Magnetoelectric coupling through the electric-field controlled ionic evolution

Electric-field control of phase transformation with ion transfer is of great interests in materials science with enormous practical applications. Due to the strong electron-ion interaction, the ionic evolution would naturally have dramatic influence ont the material magnetic properties. In this talk, I will propose two strategies/pathways to achieve the magnetoelectric coupling through the electric-field controlled ionic evolution. Firstly, I will present a reversible and nonvolatile electric-field control of oxygen and hydrogen ion evolutions within the model system of brownmillerite SrCoO_2.5 by ionic liquid gating. Due to the selectively controllable ionic evolutions, we achieved a tri-state phase transformations among SrCoO_2.5 and its counterpart of perovskite SrCoO_3-δ and a hitherto-unexplored HSrCoO_2.5 phase. Because of the extremely distinct magnetic, electrical and optical properties among these phases, this result forms solid foundations for the conceptually new tri-state magnetoelectric and electrochromic effects. Next, using Co/SrCoO_2.5 heterostructure as model system, I will introduce a room temperature electric-field control of magnetic state in the Co layer accompanied by the bipolar resistance switch. In this case, the electric field controlled oxygen evolution leads to the oxygen ion accumulation (gating) at the interface, in the same manner as the conventional charge-gating device. As the consequence, the interfacial oxygen contents modulate the magnetic interaction within the Co surface layer and eventually results in the intriguing magnetoelectric coupling. We envision that the ionic evolution brings in a new tuning knob to manipulate the coupling and correlation between charge, spin, orbital and lattice degrees of freedom and paves a new playground for the discovery of novel materials and rich functionalities.