Band structure of a two-dimensional ferromagnetic antidot lattice

The spin wave band structure of a two-dimensional square array of NiFe circular antidots having diameter of 120 nm and periodicity of 800 nm has been investigated by using Brillouin light scattering technique and micromagnetic calculations based on the dynamical matrix method [1]. The external magnetic field was applied in the plane and perpendicularly to the transferred wave vector. Extended and localized spin modes having a propagative nature were found. Opening of bandgaps is interpreted in terms of Bragg diffraction of spin waves from the antidot lattice and this effect is explained by studying the behaviour of the internal field as shown in Fig.1. The mean internal field is larger along the vertical rows of antidots and smaller between the antidots (see panel (a) for extended modes and (c) for localized modes). By developing an analytical model according to which the mean internal field is represented by means of a rectangular step function characterized by a region 1 corresponding to vertical rows of antidots and a region 2 between the antidots (see panels (b) and (d)), the relevant scattering potential for Bragg reflection is not provided by the holes themselves, but by the concomitant internal field inhomogeneity between holes [2]. This is in contrast to antidots in photonics and electronics where the back-reflection is directly caused by the presence of holes. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n228673 (MAGNONICS). [1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007). [2] R. Zivieri, S. Tacchi, F. Montoncello, L. Giovannini, F. Nizzoli, M. Madami, G. Gubbiotti, G. Carlotti, S. Neusser, G. Duerr, and D. Grundler, Phys. Rev. B 83, (2012)