Kink instability in planar ferromagnets

The 360^∘{} screw kinks in a planar ferromagnet with an in-plane magnetic field are unstable above a critical-field strength. We show that this critical-field strength decreases rapidly as the kink velocity increases toward its maximum value. As a consequence, square-root singularities appear in the dynamic structure factor S(q,ω) at field-dependent cutoff values of ωq.