Tunable frequency ferromagnetic resonance of Co nanowire arrays

In order to overcome the current limitations of metallic systems for microwave applications, columnar arrays of Co nanowires with variable wire lengths were electrochemically grown using anodic alumina membranes with thickness of ~100 μm and pore diameters around 70 nm. The Co nanowires tend to present a strong hcp-magnetocrystalline anisotropy directed in the plane perpendicular to the wires axis. The combination of shape anisotropy directed along the wire axis (depending on the wire length) and the hcp magneto-crystalline anisotropy (due to structure) causes a shift from in-plane to out-of-plane spontaneous magnetization with relevant saturation and remanence magnetization values. Different types of magnetic behavior may then lead to ferromagnetic resonance frequencies shifting from 20 to 40 GHz at zero applied field.