Theory of ferromagnetic resonance in perpendicularly magnetized nanodisks: Excitation by the Oersted field

We present theoretical studies of ferromagnetic resonance in perpendicularly magnetized nanodisks, wherein spin waves are excited through the ac modulation of the dc transport current injected into the disk. We have nanopillars in mind in our analysis, where spin-polarized current is injected from a metallic ferromagnet elsewhere in the structure. We argue that in a limit described, the modulation of the Oersted field generated by the transport current is the dominant spin-wave excitation mechanism, and our studies explore this limit. We calculate the critical current above which the nominal ferromagnetic state becomes unstable through studies of the linewidth of the lowest spin-wave mode, which vanishes when the critical current is reached. We find that as the applied Zeeman field H(0) is decreased from values above 4 pi M(S), the critical current has a minimum when H(0)similar to 4 pi M(S) to increase for values of the external field below this value.