite owing to its piezoelectricity and ferrimagnetism, coupled with magnetoelectric effects. Herein, we demonstrate large ferroelectric remanent polarization in undoped o-GFO thin films by adopting either a hexagonal strontium titanate (STO) or a cubic yttrium-stabilized zirconia (YSZ) substrate. The polarization-electric-field hysteresis curves of the polar c-axis-grown o-GFO film on a SrRuO3/STO substrate show the net switching polarization of ~35 μC cm−2 with an unusually high coercive field (Ec) of ±1400 kV cm−1 at room temperature. The positive-up and negative-down measurement also demonstrates the switching polarization of ~26 μC cm−2. The activation energy for the polarization switching, as obtained by density-functional theory calculations, is remarkably high, 1.05 eV per formula unit. We have theoretically shown that this high value accounts for the extraordinary high Ec and the stability of the polar Pna21 phase over a wide range of temperatures up to 1368 K. Researchers have clarified why the measured remanent polarization of orthorhombic GaFeO3 is about 50 times smaller than its predicted value. Multiferroics are rare materials that simultaneously show ferroelectricity and ferromagnetism, making them potentially useful for sensors and electrically controllable magnetic memories. Now, by growing thin films of the multiferroic material orthorhombic GaFeO3 on cubic and hexagonal substrates, Hyun Myung Jang of Pohang University of Science and Technology in Korea and co-workers have observed ferroelectric polarization switching that has an extraordinarily high coercive field at room temperature. Numerical calculations revealed that this switching has a remarkably high activation energy. This finding both explains the unusually high coercive field and clarifies the puzzling discrepancy between observed values for the remanent polarization of orthorhombic GaFeO3 and those calculated in previous studies. The polarization-electric-field hysteresis curves of the polar c-axis-grown orthorhombic GaFeO3 (o-GFO) film on a SrRuO3/STO substrate show the net switching polarization of ~35 μC cm−2 with an unusually high coercive field (Ec) of ±1400 kV cm−1 at room temperature. The positive-up and negative-down measurement also demonstrates the switching polarization of ~26 μC cm−2. The activation energy for the polarization switching, as obtained by density-functional theory calculations, is remarkably high, 1.05 eV per formula unit. We have theoretically shown that this high value accounts for the extraordinary high Ec and the stability of the polar Pna21 phase over a wide range of temperature up to 1368 K.