Ferromagnetic resonance in microwires and nanowires

Ferromagnetic resonance (FMR) in a single thin conducting ferromagnetic wire is investigated from theoretical and experimental points of view. It is shown that the wire radius, the symmetry of microwave magnetic field at the sample surface, and the skin depth (magnetic and nonmagnetic) should be considered as a whole for a correct interpretation of the microwave absorption. As a consequence, various resonance modes can be excited in metallic wires. The resonance fields of bulk samples satisfy the Kittel's resonance condition for a thin planar plate (FMR(0)). However, as the wire radius decreases below the nonmagnetic skin depth a weak resonance peak separates from the main resonance and moves to the field fulfilling the Kittel's resonance condition for an axially magnetized cylinder (FMR(1)). Theoretical predictions show that this insulator resonance mode should be the dominant one for a nanowire, where the radius is much smaller than the minimum magnetic skin depth. The existence of the two resonance modes is supported by experimental results on thin (down to 1.5-mu m thick) amorphous microwires.