High-frequency permeability of Ni and Co particle assemblies

A coaxial transmission line was constructed, characterized, and calibrated for the frequency dependent measurement of complex relative permeability (l r) and complex permittivity (e r). The permeability of Ni powder with a grain size of < 1 lm was measured as a function of packing density to verify the system performance. 8–10 nm diameter Co nanoparticles were synthesized, dried to a powder, and measured. The real part of the permeability for the Co nanoparticles decreased over time as a result of oxidation, and decreased the overall magnetic volume due to the formation of an antiferromagnetic CoO shell. Similarly, the imaginary part of the permeability decreased as a function of oxidation. This was attributed to the insulating CoO shell reducing eddy current losses in the nanoparticle composite.