Effect of eccentricity on the spin-wave spectrum of NiFe∕Cu∕NiFe pillars with elliptical cross section

Brillouin light scattering (BLS) has been exploited to study the effect of eccentricity on the magnetic normal-mode spectrum of five arrays of trilayered pillars with an elliptical cross section and different eccentricity ε in the range between 1 and 3. The pillars consist of a pair of 10nm thick Permalloy (NiFe) layers separated by a Cu spacer with thickness of 10nm. Several discrete spin-wave modes were observed in the BLS spectra and their frequencies tracked as a function of the in-plane direction of the applied magnetic field. All mode frequencies exhibit a uniaxial anisotropy, induced by the shape of the elements, which increases with ε. A satisfactory reproduction of the measured frequencies was achieved by three-dimensional micromagnetic simulations, solving the discretized Landau-Lifshitz-Gilbert equation over the layered structure in the time domain and performing a local Fourier transform.