Longitudinal and lateral finite size effects in NA-FMR measurements

In the linear regime, a measure of the magnetic damping can be obtained from the linewidth of the ferromagnetic resonance peak. One means of distinguishing between different classes of damping mechanisms is to examine the frequency dependence of the linewidth. With network analyzer ferromagnetic resonance (NA-FMR), the magnetic susceptibility can be measured in the 0-26 GHz range, from which one can obtain valuable information about how energy dissipation evolves at higher frequencies. The NA-FMR does, however, present a number of caveats. Under certain resonance conditions, for example, it is possible that non-uniform spin-wave modes are excited, due to a driving magnetic field that will be necessarily non-uniform when viewed by a sample whose dimensions exceed those of the central conductor of the coplanar wave guide. This results in the excitation of spin waves, which cause broadening of the resonance peak and then an apparent enhancement of the measured linewidth. We analyze quantitatively the contribution of this experimental source of error and show how to extract the value of the "intrinsic" damping.