Ferromagnetic resonances in thin films—Electrodynamic analysis and experiments employing multimode rectangular cavity and broadband coplanar waveguide techniques

Abstract

Rigorous electrodynamic analysis of resonances in rectangular TMn,m,0z mode resonators containing a thin in-plane magnetized ferromagnetic film is presented. Apart from the resonance frequencies and Q-factors, the electromagnetic field distribution is analyzed showing the appearance of a giant microwave magnetic field component perpendicular to the sample at the resonance frequencies of both the dominant and higher order modes. This confirms the plasmonic-like behavior of resonances in thin ferromagnetic films, where the term is used in the electrodynamic sense of subwavelength resonances. The conductivity of metallic ferromagnetic films is accounted for in computations. Resonance curve broadening due to conductivity is predicted for larger conductivities and/or for thicker samples. Results of rigorous computations are compared with the perturbation theory. A rectangular TMn,m,0z mode cavity is used for measurements of the saturation magnetization and Gilbert damping factor of thin CoFeB films at several frequencies in the range 4–12 GHz showing agreement with the broadband coplanar waveguide technique. The Gilbert damping factor of a single-crystal yttrium iron garnet film grown on a GGG substrate and an amorphous CoFeB thin film were determined from measurements of their Q-factors, employing a vector network analyzer, and considering the sample as a magnetic plasmon-like resonator coupled to the coplanar line.