Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface

Atomic insight into spin, charge and lattice modulations at SrFeO3-x/SrTiO3 interfaces

Novel phenomena and functionalities at interfaces of oxide heterostructures are currently of great interest in a wide range of applications. At such interfaces, charge, spin, orbital and lattice ordering coexist and correlate closely, contributing to rich functional responses. By using atomically resolved imaging and spectroscopy techniques, we investigated magnetic behaviors and structural modulation at the SrFeO_3-x/SrTiO₃ interface. Fe/Ti element intermixing and oxygen vacancies occurred across a few unit cells at the interface. Furthermore, antiferromagnetic spin ordering of Fe with different valence states in the interface of SrFeO_3-x/SrTiO₃ induced uncompensated magnetic moments. Compared to the SrFeO_3-x/La_0.3Sr_0.7Al_0.65Ta_0.35O₃ heterojunction, the variations of charge and lattice order parameters at the SrFeO_3-x/SrTiO₃ interfaces were also determined by advanced electron microscopy, which provided a good understanding of the physical origin of disparate macroscopic magnetic properties, further investigated by magnetometer measurements and X-ray magnetic circular dichroism (XMCD) spectra. These studies provide comprehensive insight into the interfacial modulation of ferrite oxide, which may be useful for designing future devices in oxide electronics.

Effects of the Heterointerface on the Growth Characteristics of a Brownmillerite SrFeO2.5 Thin Film Grown on SrRuO3 and SrTiO3 Perovskites

Manipulation of the heterointerfacial structure and/or chemistry of transition metal oxides is of great interest for the development of novel properties. However, few studies have focused on heterointerfacial effects on the growth characteristics of oxide thin films, although such interfacial engineering is crucial to determine the growth dynamics and physical properties of oxide heterostructures. Herein, we show that heterointerfacial effects play key roles in determining the growth process of oxide thin films by overcoming the simple epitaxial strain energy. Brownmillerite (SrFeO_2.5; BM-SFO) thin films are epitaxially grown along the b-axis on both SrTiO₃(001) and SrRuO₃/SrTiO₃(001) substrates, whereas growth along the a-axis is expected from conventional epitaxial strain effects originating from lattice mismatch with the substrates. Scanning transmission electron microscopy measurements and first principles calculations reveal that these peculiar growth characteristics of BM-SFO thin films originate from the heterointerfacial effects governed by their distinct interfacial structures. These include octahedral connectivity between dissimilar oxides containing different chemical species and a peculiar transition layer for BM-SFO/SrRuO₃/SrTiO₃(001) and BM-SFO/SrTiO₃(001) heterostructures, respectively. These effects enable subtle control of the growth process of oxide thin films and could facilitate the fabrication of novel functional devices.

Atomic-scale structure of misfit dislocations in CeO2/MgO heterostructures and thermodynamic stability of dopant-defect complexes at the heterointerface

Complex oxide heterostructures and thin films have found applications across the board in some of the most advanced technologies, wherein the interfaces between the two mismatched oxides influence novel functionalities. It is imperative to comprehend the atomic-scale structure of misfit dislocations, which are ubiquitous in semi-coherent oxide heterostructures, and obtain a fundamental understanding of their interaction with point defects and dopants to predict and control their interface-governed properties. Here, we report atomistic simulations elucidating the atomic-scale structure of misfit dislocations in CeO₂/MgO heterostructures. Our results demonstrate that the 45° rotation of CeO₂ thin film is one of the potential fundamental mechanisms responsible for eliminating the surface dipole, leading to the experimentally observed mixed epitaxial relationship. We further report the thermodynamic stability of diverse dopant-defect complexes near misfit dislocations, wherein various scenarios for nearest neighbor bonding environments within the complexes are explored. Complex misfit dislocation structure, asymmetry, strain, and the availability of diverse nearest neighbor bonding environments between dopants and oxygen defects at the interface are accountable for a wide dispersion in energies within a given dopant-defect arrangement. As opposed to the bulk, the thermodynamic stability of oxygen vacancies is found to be sensitive to the dopant arrangement at the heterointerface. Extended stabilities of dopant-defect complexes at misfit dislocations reveal that they would influence ionic transport at heterointerfaces of fluorite-structured thin film electrolytes. Notably, the results herein offer a fundamental atomic-scale perspective of the intricate interplay between dopants, defects, and misfit dislocations at the heterointerfaces in mismatched oxide heterostructures.

Antisite-disorder engineering in La-based oxide heterostructures via oxygen vacancy control

It has been realized lately that disorder, primarily in the form of oxygen vacancies, cation stoichiometry, atomic inter-diffusion and antisite defects, has a major effect on the electronic and transport properties of a 2D electron liquid at oxide hetero-interfaces - the first and the last being the two key players. In order to delineate the roles of these two key factors, we have investigated the effect of oxygen vacancies on the antisite disorder at a large number of interfaces separating two La-based transition metal oxides, using density functional theory. The oxygen vacancy is found to suppress antisite disorder in some heterostructures thereby stabilizing the ordered structure, while in some others, it tends to favor disorder, opening up the possibility of using it to control the order. Our calculations show that the oxygen vacancy offers an opportunity to generate new magnetic states by manipulating the inter-site coupling. Moreover, it can be used to control the electrical transport. The oxygen vacancy and antisite disorder are intrinsic to oxide heterostructures and it is therefore incumbent to engineer the latter and tune the magnetic and transport properties by controlling the oxygen partial pressure during growth.