Unconventional Ferroelectric Switching via Local Domain Wall Motion in Multiferroic ε‐Fe2O3 Films

Xiangxiang Guan,Richeng Yu,Sampo Inkinen,Konstantin Z Rushchanskii,Florencio Sánchez,Martí Gich,Lide Yao,Marjana Ležaić,Sebastiaan Van Dijken

doi:10.1002/aelm.201901134

Abstract

AbstractDeterministic polarization reversal in ferroelectric and multiferroic films is critical for their exploitation in nanoelectronic devices. While ferroelectricity has been studied for nearly a century, major discrepancies in the reported values of coercive fields and saturation polarization persist in literature for many materials. This raises questions about the atomic‐scale mechanisms behind polarization reversal. Unconventional ferroelectric switching in ε‐Fe2O3 films, a material that combines ferrimagnetism and ferroelectricity at room temperature, is reported. High‐resolution in situ scanning transmission electron microscopy experiments and first‐principles calculations demonstrate that polarization reversal in ε‐Fe2O3 occurs around pre‐existing domain walls only, triggering local domain wall motion in moderate electric fields of 250–500 kV cm−1. Calculations indicate that the activation barrier for switching at domain walls is nearly a quarter of that corresponding to the most likely transition paths inside ε‐Fe2O3 domains. Moreover, domain walls provide symmetry lowering of the polar structure near the domain boundary, which is shown to be necessary for ferroelectric switching in ε‐Fe2O3. Local polarization reversal in ε‐Fe2O3 limits the macroscopic ferroelectric response and offers important hints on how to tailor ferroelectric properties by domain structure design in other relevant ferroelectric materials.

Highlights

Guan, Xiangxiang; Yao, Lide; Rushchanskii, Konstantin Z.; Inkinen, Sampo; Yu, Richeng; Ležaí, Marjana; Sánchez, Florencio; Gich, Martí; van Dijken, Sebastiaan Unconventional Ferroelectric Switching via Local Domain Wall Motion in Multiferroic ε-Fe2O3 Films
We combined in situ scanning transmission electron microscopy (STEM) experiments and first-principles calculations to elucidate the origin of ferroelectric switching in multiferroic ε-Fe2O3 films
We find that ferroelectric switching proceeds locally via the motion of non-polar boundaries

Summary

In Situ STEM Experiments

We performed in situ STEM experiments on 50 nm thick εFe2O3 films. The films were grown by pulsed laser deposition (PLD) on an isostructural 8 nm thick AlFeO3 (AFO) buffer layer and a conducting (111)-oriented Nb-doped SrTiO3 (STO) substrate (see Experimental Section for details). The relative vertical displacement of the Fe2 and Fe3 ions near the domain boundary is consistent with the magnitude of smiles and cries inside the domains This observation suggests that applying 1.5–3.0 V, corresponding to an electric field of 250–500 kV cm−1, is sufficient to saturate the local polarization, in agreement with the polarization versus electric field hysteresis loop previously reported for this film.[24]. The depicted HAADF-STEM images show how the cry domain with upward polarization grows at the expense of the smile domain when we apply positive voltage pulses of increasing amplitude to the Nb-doped STO substrate. This behavior, driven by local polarization reversal at the domain wall, produces a macroscopic ferroelectric response. The restriction of polarization reversal to the vicinity of pre-existing domain walls might explain existing discrepancies between theory and experiments on ε-Fe2O3 and possibly GaFeO3 in literature

Ferroelectric Switching without Pre-Existing Domain Walls

Ferroelectric Switching with Pre-Existing Domain Walls

Conclusions

Experimental Section

Conflict of Interest