Noncontact Evanescent Microwave Magnetic Dipole Probe Imaging of Ferromagnets

We describe a 2-GHz evanescent microwave magnetic dipole probe and its applications in imaging high-frequency electromagnetic properties of metallic magnetic samples. The probe offers the advantages of low cost and ease of implementation. It consists of a small inductive metallic loop fed by a microstripline resonator that is scanned over ferromagnetic samples. In contrast to the electric dipole probe reported previously, the magnetic dipole probe has higher input admittance, making it more suitable for quantitative imaging of high-conductivity metallic samples. When calibrated with a Gauss probe, the magnetic dipole probe was insensitive to electric field while it detected the magnetic field above the magnetized ferromagnetic sample with 1% accuracy and 0.06% resolution at 2 GHz. Using a simplified circuit model of the resonator, we were able to interpret the probe´s output and predict the magnitude of the reflected wave and relate it to the magnetic properties of the samples under investigation.