ABSTRACT:Strain is known to greatly influence low temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity essential for fuel cells and metal-air batteries. However, its catalytic impact on transition metal oxide (TMO) thin films, such as perovskites, is not widely understood. Here, we epitaxially strain the conducting perovskite LaNiO 3 to systematically determine its influence on both the oxygen reduction (ORR) and oxygen evolution reaction (OER). Uniquely, we found that compressive strain could significantly enhance both reactions, yielding a bifunctional catalyst that surpasses the performance of noble metals such as Pt. We attribute the improved bifunctionality to straininduced splitting of the e g orbitals, which can customize orbital asymmetry at the surface. Analogous to straininduced shifts in the d-band center of noble metals relative to Fermi level, such splitting can dramatically affect catalytic activity in this perovskite and other potentially more active oxides. Advancements in energy storage are essential for driving the development of more sophisticated mobile technologies as well as continuing the trend towards a greener economy. At the forefront of this push are high energy density devices, such as regenerative fuel cells and metal-air batteries. 1 In these and related electrochemical systems, both the oxygen reduction and oxygen evolution reactions (ORR and OER, respectively) are crucial towards successful operation. 2 Traditionally, conductive catalysts incorporating noble metals (e.g., Pt and IrO 2 ) have been used to facilitate these reactions near room temperature. 3 To alleviate costs and poor stabilities during OER in alkaline solutions, significant efforts have focused on transition metal oxides (TMOs) with multivalent Ni, Fe, Co, and Mn, such as NiFeO x .4 Similar to alloying in noble metals, the majority of research into increasing oxygen activities of TMOs involves cationic doping, which often promotes either the ORR or OER but not bifunctionality. 5Here, we explore how another factor, i.e. strain, can influence bifunctionality in TMOs. Contemporary work on high-temperature (>500 C) oxide catalysis in an aprotic environment (e.g. ORR: O 2 + 4e -→ 2O 2-) has emphasized the importance of tensile strain for activating defects to improve catalytic reactions. 6 In an alkaline environment (e. . Among TMOs, the ABO 3 perovskites, in which A is traditionally from Groups I-III and B is a transition metal ion with six-fold octahedral coordination, also have electronic structures that are sensitive to strain. Due to strong hybridization between the O 2p and the transition metal d z 2 and d x 2 -y 2 lobes in the BO 6 octahedra, the σ* states near E F consist of e g orbitals. 10,10b Similar to the Jahn-Teller distortion, strain is known to lift the degeneracy in these symmetry-localized d z 2 and d x 2 -y 2 orbitals, yielding changes in the orbital occupancy, or polarization.7a, 11 By using strain to control the degree of this ...

Oxygen vacancies in transition-metal oxides facilitate catalysis critical for energy storage and generation. However, promoting vacancies at the lower temperatures required for operation in devices such as metal-air batteries and portable fuel cells has proven elusive. Here we used thin films of perovskite-based strontium cobaltite (SrCoOx) to show that epitaxial strain is a powerful tool for manipulating the oxygen content under conditions consistent with the oxygen evolution reaction, yielding increasingly oxygen-deficient states in an environment where the cobaltite would normally be fully oxidized. The additional oxygen vacancies created through tensile strain enhance the cobaltite's catalytic activity toward this important reaction by over an order of magnitude, equaling that of precious-metal catalysts, including IrO2. Our findings demonstrate that strain in these oxides can dictate the oxygen stoichiometry independent of ambient conditions, allowing unprecedented control over oxygen vacancies essential in catalysis near room temperature.

The ability to manipulate oxygen anion defects rather than metal cations in complex oxides is facilitating new functionalities critical for emerging energy and device technologies.However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of an important defect, oxygen vacancies. Here, strontium cobaltite (SrCoO x ) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 °C. By applying a small biaxial tensile strain (2%), the oxygen activation energy barrier decreases by ~30%, resulting in a tunable oxygen deficient steady-state under conditions that would normally fully oxidize unstrained cobaltite. These strain-induced changes in oxygen stoichiometry drive the cobaltite from a ferromagnetic metal towards an antiferromagnetic insulator. The ability to decouple the oxygen vacancy concentration from its typical dependence on the operational environment is useful for effectively designing oxides materials with a specific oxygen stoichiometry.2

Strong Coulomb repulsion and spin–orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems, where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin–orbit entangled 3d–5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials.

Spin waves are investigated in Yttrium Iron Garnet (YIG) waveguides with a thickness of 39 nm and widths ranging down to 50 nm, i.e., with aspect ratios thickness over width approaching unity, using Brillouin Light Scattering spectroscopy. The experimental results are verified by a semi-analytical theory and micromagnetic simulations. A critical width is found, below which the exchange interaction suppresses the dipolar pinning phenomenon. This changes the quantization criterion for the spin-wave eigenmodes and results in a pronounced modification of the spin-wave characteristics. The presented semi-analytical theory allows for the calculation of spin-wave mode profiles and dispersion relations in nano-structures.Spin waves and their quanta, magnons, typically feature frequencies in the GHz to THz range and wavelengths in the micrometer to nanometer range. They are envisioned for the design of faster and smaller next generational information processing devices where information is carried by magnons instead of electrons [1][2][3][4][5][6][7][8][9]. In the past, spin-wave modes in thin films or rather planar waveguides with thickness-towidth aspect ratios ar = h/w << 1 have been studied. Such thin waveguides demonstrate the effect of "dipolar pinning" at the lateral edges, and for its theoretical description the thin strip approximation was developed, in which only pinning of the much-larger-in-amplitude dynamic in-plane magnetization component is taken into account [10][11][12][13][14][15]. The recent progress in fabrication technology leads to the development of nanoscopic magnetic devices in which the width w and the thickness h become comparable [16][17][18][19][20][21][22][23]. The description of such waveguides is beyond the thin strip model of effective pinning, because the scale of nonuniformity of the dynamic dipolar fields, which is described as "effective dipolar boundary conditions", becomes comparable to the waveguide width. Additionally, both, in-plane and out-of-plane dynamic magnetization components, become involved in the effective dipolar pinning, as they become of comparable amplitude.Thus, a more general model should be developed and verified experimentally. In addition, such nanoscopic feature sizes imply that the spin-wave modes bear a strong exchange character, since the widths of the structures are now comparable to the exchange length [24]. A proper description of the spin-wave eigenmodes in nanoscopic strips which considers the influence of the exchange interaction, as well as the shape of the structure, is fundamental for the field of magnonics.In this Letter, we discuss the evolution of the frequencies and profiles of the spin-wave modes in nanoscopic waveguides where the aspect ratio ar evolves from the thin film case ar → 0 to a rectangular bar with ar → 1. Yttrium Iron Garnet (YIG) waveguides with a thickness of 39 nm and widths ranging down to 50 nm are fabricated and the quasi-ferromagnetic resonance (quasi-FMR) frequencies within them are measured using microfocused Brillouin Ligh...