Ferromagnetic resonance of transversally magnetized amorphous microwires and nanowires

Ferromagnetic resonance of very thin glass covered wires with diameters ranging from 25 μm down to 133 nm was investigated at microwave frequencies 49.1 and 69.7 GHz at room temperature. The static magnetic field was applied perpendicular to the wire axis. The resonance spectra substantially change when the wire diameter decreases below the electromagnetic skin depth. Depending on the wire thickness and the experimental arrangement, various resonance modes can be excited. In thick wires, an inhomogeneously broadened resonance curve, with two distinct peaks at minimum and maximum resonance fields, can be seen. In submicron wires, generally three narrow resonances can be observed. The weak central resonance peak corresponds to the uniform precession (Kittel) resonance mode and is excited by the uniform component of microwave magnetic field. The other two belong to non-uniform magnetostatic modes, which are excited by the strong circumferential magnetic field due to electric polarization of the wire. The experimental results are explained using a strong skin effect limit and a quasistatic approximation for the bulk and submicron wires, respectively.