IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 3063 Broadband Magnetic Response of Periodic Arrays of FeNi Dots Juan Francisco Sierra , Ahmad A. Awad , Gleb N. Kakazei , Francisco Javier Palomares , and Farkhad G. Aliev Departamento de Física de la Materia Condensada. C III, Universidad Autónoma de Madrid, Madrid 28049, Spain IFIMUP-IN, Departamento do Física, Universidade do Porto, Porto, Portugal ICMM-CSIC, Cantoblanco, Madrid 28049, Spain We present a study of magnetization dynamics at room temperature in periodic arrays of 50 nm thick FeNi (Py) circular magnetic dots of 500 nm radius and different center to center distance (1200 and 2500 nm), by using a broadband magnetometer based on Vector Network Analyzer which works between 300 kHz and 8.5 GHz. We also present a comparison between the dynamic response, ferro- magnetic resonance (FMR) and its linewidth, with static magnetic characteristics such as magnetization curves. The FMR peak appears just above the nucleation field and is perfectly described by Kittel formula taking into account the demagnetizing factor of an individual magnetic dot. In addition to FMR we observed a spin wave resonance below the uniform mode, which could be attributed to spin waves in confined systems. The FMR linewidth shows a significant broadening close to the field region corresponding to nucleation of magnetic vortex. Index Terms—Magnetic devices, magnetic resonance, nanotechnology. I. INTRODUCTION M AGNETIZATION dynamics in magnetic nanostructures such as thin films, magnetic multilayers, magnetic tunnel junctions and spin valves have attracted much attention due to their technological applications in magnetic random access memories (MRAM) [1] and patterned recording media [2]. During last decade spin dynamics in magnetic dots with different shapes and sizes have been intensively studied the- oretically and experimentally. Spatial regularity of arrays of magnetic elements permits the investigation of interdot interac- tions [3] and collective excitations [4]. Depending on the aspect ratio where L is the thick- ness and R is the dot radius, the circular element could have in-plane or out-of-plane magnetization [5]. For all spins trend to align in-plane to minimize both exchange and total dipole energies. Reduction of the dot radius to micrometer or submicrometer length scales induces therefore appearance of a curling spin configuration in the dot and corresponding vortex state formation where spins are aligned out-of-plane close to the vortex core. This core has extension of the order of exchange length which depends on the exchange stiffness A and the saturation magnetization as: . Pres- ence of such unusual topologic anomaly, the magnetic vortex, is expected to give rise to a rich variety of interesting dynamic properties including excitation of translational, radial (RM) and azimuthal (AM) modes [6]. To study the dipolar interactions in the array of the mag- netic dots in the saturated state, it is very important to examine the magnetization dynamics at high frequencies (GHz range) with the sample magnetized in plane. Different experimental Digital Object Identifier 10.1109/TMAG.2008.2002527 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. techniques have been used to probe high frequency magnetiza- tion dynamics in Permalloy dots including Brillouin Light Scat- tering (BLS) [7] and conventional Ferromagnetic Resonance (FMR) [8]. Theoretical studies of saturated in-plane [9] and out-of-plane [10] dots predict multiresonance eigenmodes. In addition to the uniform Kittel resonance, the lateral confinement of spins within each dot may cause a marked discretization of the spin wave spectrum. Here, we present broadband magnetization dynamics, both above and below the vortex nucleation field, measured in arrays of magnetic dots with 500 nm radius and 50 nm thickness. These parameters (thickness of about 3 and aspect ratio of about 0.1) ensure vortex configuration in the ground state and uniform magnetization along the dot thickness in the saturated state. In order to investigate possible influence of dipolar coupling we have measured response from the arrays with different center to center (CTC) dots distance: the high density (HD) sample had and the low density (LD) sample had . II. EXPERIMENTAL DETAILS LD and HD arrays of polycrystalline Py dots were fabricated on silicon wafers by using electron beam (EB) lithography and lift-off techniques. A double layered resist spin coating and highly directional EB evaporation were used to obtain circular dots with sharp edges. This technique is very convenient to fabricate arrays of submicron dots with different diameters and periods, within area limited by substrate and with identical properties of magnetic material: grain size, distribution, ori- entation and film thickness over the whole sample (for more details in sample preparation see [12]). The patterned area has about with sufficient amount of magnetic material for a good detection of magnetization dynamics of the samples in our experimental set-up. The measurements of high frequency magnetization dy- namics were carried out by using Agilent E5071B Vector Network Analyzer (VNA) working at frequencies up to 8.5 GHz. A VNA-FMR inductive technique was used to determine the FMR frequency and the linewidth. Once the M-H loops were measured, both samples were covered with a thin layer of 0018-9464/$25.00 © 2008 IEEE Authorized licensed use limited to: IEEE Xplore. Downloaded on December 26, 2008 at 11:12 from IEEE Xplore. Restrictions apply.