Properties of thin IrMn in exchange biased multilayers

We report striking behavior in thin IrMn exchange-biased heterostructures. We have studied exchange-biased multilayers with IrMn thicknesses from 2 to 80 Angstrom with 50-Angstrom Ti over- and underlayers. The resistance of spin valves with Si/50-Angstrom Ti/40-Angstrom NiFe/8-parallel to Co/30-Angstrom Cu/30-Angstrom Co/t(IrMn)/50-Angstrom Ti as a function of temperature shows anomalous behavior for 12 Angstrom<t(IrMn)<26 Angstrom. These features are not seen for spin valves in which the ferromagnetic layers are replaced with CoFe. To isolate the effects of the IrMn, resistance and magnetization versus temperature for structures of 50 Angstrom Ti/ t(IrMn) /50 Angstrom Ti were measured. Our measurements are suggestive of a model involving a mixed hcp/fcc phase of IrMn, in which the hcp phase undergoes a magnetic phase transition, and the resistance is influenced by a combination of the magnetic transition and the structural properties of the IrMn layer. (C) 2003 American Institute of Physics.