Microwave permeability study for antiferromagnet thickness dependence on exchange bias field in NiFe/lrMn layers

Exchange biasing properties were investigated in NiFe/lrMn bilayers tuning the IrMn thickness from 17 to 150 Å. The microwave permeability of these layers has been investigated in the 100 MHz to 4 GHz range using a permeameter based on a microstrip cell. The effective field deduced from the complex permeability spectra resonance has been compared with the exchange bias and coercive field deduced from vibrating sample magnetometer measurements. An effective field enhancement appears for a very thin IrMn (20 Å) layer but no exchange bias can be evidenced. We assign this critical thickness as the first effect resulting from exchange coupling.