Parallel pumping of a ferromagetic nanostripe: confinement quantization and off-resonance driving

The parametric excitation of spin waves in a rectangular, ferromagnetic nanowire in the parallel pump configuration and with applied field along the long axis of the wire is studied theoretically, using a semi-classical and semi-analytic Hamiltonian approach. We find that as a function of static applied field strength, there are jumps in the pump power needed to excite thermal spin waves. At these jumps, there is the possibility to non-resonantly excite spin waves near $k_z=0$. Spin waves with negative or positive group velocity and with different standing wave structure across the wire width can be excited by tuning the applied field. By using a magnetostatic Green's function that depends on both the nanowire's width and thickness -- rather than just its aspect ratio -- we also find that the threshold field strength varies considerably for nanowires with the same aspect ratio but of different sizes, in contrast to previous work.