Magnetic viscosity phenomena in exchange coupled CoFe/MnIr bilayers

摘要

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