Electric field control of the exchange spring effect in perpendicularly magnetized FePt/NiFe bilayers

The electric field control of the exchange spring (ES) effect in perpendicularly magnetized bilayers is studied using micromagnetic simulation. The ES bilayer with a total thickness of 20 nm is composed of a FePt layer with perpendicular magnetic anisotropy (PMA) and a NiFe layer with in-plane anisotropy. The thickness ratio between the FePt and NiFe layers (TRFN) is varied. Single-crystal lead zinc niobate-lead titanate (PZN-PT) forms the piezoelectric substrate, and electric fields are applied across the thickness of the substrate to induce in-plane stress. The results reveal that application of a negative electric field E reduces the switching field (HS) and squareness ratio (MR/MS) but enhances the nucleation field (HN), while applying a positive E has a negligible effect on these parameters. When the TRFN is 18:2, an electric field of −2 MV/m reduces HS by 5.5% while MR/MS is maintained at 1. This indicates that when the FePt content is sufficiently large, applying a strong negative E can reduce HS without changing MR/MS. Based on the simulation results, we propose a model for electrically assisted magnetic memory.