In complex oxides, oxygen defects play a crucial role for physical functionalities. For instance, the presence of vacant anion sites can induce a large ionic conductivity, with relevant applications including solid oxide fuel cells, electrolyte membranes, catalysts, and oxygen sensors. [1][2][3][4] In addition, oxygen vacancy migration driven by low voltages possesses excellent resistance switching characteristics, promoting the emergence of novel oxide devices for resistive switching random access memory. [5][6][7][8] Beyond these, especially among transition metal oxides with mixed valence, oxygen off-stoichiometry alters oxidation states of the transition metal, the metal-oxygenmetal bond angles and the overall charge carrier concentration, which are essential for a multitude of unique phenomena, such as various forms of magnetism, superconductivity, magnetoresistance, and multiferroics. [9,10] Due to the profound influence of oxygen defects, particular attention has been devoted to functional oxides accommodating oxygen content variations. A significant development in recent studies is the realization of a layered vacancy ordered brownmillerite (BM) structure that can be topotactically transformed from a perovskite (PV) structure framework, in which one-sixth of the oxygen anions have been removed. [11][12][13] The special feature of this structure type is demonstrated to provide additional mechanisms for proton and oxide-ion conduction. [14][15][16] However, very few studies have been conducted regarding the dynamic behavior of oxygen vacancies and the transformation mechanisms between distinctive topotactic phases. Such mechanistic details will not only aid the fundamental understanding of oxygen vacancy physics, but also open comprehensive prospects for novel ionic conductor devices. In this view, a real-time investigation of the structural evolution is crucial for addressing the full details of the phase transition process.Strontium-doped lanthanum manganites (La 1−x Sr x MnO 3 ) exhibit a variety of unique tunable functionalities such as colossal magnetoresistance, half-metallicity, and ferromagnetism above room temperature, which can be considered as a novel platform to study the topotactic transition mechanism andThe vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure-property relationship in such systems. Here, an oxygen-vacancy-induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La 0.7 Sr 0.3 MnO 3−δ thin films is identified by real-time X-ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen-deficient in...

Synthesizing antiferromagnets with correlated oxides has been challenging, owing partly to the markedly degraded ferromagnetism of the magnetic layer at nanoscale thicknesses. Here we report on the engineering of an antiferromagnetic interlayer exchange coupling (AF-IEC) between ultrathin but ferromagnetic LaCaMnO layers across an insulating CaRuTiO spacer. The layer-resolved magnetic switching leads to sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. The magnetization configurations can be switched at moderate fields of hundreds of oersted. Moreover, the AF-IEC can also be realized with an alternative magnetic layer of LaSrMnO that possesses a Curie temperature near room temperature. The findings will add functionalities to devices with correlated-oxide interfaces.

Direct structural evidence of the periodic ordering of neutral and ionic planes is reported for 2,6dimethyltetrathiafulvalene-p-chloranil, detected by high-resolution x-ray diffraction experiments. Simultaneous to the neutral-ionic layered ordering, theoretically predicted a long time ago to occur in stages, an antiparallel dimerization ordering takes place, leading to a ''ferrielectric'' structure. Both intramolecular deformations and intermolecular contacts, related to the charge-transfer ordering and to the dimerization process, are discussed and compared with results obtained for the prototype compound of the neutral to ionic transition, tetrathiafulvalene-p-chloranil. Thus essential insights are provided for an understanding of the mechanism of this unusual electronic-structural instability, and important physical features are discussed such as the interstack interactions, the coupled order parameters, and the interplay between the staging and condensation mechanisms.

MARIA is a world class vertical sample reflectometer dedicated to the investigation of thin films in the fields of magnetism, soft matter and biology. With the elliptical vertically focusing guide and a wavelength resolution of Δλ/λ = 10%, the non-polarized flux at the sample position amounts to 1.2 × 108 n (s cm2)−1. Besides the polarized and non-polarized reflectivity mode for specular and off-specular reflectivity measurements, MARIA can also be used to carry out grazing-incidence small-angle neutron scattering investigations.