Reversible Magneto-Ionic Control of Exchange Bias in Coupled Spin-Valve-Like Heterostructures

Voltage control of exchange bias (EB) is an important technological goal for low-power spintronic sensor and memory devices. The magneto-ionic (MI) approach for voltage-controlled EB is a promising strategy to achieve this goal, utilizing electrochemical reactions at low operational voltages. In typical MI devices, however, the sensitive EB layers are directly targeted by the electrochemical reactions, which often impairs reversibility. Here, we introduce an alternative device structure by isolating the EB layers from the active MI layer. Making use of the interlayer (IL) coupling through a spacer layer in an IrMn/Fe/Au/Fe spin-valve-like heterostructure, we show that EB can be reversibly controlled by an electrochemical modification of the top layer. Using the same device structure, we also realize an MI switching between single-step and double-step hysteresis loops. We interpret the observed MI effects via an increasing top Fe layer thickness, caused by the electrochemical reduction of FeO_x to ferromagnetic Fe. Modeling of hysteresis loops as a function of top layer thickness in an extended Stoner-Wohlfarth approach corroborates this interpretation. Our results highlight an advanced strategy for improving reversibility in MI devices and open a novel pathway toward voltage-controlled spin valves.