Forbidden Band Gaps in the Spin-Wave Spectrum of a Two-Dimensional Bicomponent Magnonic Crystal S. Tacchi, 1, * G. Duerr, 2 J. W. Klos, 3 M. Madami, 1 S. Neusser, 2 G. Gubbiotti, 1,4 G. Carlotti, 1,5 M. Krawczyk, 3 and D. Grundler 2,† 1 CNISM, Unita ` di Perugia and Dipartimento di Fisica, Via A. Pascoli, I-06123 Perugia, Italy 2 Lehrstuhl fu ¨r Physik funktionaler Schichtsysteme, Technische Universita ¨t Mu ¨nchen, Physik Department, James-Franck-Strasse 1, D-85747 Garching bei Mu ¨nchen, Germany 3 Faculty of Physics, Adam Mickiewicz University, Umultowska 85, Poznan ´, 61-614, Poland 4 Istituto Officina dei Materiali del CNR (CNR-IOM), Unita ` di Peruia, c/o Dipartimento di Fisica, Via A. Pascoli, I-06123 Perugia, Italy 5 Centro S3, CNR-Instituto di Nanoscienze, Via Campi 213A, I-41125 Modena, Italy (Received 21 June 2012; published 28 September 2012) The spin-wave band structure of a two-dimensional bicomponent magnonic crystal, consisting of Co nanodisks partially embedded in a Permalloy thin film, is experimentally investigated along a high- symmetry direction by Brillouin light scattering. The eigenfrequencies and scattering cross sections are interpreted using plane wave method calculations and micromagnetic simulations. At the boundary of both the first and the second Brillouin zones, we measure a forbidden frequency gap whose width depends on the magnetic contrast between the constituent materials. The modes above and below the gap exhibit resonant spin-precession amplitudes in the complementary regions of periodically varying magnetic parameters. Our findings are key to advance both the physics and the technology of band gap engineering in magnonics. DOI: 10.1103/PhysRevLett.109.137202 PACS numbers: 75.30.Ds, 75.40.Gb, 75.78.n, 78.35.+c Magnonic crystals (MCs) are a new class of metamate- rials where a periodic modulation of magnetic properties allows for manipulation of the band structure of spin waves (SWs) [1–3]. This is similar to photonic crystals [4], where allowed frequency bands and forbidden frequency gaps (band gaps) are tailored for light by a smart choice of the geometrical symmetry and/or the materials [5]. For two- dimensional (2D) photonic crystals, air holes periodically arranged in a dielectric film have been a powerful concept to induce the Bragg reflection of light. Contrarily to elec- tromagnetic waves, SWs, i.e., the low lying excitations in magnetically ordered substances, do not exist in air, and the incorporation of a second magnetic material is key to provide the relevant material contrast for continuously propagating SWs in MCs. Such so-called bicomponent magnonic crystals (BMCs) offer magnonic band structures that can be tuned by changing either the spatial symmetry, the filling fraction, or the magnetic contrast of constituent materials [6–10]. Pioneering experiments have been fo- cused on 1D BMCs consisting of an array of longitudinally magnetized Co and Ni 80 Fe 20 (Py) alternated nanostripes [11,12]. Here, the Co stripes exhibited forced spin preces- sion, thereby enhancing the dynamic dipolar coupling between confined modes in the Py stripes [11–13]. Very recently, it has been shown that propagation of SWs in 2D bicomponent antidot lattices is guided into channels of nanometric width which reside in complementary regions depending on the eigenfrequency [14]. In the case of a 2D chessboardlike square array of dots [15], the periodically modulated internal fields and damping parameters were decisive to understand the modes. A band gap for SWs at Brillouin zone (BZ) boundaries, however, was not ob- served, and it is an open question whether SWs in such 2D BMCs reflect the magnetic-contrast-induced band for- mation due to different material parameters. In this Letter, the SW dispersion along one of the principal symmetry directions in a 2D BMC, consisting of Co nanodisks (so-called dots) partially embedded in a Py thin film, has been measured by Brillouin light scattering (BLS). The experimental data are successfully explained by both mi- cromagnetic simulations and plane wave method (PWM) calculations. We find that the composite structure supports propagating modes whose spatial profiles are calculated and discussed. Evidence is given for the opening of band gaps, caused by Bragg diffraction of SWs induced by the magnetic contrast effect, creating great perspectives for tailored band structures in magnonics similar to photonics and electronics. The bicomponent investigated sample consists of a square lattice of shallow circular holes (8 nm deep) etched into a 24 nm thick Py film and filled with 15 nm thick Co dots [Fig. 1(a)]. The Co dot radius is R ¼ 155 nm, and the lattice parameter is a ¼ 600 nm. The corresponding first BZ is a square of side length 2q BZ ¼ 2%=a ¼ 1:046  10 5 rad=cm. The dispersion of SW modes has been measured by BLS for the in-plane transferred wave vector (q y ) up to 1:8  10 5 rad=cm, i.e., up to the fourth BZ. The magnetic field H 0 ¼ 200 Oe was applied along the z axis perpendicularly to q y . PRL 109, 137202 (2012) PHYSICAL REVIEW LETTERS week ending 28 SEPTEMBER 2012 0031-9007= 12=109(13)=137202(5) 137202-1 Ó 2012 American Physical Society