Ionic tuning of cobaltites at the nanoscale

Abstract

Control of materials through custom design of ionic distributions represents a powerful new approach to develop future technologies ranging from spintronic logic and memory devices to energy storage. Perovskites have shown particular promise for ionic devices due to their high ion mobility and sensitivity to chemical stoichiometry. In this work, we demonstrate a solid-state approach to control of ionic distributions in (La,Sr)CoO 3 thin films.Depositing a Gd capping layer on the perovskite film, oxygen is controllably extracted from the structure, up-to 0.5 O/u.c. throughout the entire 35 nm thickness. Commensurate with the oxygen extraction, the Co valence state and saturation magnetization show a smooth continuous variation. In contrast, magnetoresistance measurements show no-change in the magnetic anisotropy and a rapid increase in the resistivity over the same range of oxygen stoichiometry.These results suggest significant phase separation, with metallic ferromagnetic regions and oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering, including transformation to a brownmillerite crystal structure. The unexpected transformation to the brownmillerite phase at ambient temperature is further confirmed by high-resolution scanning transmission electron microscopy which shows significant structural -and correspondingly chemical -phase separation. This work demonstrates room-temperature ionic control of magnetism, electrical resistivity, and crystalline structure in a 36 nm thick film, presenting new opportunities for ionic devices that leverage multiple material functionalities.

X-ray nanodiffraction studies of ionically controlled nanoscale phase separation in cobaltites

“…Therefore, the BM phase can be characterized in an x-ray diffraction (XRD) experiment by a shift of the main film Bragg reflection to lower 2 values as well as the appearance of half order reflections. Figure 1 Å ± 0.002 Å for the as-grown LSCO sample, and 3.802 Å ± 0.002 Å and 3.817 Å ± 0.002 Å for the LSCO/Gd(0.5 nm) and LSCO/Gd(3 nm) heterostructures, respectively, in agreement with literature values [27]. Unless otherwise noted, all uncertainties represent ±1 standard deviation.…”

“…The deposition of a strong oxygen getter on top of an oxide thin film has recently emerged as a novel way to tailor oxygen stoichiometry and nanoscale functional properties [7,27,28]. For example, the presence of a strong oxygen getter (e.g.…”

“…For example, the presence of a strong oxygen getter (e.g. Gd) led to the suppression of the magnetization of La0.67Sr0.33CoO3 (LSCO)/Gd heterostructures throughout the full film thickness (36 nm) [27]. In contrast, applying a similar approach to material systems such as (Ni,Co)O/GdFe heterostructures produced a thin (3 nm) layer of ferromagnetic NiCo at the interface [29].…”

“…The Au layer was deposited before breaking vacuum to prevent oxygen exchange with the ambient environment. Detailed characterization information for these films can be found in Gilbert [27].…”

Understanding the metal-to-insulator transition in La1−xSrxCoO3−δ and its applications for neuromorphic computing

“…13,14 ). Recently, an experimental study 15 suggested to control the MIT by tuning the oxygen vacancy concentration in LSCO, and a topotactic transition was realized from a paramagnetic metallic perovskite (δ%0) to an antiferromagnetic (AFM) semiconducting brownmillerite (BM) structure (δ = 0.5). Interestingly, a similar transition was also observed between SrCoO 3 and SrCoO 2.5 (SCO 2.5 ) 16,17 and oxygen vacancy concentrations have been varied in multiple ways in cobaltites, e.g.…”