Absence of critical thickness in improper ferroelectric hexagonal-YbFeO3 thin films

Improper ferroelectrics are highly promising for technological applications due to their expected persisting polarizations even at ultrathin limits. However, the evolution of their electrical behaviors with thickness, including the magnitude of the polarization and the switching mechanism, remain unresolved experimentally. This is primarily due to the difficulty in growth and characterization of ultrathin improper ferroelectric films. Here, we investigated the spontaneous polarization and switching mechanism in ferroelectric/dielectric bilayer structures, which allows the decoupling of the electrostatic and interfacial effects and circumventing the need for ultrathin films. We show that, for the bilayer structures of prototypical improper ferroelectrics h-YbFeO3 and dielectrics CoFe2O4, although the effective spontaneous polarization is significantly reduced by the dielectric layer due to the electrostatic under-screening, it persists at least down to a ferroelectric/dielectric thickness ratio of about 2, with no evidence of critical thickness. Interfacial clamping that suppresses the primary structural distortion of h-YbFeO3 have been observed, which appears to play an important role in ferroelectric domain pinning. The microstructure caused by the heteroepitaxy favors a nucleation-limited polarization switching dynamics. These results demonstrate the much-desired absence of critical thickness in improper ferroelectrics for the scalable thin-film device applications; they also reveal significant impact of the mismatched film/substrate epitaxy on the polarization switching mechanism.

Investigation on Domain Pinning Mechanism in Nanometer Hard Magnetic Materials

The domain pinning mechanism was investigated in nanometer hard magnetic materials. The pinning fields of domain wall at different inhomogeneities were studied respectively. And exchange-coupling coefficient αex was investigated too. The results showed that αex is proportional to the ratio of inhomogeneity thickness ro to wall width δm for narrow inhomogeneity, while it decreases with enhancement of ro/δm for extended inhomogeneity. At a certain value of ro, exchange-coupling coefficient and pinning fields will reach the maximum. Exchange-coupling interaction and pinning fields are greatly influenced by inhomogeneity. Control range of inhomogeneity may obtain higher coercivity.

Domain Pinning and Valence State Switching in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 Heterostructures

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.