Abstract
The magnetization reversal process and the spectrum of magnetic excitations of NiFe(10 nm)/Cu(10 nm)/NiFe(20 nm) circular nanorings have been investigated by magneto-optic Kerr effect, magnetic force microscopy (MFM), and Brillouin light scattering (BLS). These measurements reveal several magnetic ground states as a function of the applied field strength, corresponding to the different magnetization distributions in the two ferromagnetic layers with switching fields influenced by the sizable interlayer dipolar coupling. A satisfactory reproduction of the measured hysteresis loop as well as of the main features of the MFM images was achieved by the combined use of three-dimensional micromagnetic simulations and analytical calculations of the dipolar stray field felt by the magnetic tip. The spectrum of thermal spin waves reveals the presence of several discrete and stationary modes, whose frequency evolution with the applied field was measured by BLS, encompassing different magnetic ground states. The frequency and the spatial distribution of the dynamical magnetization of each mode were calculated by micromagnetic simulations. The presence of dipolar coupling between the layers causes the appearance of modes, which are either in-phase (acoustic) or out-of-phase (optical) in the two layers, as well as the presence of complicated mode profiles when the ground states in the two layers are different.
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