Method for measuring magnetic susceptibility of magnetic nanoparticles up to millimeter-wave frequency range

A novel method for measuring the complex magnetic susceptibility of magnetic nanoparticles (MNPs) has been developed by a broadband microstrip line-type probe with a frequency range of up to 50 GHz. Two external orthogonal DC fields, Hhard and Heasy, were applied to change the susceptibility of MNPs (Streptavidin coated, 200 nmϕ, number of MNPs about 1010) based on magnetization switching theory. The susceptibility of MNPs in the liquid, solid state, and after biotin-avidin reaction with polymer beads (2 μmϕ, number of polymer beads about 108) was evaluated. A clear FMR frequency of MNPs and the same FMR frequency shift trend as Hhard changed was observed around 2–10 GHz in both liquid and solid states. The susceptibility of MNPs reached maximum when Hhard was 0.1 T which is approximately equal to Heasy (0.135 T). The maximum values of real and imaginary parts increased by a factor of 3.39 and 1.66 compared to when Hhard was 0. The obtained results indicate that the magnetic anisotropy was successfully controlled in the liquid and solid states of MNPs aggregate. Furthermore, after the biotin-avidin reaction of MNPs with polymer beads, the imaginary part of susceptibility showed a decreasing trend regardless of the increase in Hhard, and its half-width expanded by 35.1% at maximum when Hhard was 0.15 T.