Spin waves in stripes submitted to a perpendicular applied field

In the dipolar approximation, we present an analytical study of the magnetic modes occurring in a stripe submitted to a magnetic field H parallel to a principal axis of its elliptical cross section. The studied excitations oscillate within the cross section. The results differ greatly from those obtained in the previously treated case of H parallel to the stripe direction. The dependences of the frequencies upon the applied field are opposite below and above the saturation field Hs, where Hs defines the limit between the oblique phase (H Hs): the frequencies slow down until Hs, they vanish at Hs and they increase above. This softening was previously observed on stripes with a rectangular cross section and the measured critical field lies near the saturation field, in agreement with our calculations.