Dual band microwave ferromagnetic resonance absorption in annealed cobalt nanowire arrays

In this letter, we present a dual band microwave absorption phenomenon in 60 nm diameter Co ferromagnetic nanowire arrays annealed at 500 °C in nitrogen atmosphere. The frequency dependent microwave response of the wires is obtained for applied field parallel to the nanowire axis, below 4 kOe and above saturation magnetization up to 8 kOe, in steps of 1 kOe. For applied magnetic fields above saturation magnetization, two sets of absorption peaks are observed, while below saturation, only one single peak is obtained. Combining structural characterization and ferromagnetic resonance measurements, it is proven that the two ferromagnetic resonance peaks are associated with a coexistence of two kinds of nanopillars with different crystal structure in annealed nanowires, one for nanopillars with hexagonal close packed structure at higher ferromagnetic resonance frequency, and the other with face center cubic structure.