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

Deterministic 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.
This work was supported by the Academy of Finland (Grant Nos. 293929, 304291, 319218 and 316857) and by the European Research Council (ERC‐2012‐StG 307502 and ERC‐2018‐CoG 819623). STEM analysis was conducted at the Aalto University OtaNano‐Nanomicroscopy Center (Aalto‐NMC). ICMAB acknowledges financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV‐ 2015‐0496) and the project MAT2017‐85232‐R co‐financed with the EU FEDER program, as well as the Generalitat de Catalunya (projects 2017SGR1377 and 2017SGR765). The authors thank Jaume Gàzquez for discussions about the work and critical reading of the manuscript. K.Z.R. and M.L. gratefully acknowledge the financial support by Deutsche Forschungsgemeinschaft (DFG) through DFG‐ANR GALIMEO project (LE 2504/2‐1), as well as the support of Jülich Supercomputing Centre (JSC, project jiff38) and JARA‐HPC Partition (projects jara0081 and jara0126). R.C.Y. and X.X.G. acknowledge the financial support by the National Key Research Program of China (Grant No. 2017YFA0206200) and the National Natural Science Foundation of China (Grant No. 11874413). We thank Prof. Nathalie Viart for valuable discussions.