Frequency softening of collective modes in two-dimensional ferromagnetic antidot arrays -- Presentazione poster by R. Zivieri -- Conferenza internazionale

The softening of collective mode frequencies in two-dimensional Permalloy antidot arrays with circulat holes have been investigated. The antidots have fixed lattice constant a = 420 nm and thickness of L= 30 nm. The diameters of the holes range between 140 nm and 260 nm [1]. The magnonic mode frequencies calculated by means of the dynamical matrix method (DMM) as a function of the in-plane magnetic field along the y-direction at the centre of the first Brillouin zone are compared with those measured by Brillouin light scattering technique. The two lowest spin-wave mode frequencies, namely the edge mode (EM) localized at the antidot borders [2] and the F mode, whose spatial profile is confined in the horizontal channels between holes, become soft at a given critical field Hc. High-frequency mode frequencies do not soften including that of the Floc mode that is mainly confined in the horizontal rows between holes. The softening mechanism is strictly related to the rotation of the static magnetization from the hard to the easy axis marking a reorientational and continuous phase transition. By means of an analytical model it is shown that the critical field is approximately equal and opposite to the demagnetizing field averaged over the primitive cell. The different frequency trend of the F mode and the Floc mode as a function of the aspect ratio L/s with s the antidot separation is explained in terms of the opposite demagnetizing field experienced by the two modes. This work was partially supported by MIURPRIN 2010-11 Project2010ECA8P3 "DyNanoMag"