Structural origin of magnetic biased field in NiMn/NiFe exchange coupled films

The factors which determine the strength of the magnetic biased field in NiMn/NiFe exchange biased system have been examined. It was found that a strong NiFe (111) texture and columnar growth are essential to obtaining a high exchange field as they facilitate epitaxy between the NiFe and NiMn layers. The application of an amorphous Ta nucleation layer and substrate biasing during NiFe deposition have been found to promote these structures. Fee NiMn replicates the (111) texture through epitaxial growth on NiFe according to a cube on cube orientation relationship. Upon annealing, atomic ordering takes place in the NiMn layer and gives rise to an exchange biased field. However, a fully L1(0) ordered structure cannot be achieved at the NiMn/NiFe interface due to the stabilizing effect of fee NiFe. Away from the interface, the transformation proceeds and the ordered tetragonal unit cell creates a lattice strain within the NiMn film. This strain is relieved by twinning within the NiMn grains of the film. The strength of the exchange field has been found to be closely associated with the density of twins and their proximity to the NiMn/NiFe interface. (C) 1996 American Institute of Physics.