Route to Achieving Giant Magnetoelectric Coupling in BaTiO$_3$/Sr$_2$CoO$_3$F Perovskite Heterostructures

Polarization induced spin switching of atoms in magnetic materials opens for possibilities to design and develop advanced spintronic devices, in particular, storage devices where the magnetic state can be controlled by an electric field. We employ density-functional theory calculations to study the magnetic properties of a perovskite strontium cobalt oxyfluoride Sr$_2$CoO$_3$F (SCOF) in a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectic BaTiO$_3$ (BTO) layers. Our calculations show that the spin state of the central Co atom in SCOF can be controlled by altering the polarization direction of the BTO, specifically, to switch from high-spin state to low-spin state by changing the relative orientation of the ferroelectric polarization of BTO with respect to SCOF, leading to an unexpected, giant magnetoelectric coupling, $\alpha_s \approx 21 \times 10 ^{-10}$ Gcm$^2$/V.