  Orthorhombic GaFeO3 (o-GFO) with the polar Pna21 space group is a prominent
ferrite by virtue of its piezoelectricity and ferrimagnetism, coupled with
magneto-electric effects. Herein, we unequivocally demonstrate a 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 SrTiO3/STO substrate show the net
switching polarization of ~35 {\mu}C/cm2 with an unusually high coercive field
of +-1400 kV/cm at room temperature. The PUND measurement also demonstrates the
switching polarization of ~26 {\mu}C/cm2. The activation energy for the
polarization switching, as obtained by density-functional theory calculations,
is remarkably high, 1.05 eV per formula unit. This high value accounts for the
observed stability of the polar Pna21 phase over a wide range of temperature up
to 1368 K.
