Asymmetry in magnetic behavior caused by superposition of unidirectional and four-fold magnetic anisotropies in CoPt/FeMn bilayers

Abstract

We investigated magnetic anisotropy and magnetization reversal process in CoPt/FeMn bilayers by four-probe measurements of anisotropic magnetoresistance (AMR) effect. Here, a single crystal-CoPt/polycrystal-FeMn interface has been established, where A1-CoPt layer with four-fold magnetic anisotropy was epitaxially grown on MgO(100) substrate, while the top FeMn layer was 111 textured showing in-plane polycrystalline feature. Due to the noncollinear 3Q AFM spin structure and existence of uncompensated spins in ultrathin FeMn, the magnetic anisotropy of the bilayer can be tuned through interfacial exchange coupling, controlled by field cooling treatment. As a result, it gives rise to asymmetric AMR curves. We show that this asymmetry strongly relies on the direction of induced unidirectional magnetic anisotropy with respect to the easy axis of CoPt layer, leading to a modulated magnetization reversal process.