Magnetization dynamics of Co antidot lattices

We have systematically investigated the static and dynamic magnetic properties of Co(t nm)/CoO(5 nm)/Cu(2 nm) antidot square lattices as a function of Co thickness [1]. The Co film thicknesses in the different samples are 25, 50, 75, and 100 nm, respectively. The antidot pattern forms a square lattice with a wavelength of 415 nm (center to center) and the hole diameters were varied in the different samples (145, 185, 225, and 265 nm, respectively).
The static magnetic properties were measured using in-plane magneto-optical Kerr effect (MOKE) magnetometry. Measuring magnetization loops at different in-plane angles we could determine the orientation dependence of coercive (Hc) and saturation (Hs) fields. Both fields show angular dependencies that form a 4-fold anisotropy with hard axes along the [10] directions. Those anisotropies in Hc and Hs can be explained by the geometry of the lattice by taking into account the hole diameter.
Dynamic measurements were done using time-resolved MOKE experiments and different ferromagnetic resonance (FMR) setups (Vector Network Analyzer (VNA)-based and classical detection using a shortened microwave cable). For the 50 nm thick sample with holes of 145 nm diameter a transition of the main mode from a lower frequency branch to a higher one was observed at a field of around 900 Oe and a frequency of around 6 GHz along the [10] direction. A similar transition between main modes is also visible for larger hole diameters as well as for larger film thicknesses. Due to the coexistence of 2 modes at a fixed frequency it is not a real frequency gap but on the other hand the two branches are also not connected, rather the main precessional magnitude transitions from the lower branch to the higher branch.
Angular FMR scans show a strong signal for a 4-fold anisotropy with the hard axes along the [10] directions and a weak signal for additional hard axes along the [11] directions, confirming the findings of the static angular loop measurements done with MOKE magnetometry.