Large Exchange Bias Field in (Pt/Co) $_{\bf 3}$ /IrMn/Co Trilayers With Ultrathin IrMn Layers

The field shift of the hysteresis loop, called exchange bias field, in ferromagnetic/antiferromagnetic (F/AF) Co/IrMn bilayers, or (Pt/Co),,/IrMn structures, appears only above a critical thickness of IrMn, which is related to the IrMn magnetocrystalline anisotropy and to the interfacial coupling between the F and AF layers. In this letter, we show that this critical IrMn thickness can be considerably reduced by sandwiching the IrMn layer between two F layers or multilayers having parallel or orthogonal anisotropy, i.e., the first being magnetized in plane and the second out of plane. An in-plane exchange bias field of 20 mT could be measured in structures of the form (Pt/Co)₃/IrMn/Co (5 nm) having an IrMn layer as thin as 3 nm at room temperature. Several explanations are proposed and discussed. The first is associated with structural variations of texture resulting in changes in the antiferromagnet grains´ anisotropy energy. The second is based on an enhancement of the stability of the antiferromagnet spin lattice resulting from an indirect intergrain coupling. The third relies on a decrease of the Co/IrMn interfacial coupling due to an out-of-plane canting of the interfacial spin.