Electrochemical control of oxygen stoichiometry and materials properties in ion-gel-gated cobaltite thin films

Abstract

Wide-ranging control of materials properties using applied voltages represents a longstanding goal in physics and technology, particularly for low-power applications. To this end, substantial interest has developed around electric-double-layer transistors (EDLTs) based on functional materials. More recently, electrochemical EDLTs, where ions such as O2-, H+, Li+, etc., are driven into / out of a channel material via voltage, have proven capable of offering unique benefits (including non-volatility) for a variety of novel applications. Cobaltites, such as SrCoO3-δ (SCO) have recently emerged as an archetypal example of electrochemical control of materials properties in electrolyte-gate devices. This is accomplished by voltage-driven redox cycling between two distinct phases: fully oxygenated perovskite (P) (δ ≈ 0) and oxygen-vacancy-ordered brownmillerite (BM) (δ = 0.5). To date, SCO has received the most attention in this regard, despite significant issues with air stability in the P phase, and few alternatives have been considered. Additionally, critical issues of voltage hysteresis and fundamental limits on reversibility and cycling endurance remain unaddressed.To address this, using EDLTs based on epitaxial La1-xSrxCoO3-δ (LSCO) thin films, we first investigate the electrochemical reduction that is known to occur at positive gate voltages (Vg) in such systems, establishing that the P → BM transformation occurs in LSCO over a wide doping range. Importantly, both the P and BM phase of x = 0.5 LSCO are robustly air stable, and the electrochemical reduction behavior was found to be voltage-tunable with both doping and strain. We then leverage this voltage-tuned P → BM transformation to demonstrate large property modulations in electronic transport, magnetism, thermal transport, and optical properties, achieving similar or greater ranges of control than in SCO. Next, to explore the reversibility of the transformation, we performed detailed analysis of Vg hysteresis loops, revealing a wealth of new mechanistic findings, including asymmetric transformations due to differing oxygen diffusivities in the P vs. the BM phase, non-monotonic transformation rates due to the first-order nature of the P-BM transformation, and limits on reversibility due to first-cycle structural degradation. Additionally, using minor hysteresis loops, we demonstrate the first rational design of an optimal Vg cycle, leading to state-of-the-art cycling of electronic and magnetic properties, encompassing >105 transport ON/OFF ratios at room temperature, reversible and non-volatile metal-insulator-metal and ferromagnet-nonferromagnet-ferromagnet cycling, all at ultrathin 10-unit-cell thickness. Finally, to further investigate the magnetic properties of the BM nonferromagnet “OFF” state, we performed neutron diffraction experiments, finding the first direct evidence of antiferromagnetic order in BM-SCO films and identifying weak ferromagnetism in x = 0.5 BM-LSCO. These findings thus significantly advance the understanding of voltage-induced P ↔ BM transformations in cobaltite films and pave the way for future work establishing the ultimate cycling frequency and endurance in such electrolyte-gated devices.