Voltage Control of Exchange Bias in LaCoO₃/SrCoO_3-δ Heterostructures via an Electrolyte-Gated Topotactic Phase Transformation

Voltage control of magnetism by ionic motion (magnetoionics) offers an energy-efficient route to spintronic devices, neuromorphics, etc. Among various approaches, electrolyte gating is particularly effective as it creates strong interfacial electric fields via an electric double layer, driving reversible ion migration and electrochemical control of material properties. Perovskite oxides such as SrCoO_3-δ are particularly attractive for such devices due to their low oxygen vacancy formation energies, high oxygen vacancy diffusivity, and voltage-induced transitions between perovskite (P) SrCoO₃ and brownmillerite (BM) SrCoO_2.5 phases. The P phase here is ferromagnetic (FM), while the BM phase is a room-temperature antiferromagnet (AFM). In this work, we thus study epitaxial LSAT/LaCoO₃/SrCoO_3-δ heterostructures in which the tensile-strained LaCoO₃ is FM and the SrCoO_3-δ can be toggled between AFM and FM states. With the SrCoO_3-δ in the as-deposited BM state, clear exchange bias is detected, which we have tracked vs. FM thickness, AFM thickness, temperature, and cooling field. Under electrolyte gating, a reversible topotactic transformation to P SrCoO_3-δ is then evidenced, via detailed electronic transport measurements. The ensuing evolution of magnetic properties will be discussed in detail, establishing voltage control of exchange bias among other features.