Double ferromagnetic resonance in nanowire arrays

Microstrip line measurements are used to determine the frequency dependent microwave response of 40 nm diameter CoFeB ferromagnetic nanowire arrays, with external static applied field parallel to the nanowire axis. The ferromagnetic resonance (FMR) response of the wires is obtained for applied fields below and above magnetization saturation. For applied magnetic fields above saturation, a single FMR peak is observed, while below saturation, two sets of peaks are obtained. The two FMR peaks below saturation are associated with two magnetization populations, one for nanowires with upward magnetization and one with downward magnetization. A model based on a Maxwell-Garnett homogenization procedure has been established and used to predict the frequency response of the FMR peaks. There is good agreement between the model and experimental results.