Control of Oxygen Vacancy Ordering in Brownmillerite Thin Films via Ionic Liquid Gating

Abstract

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO_3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO_2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Keywords: brownmillerite; ionic liquid gating; oxygen vacancy channel; strain; strontium cobaltite.

Figure 1

Electrical control of oxygen vacancy ordering in SrCoO_x thin films. (a) Crystal structure of brownmillerite SrCoO_2.5 having horizontal OVCs (H-SCO) (left), perovskite SrCoO_3-δ (P-SCO) (center), and brownmillerite SrCoO_2.5 having vertical OVCs (V-SCO) (right). The structure can be sequentially transformed by ionic liquid gating (ILG) at different gate voltages. H-SCO transforms to P-SCO at negative gate voltages (V_g). Then, P-SCO transforms to V-SCO at positive V_g. V-SCO is transformed back to P-SCO at negative V_g. The black, blue, and red spheres denote Sr, Co, and O ions, respectively. The blue and yellow polyhedra represent octahedra and tetrahedra, respectively. (b) STEM-HAADF images of H-SCO, P-SCO, and V-SCO (from left to right) grown on STO (001) substrates. (c) Magnetization versus field curves for H-SCO, P-SCO, and V-SCO (from left to right) measured at 100 K.

Figure 2

Electronic, magnetic, and structural properties of three distinct SrCoO_x films. (a) Temperature-dependent magnetization at 1000 Oe. (b) Magnetization versus in-plane magnetic field at 100 K. (c) Temperature dependent resistivity. (d) Normalized XAS spectra in the vicinity of the Co L_2,3 absorption edge. (e) XMCD spectra recorded in remanence after applying an in-plane magnetic field of 6 T. (f) Normalized XAS O K edge spectra. XAS and XMCD were recorded in grazing incidence. (g) Reciprocal space maps of three SCO_x films on STO (001).

Figure 3

Substrate-independent oxygen vacancy ordering in SrCoO_x thin films. θ–2θ XRD scans for ionic liquid gated SCO_x films grown on (a) STO (001), (b) LSAT (001), and (c) LAO (001). Asterisks denote substrate peaks. All films grown on the three different substrates show the same phase transformation sequence from H-SCO to P-SCO at V_g = −2.5, followed by P-SCO to V-SCO at V_g = 2.7. (d) Lattice mismatch (ε) of the three phases for each substrate. The lattice mismatch was calculated based on the effective lattice constants of each phase, as summarized in SI Table S2. Positive (negative) lattice mismatch corresponds to tensile (compressive) strain in the film.

Figure 4

Phase transformations induced by thermal annealing. (a) θ–2θ XRD scans for 30 nm thick P-SCO films grown on three different substrates STO (001), LSAT (001), LAO (001). (b) θ–2θ XRD scans after annealing at 750 °C in an oxygen pressure (pO₂) of 10^–2 Torr for 30 min. All films are transformed from P-SCO to H-SCO after this annealing process.

Figure 5

In situ monitoring of the electric-field control of oxygen vacancy ordering (a) Schematic representation of the formation of V-SCO in a P-SCO film by ILG at a positive gate voltage. The green and red spheres in EDL represent positive and negative ions, respectively. The black, blue, and red spheres in the lattice denote Sr, Co, and O ions, respectively. The blue and yellow polyhedra represent octahedra and tetrahedra, respectively. (b) θ–2θ scans of an as-deposited H-SCO/STO(001) film and the same film after it was subjected to negative voltage gating (−2.5 V) followed by positive voltage gating (2.5 V). This process leads to the formation of V-SCO via an intermediate P-SCO phase. Asterisks denote substrate peaks. (c) Schematic of an in situ XRD measurement during ILG. To thin down the IL on the film surface, a Kapton film was attached to the device after applying the IL. (d) Time dependent in situ XRD measurements of several reflections characteristic for the different phases during ILG of H-SCO/LSAT (001), demonstrating electrical control of the formation of OVCs. Red arrows indicate the onset of the applied gate voltage.