  Controlling magnetism and spin structures in strongly correlated systems by
using electric field is of fundamental importance but challenging. Here, a
high-spin ruthenate phase is achieved via a solid ionic chemical junction at
SrRuO3/SrTiO3 interface with distinct formation energies and diffusion barriers
of oxygen vacancies, analogue to electronic band alignment in semiconductor
heterojunction. Oxygen vacancies trapped within this interfacial SrRuO3
reconstruct Ru-4d electronic structure and orbital occupancy, leading to an
enhanced magnetic moment. Furthermore, an interfacial magnetic phase can be
switched reversibly by electric-field-rectifying oxygen migration in a
solid-state ionic gating device, providing a framework for atomic design of
functionalities in strongly correlated oxides using a way of solid chemistry.
