Magnetic viscosity phenomena in exchange coupled CoFeMnIr bilayers

The complex permeability spectra were measured in the low (10 Hz-1 MHz) and microwave (100 MHz-9 GHz) frequency ranges in CoFeMnIr bilayers. The low frequency permeability spectra showed the magnetic viscosity effect below the critical antiferromagnet thickness (tcAF), but not at tAF < tcAF. This effect was analyzed with the dispersed relaxation behavior of an antiferromagnetic (AF) layer coupled with a ferromagnetic layer across the exchange anisotropy (Jc). Over the AF relaxation frequency, the AF spin becomes stable and nearly fixed. Therefore, the Jc can be estimated from the resonance frequency of microwave permeability at tAF < tcAF as well as tAF > tcAF. The discrepancies between dynamic and quasistatic measurements of the Jc only begin to appear in the vicinity of the tcAF and dominate at tAF < tcAF.