Authors: B.K. Kuanr, L.M. Malkinski, M. Yu, D. Scherer, R. Camley, Z. Celinski
Affilation: University of Colorado at Colorado Springs, United States
Pages: 542 - 545
Keywords: metallic nano structures
Arrays of magnetic antidots (holes) have attracted extensive attention these days due to their potential application for high density data storage. In this investigation we fabricated antidots by means of electron-beam lithography and lift-off techniques from a 100 nm thick Permalloy film grown by sputtering. We report here the dynamic properties of nanometer sized Permalloy antidot arrays (S1; 400x400 nm2, S2; 800x800 nm2 and S3; 1200x1200 nm2) at GHz frequencies by using a Network Analyzer based Ferromagnetic Resonance (NA-FMR) technique. The dynamic excitations in the antidot arrays were studied using a flip-chip geometry on top of a co-planar waveguide. We have demonstrated the possible application of antidot arrays as a magnetically tunable localized band-pass filter (Fig.1). The transmission response of the all the three antidot structures exhibit double resonance modes of almost equal intensity (see Fig.1); one at a higher frequency and the second at a lower frequency in comparison to the single uniform resonance mode observed for a continuous Permalloy film of same thickness. For the antidote arrays, the uniform mode splits into two distinct resonance modes. In between these two resonance modes we observed a band-pass region (shown by a double arrow line in Fig.1), which can be tuned by an external magnetic field. It is also observed that the bandwidth of the pass-band increases with the increase in the square hole-size. This is because at a constant magnetic field (say 4 kOe); the higher resonance mode for all the three antidotes arrays (S1-S3) occurs at the same frequency of 21 GHz, whereas the lower resonance occurs at 16, 15 and 14 GHz for S1 to S3, respectively. In contrast, the continuous Py film resonance occurs at 18.5 GHz, i.e. in between the two modes of the antidot array. We also observed that the higher mode is broader than the lower mode. This may be due to multiple additional modes around that frequency, which are the outcome of the localized edge modes due to the sharp drop of the effective field near the hole edges. Fig.2 shows the in-plane magnetic field dependence of both the modes for the S2 antidot array along with the resonance frequency of the continuous Py film. The theoretical fitting to these modes includes uniaxial in-plane anisotropy which is the consequence of dipolar fields with orthogonal symmetry. Fig.3 shows the effective anisotropy field arising from the dipolar field distribution of the different antidot arrays as a function of the size of the antidot. The anisotropy field is observed to decrease with the increase of hole size. Due to the generation of demagnetizing field by the array of antidots the internal field reduces the frequency for the lower order mode whereas the internal field increases the frequency for the higher order mode. It is also observed that the frequency linewidth of the notches increases with the decrease in hole size.