Microwave Interferometry for High Sensitivity VNA-FMR Measurements

Two-port vector network analyzer ferromagnetic resonance (VNA-FMR) measurements using coplanar waveguides (CPWs) are a fast and powerful tool for the analysis of ferromagnetic film parameters, including damping and effective field contributions like anisotropies, exchange coupling, and demagnetization fields. Thereby, VNA-FMR appears to be a good choice for the analysis of patterned magnetic nanostructures showing a rich spectrum of different anisotropy terms. However, due to their small volume, the absorption of nanostructures in FMR is low and the measurement signal is superimposed by the excitation signal. To eliminate the background signal, we propose an interferometric approach with the following two key features: 1) the reference path has the same geometry as the measurement path, including an identical but empty CPW parallel to the measurement CPW to remove spurious field dependent contributions to the signal and 2) additionally, we extend the reference path by 0.5 m of an RF coaxial cable. This path length difference leads to a frequency-dependent phase difference of the two paths, assuming a value of 180 ° in ≈ 500 MHz intervals. Here, the excitation RF signals from both paths add to zero. In broadband measurements, this simple approach allows to obtain a sufficient number of grid points for dispersion analysis without the need for a phase shifter. Our experiments reveal an improvement of the sensitivity by up to two orders in magnitude under optimum conditions. The amplitude and the shape of the resonance peaks will be discussed.