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

Abstract

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.

Dependence of the Structural and Magnetic Properties on the Growth Sequence in Heterostructures Designed by YbFeO3 and BaFe12O19

“…Xu et al [ 20 ] designed a ferroelectric/dielectric bilayer structure with h-YbFeO 3 as the improper ferroelectric (FE) and CoFe 2 O 4 as the dielectric (DE) with the goal to tune the electrostatic energy of the (FE/DE) bilayer system and to study the electrostatic effect of the DE layer on the spontaneous polarization of the FE layer. Their atomic-resolution cross-sectional scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) imaging demonstrated a periodic arrangement with “two-up-one-down” and “two-down-one-up”, corresponding to the FE polarization [ 20 , 21 , 22 ]. The influence of the film thickness ratio of the FE to DE layers on the ferroelectric polarization has been discussed [ 21 ].…”

Relevance of Platinum Underlayer Crystal Quality for the Microstructure and Magnetic Properties of the Heterostructures YbFeO3/Pt/YSZ(111)

“…Epitaxial growth of rare earth hexaferrite (h-RFeO 3 , R = Y, Dy-Lu) thin films including h-YbFeO 3 have recently attracted the attention of several researchers as it has been successfully stabilized in a metastable hexagonal structure (P63 cm) [ 1 , 2 , 3 ] by depositing it onto substrates with a hexagonal crystal structure like Al 2 O 3 [ 4 , 5 , 6 , 7 ], YSZ(111) [ 6 , 7 , 8 , 9 ], MgO [ 5 ], and substrates buffered with platinum (Pt) [ 4 , 5 , 9 ], Fe 3 O 4 [ 10 ], indium tin oxide (ITO) [ 8 , 9 , 11 ], SrTiO 3 (STO), and others [ 7 , 12 , 13 ]. Jeong et al [ 4 ] demonstrated the ferroelectricity of h-YbFeO 3 films at room temperature (RT) for a film thickness of about 60 nm, where the films were grown by pulsed laser deposition (PLD) on sputtered Pt-buffered sapphire.…”

“…There have recently been several attempts to produce h-YbFeO 3 -based devices since the growth of a bottom electrode, such as Pt [ 4 , 5 , 9 ], ITO [ 8 , 9 , 11 ] or La 2/3 Sr 1/3 MnO 3 (LSMO) [ 12 , 13 ], has become crucial for the characterization of ferroelectric and dielectric properties. Moreover, it is worthwhile to highlight the importance of tailoring the interfacial strain either by examining different substrates [ 7 ] or by introducing an underlayer, which could be simultaneously utilized as a bottom electrode.…”