Frequencies of Radially Symmetric Excitations in Vortex State Disks

Submicron ferromagnetic disks with a vortex ground state exhibit interesting and potentially useful dynamic properties arising from excitations on the ground state. In particular, a magnetic pulse applied perpendicular to the vortex plane will excite radially symmetric modes. Previous calculations of the frequencies of these modes based on the linearized Landau-Lifschitz equation and the magnetostatic Green's function give eigenfrequencies proportional to the square root of the aspect ratio radic(
L/R)
, where
L
is the disk thickness and
R
is the disk radius. However, experimental frequency data show significant deviation from the square root dependence. An improved calculation of the frequency is done through a collective variable approach by exploiting the high symmetry of these modes. In the linear approximation, this approach gives the main contribution to the frequency proportional to radic((
L/R
)ln(
R/L
)), which is closer to the observed aspect ratio dependence.