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Active control over the concentration or distribution of oxygen vacancies in transition metal oxides enables manipulation of their structural, magnetic, electronic transport, and optical properties. Topotactic oxidation or reduction reactions involving annealing under different ambient conditions are often used to manipulate the oxygen concentration. In this paper, using in situ scanning transmission electron microscopy, the authors demonstrate a reversible structural phase transition in perovskite La${}_{0.5}$Sr${}_{0.5}$CoO${}_{3\ensuremath{-}\ensuremath{\delta}}$ films during heating/cooling cycles while the environment and oxygen vacancy concentration are kept constant. Switching between two oxygen-deficient structures is shown to arise from a local reordering of the oxygen vacancies by thermal strain imposed by the substrate. This approach presents new opportunities for switchable ionic devices.

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.

Resistive switching in transition metal oxides involves intricate physical and chemical behaviours with potential for non-volatile memory and memristive devices. Although oxygen vacancy migration is known to play a crucial role in resistive switching of oxides, an in-depth understanding of oxygen vacancy-driven effects requires direct imaging of atomic-scale dynamic processes and their real-time impact on resistance changes. Here we use in situ transmission electron microscopy to demonstrate reversible switching between three resistance states in epitaxial La2/3Sr1/3MnO3 films. Simultaneous high-resolution imaging and resistance probing indicate that the switching events are caused by the formation of uniform structural phases. Reversible horizontal migration of oxygen vacancies within the manganite film, driven by combined effects of Joule heating and bias voltage, predominantly triggers the structural and resistive transitions. Our findings open prospects for ionotronic devices based on dynamic control of physical properties in complex oxide nanostructures.

Epitaxial films and heterostructures of perovskite oxides attract tremendous scientific interest because of the unique phenomena therein. Especially important is the epitaxial growth of films subjected to substrate-induced misfit strain. We show here that in contrast to conventional misfit-controlled epitaxy, chemical bonds determine the crystal stability and strain in epitaxial films of sodium niobate on different cubic substrates. Strain relaxation in sodium niobate is independent of misfit magnitude and proceeds through perovskite-specific tilting of oxygen octahedra in addition to common defect formation. The observed structural relaxation evidences a major role of a large internal strain that originates from chemical bonds in the perovskite cell. The effect of chemical bonds on film strain is anticipated to also control the epitaxy of other perovskite oxides and related compounds.

The angular dependencies of the critical current of BaCeO3 (BCO) doped YBa2Cu3O6+x (YBCO) thin films grown with pulsed laser deposition (PLD) on SrTiO3 (001) were systematically investigated. The BCO concentration in the YBCO matrix was varied between 0-8 wt.%. Transmission electron microscopy confirmed that the 4% BCO containing sample has point-like, partly agglomerated BCO particles with diameter of 2.5 nm. The dopant degrades the crystal quality of YBCO but improves its in-field current carrying performance especially at low temperatures. The BCO addition affects the angular dependence of the critical current by broadening the B parallel to ab-axis peak of YCBO but no c-axis peak is seen, which is contrary to BaZrO3 doped films made by PLD. Although the particles are point-like, it is found that a model featuring only isotropic pinning centers is not sufficient to describe the angular dependence of the samples where B parallel to ab-axis of YCBO.

Resistive switching in transition metal oxides involves intricate physical and chemical behaviours with potential for non-volatile memory and memristive devices. Although oxygen vacancy migration is known to play a crucial role in resistive switching of oxides, an in-depth understanding of oxygen vacancy-driven effects requires direct imaging of atomic-scale dynamic processes and their real-time impact on resistance changes. Here we use in situ transmission electron microscopy to demonstrate reversible switching between three resistance states in epitaxial La2/3Sr1/3MnO3 films. Simultaneous high-resolution imaging and resistance probing indicate that the switching events are caused by the formation of uniform structural phases. Reversible horizontal migration of oxygen vacancies within the manganite film, driven by combined effects of Joule heating and bias voltage, predominantly triggers the structural and resistive transitions. Our findings open prospects for ionotronic devices based on dynamic control of physical properties in complex oxide nanostructures., An in-depth understanding of oxygen vacancy-driven effects is necessary for the development of functional ionic devices. Using simultaneous high-resolution imaging and resistance probing, Yao et al. demonstrate oxygen vacancy-driven structural and resistive transitions between three distinct phases in La2/3Sr1/3MnO3.

We measured the resistivity of pulsed-laser-deposited BaCeO3 (BCO)-doped YBCO thin films containing spherical BCO particles in fields up to 30 T. The average diameter of the particles depends on the dopant concentration being below 4 nm in all the samples. Raised values of the upper critical field, B-c2, were observed in all the samples. Additionally, the parameter gamma, describing the electron mass anisotropy, decreased from 6.2 in the undoped sample to 3.1 in the 8 wt.% BCO-doped sample. These results can be explained by the increased number of defects decreasing the mean free path of electrons and thus lowering the coherence length, which in turn increases B-c2.

Universal, giant and nonvolatile resistive switching is demonstrated for oxide tunnel junctions with ferroelectric PbZr0.2 Ti0.8 O3 , ferroelectric BaTiO3, and paraelectric SrTiO3 tunnel barriers. The effects are caused by reversible migration of oxygen vacancies between the tunnel barrier and bottom La2/3 Sr1/3 MnO3 electrode. The switching process, which is driven by large electric fields, is efficient down to a temperature of 5 K.

Cube-on-cube epitaxy of perovskite sub-cell of Pr-doped and undoped NaNbO3 is obtained in 130-nm-thick films on top of (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) substrates. Experimental studies show that the edge of optical absorption red-shifts and some interband transitions change in the films compared to crystals. Bright red luminescence is achieved at room-temperature under ultraviolet excitation in the Pr-doped film. An interband mechanism of luminescence excitation is detected in the film, which is in contrast to the intervalence charge transfer mechanism in the crystal. The results are discussed in terms of epitaxially induced changes of crystal symmetry and ferroelectric polarization in the films. It is suggested that the band structure and interband transitions in NaNbO3and the transition probabilities in the Pr ions can be significantly modified by these changes.

Structural phase transitions driven by oxygen-vacancy ordering can drastically affect the properties of transition metal oxides. The focused electron beam of a transmission electron microscope (TEM) can be used to control structural phase transitions in epitaxial La2/3Sr1/3MnO3. The ability to induce and characterize oxygen-deficient structural phases simultaneously in a continuous and controllable manner opens up new pathways for atomic-scale studies of transition metal oxides and other complex materials.