Exchange Bias Effect on the Relaxation Behavior of the IrMn/NiFe Bilayer System

Magnetization dynamic of exchange bias IrMn/NiFe bilayers system were investigated. Samples with fixed ferromagnetic (FM) layer thickness of 25 nm and antiferromagnetic (AFM) layer thickness t_AFM of 5, 10, 15 and 20 nm were deposited by dc-magnetron sputtering on buffered silicon substrates. The static exchange bias field extracted from the magnetization curves increased at the beginning with increasing AFM layer thickness then slightly decreased. The dynamic behavior was studied from the ferromagnetic resonance (FMR) spectra of the samples under external magnetic field in the range 50-750 Oe. The linewidth versus frequency was found to have two distinct slope regions for the samples with high exchange bias values. The damping coefficient at frequencies up to about 7 GHz is found to be generated from the intrinsic linewidth broadening and it has a similar tendency to the exchange bias field with increasing AFM layer thickness. At higher frequencies, the damping coefficient is generated from the extrinsic contributions to the linewidth broadening and its behavior with increasing AFM layer thickness follows the dynamic anisotropy.