Ferromagnetic Resonance Spectroscopy of Arrays of Coupled Nanomagnets
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Magnetodynamic properties of large area magnetic nanopatterns are of the great interest for magnetic hard drive storage industry and novel magnetic logic devices. With general trend for miniaturization and increase of areal density of magnetic nanodevices, collective magnetodynamic behavior will play increasingly important role. Thus, understanding and identification of the key factors affecting performance of coupled nanomagnetic system is the crucial part in the future success of magnetism related industries. In this work, we look at phenomena of ferromagnetic and spin wave (SW) resonances in magnetic nanopatterned films. Also, within the scope of this study are conditions of emergence, magnetic properties, and stability of the magnetic vortices in large arrays of Ni19Fe81 (permalloy) dipole coupled nanomagnets. In our experimental studies, we survey magnetodynamic properties and magnetic texture of permalloy nanopatterns by means of the field sweep FMR spectrometry, Alternating Gradient Field Magnetometry, Polar Magneto-Optical Kerr Effect Magnetometry, and Magnetic Force Microscopy. Using electron-beam lithography and lift-off process, we fabricate and characterize magnetic nanopatterned films with a wide range of geometrical parameters such as lateral size of rectangular nanomagnets, nanomagnet aspect ratio and the duty cycle of the square pattern as well as type of 2D lattice. By changing the geometrical parameters of the nanostructures we achieve control over the ferromagnetic and spin-wave resonance modes in patterned films at various directions of external bias magnetic field. Using FMR spectrometry, we measure the critical angles between the DC magnetic field and the plane of the nanopattern at which quantized standing spin wave modes are excited (resonance mode splitting). Our experimental results were supplemented with analytic calculations and micromagnetic simulations. Proposed analytic model allows distinguishing between the observed resonance modes based on effective demagnetizing factors which in their turn represent geometries of individual nanomagnets and geometrical properties of arrays. Our micromagnetic simulations are in good agreement with experimental observations and confirm our assumptions about significant contribution of long range dipolar interdot coupling to magnetic textures and spin wave resonance spectrum of nanodot arrays.