Impact of Complex Permeability Measurements Up to Millimeter-Wave Frequency Range

A highly sensitive microstrip-line-type probe using a flexible substrate was developed to measure thin-film permeability continuously up to the millimeter-wave frequency range (over 30 GHz). The probe enables broad bandwidth and highly sensitive permeability measurements without sample size limitations. The complex permeability of the CoFeB film (45 mm × 25 mm and 0.5 μm in thickness), CoNbZr film (25 mm × 25 mm and 3 nm in thickness), and nickel ferrite film (8 mm × 5 mm, and 1.2 μm in thickness) were optimized. The measured permeability spectrum and ferromagnetic resonance frequency showed good agreement with the theoretical values based on the Landau-Lifshitz-Gilbert equation and eddy current generation up to 67 GHz.