Finite size effects in patterned magnetic permalloy films

Electron beam lithography has been used to prepare 250-Å-thick square-shaped permalloy dots whose width and separation are in the range between 1 and 4 mm. The static and dynamic magnetic properties of these structures have been investigated by complementary techniques such as magneto-optic Kerr effect ~MOKE!, magnetic force microscopy ~MFM!, and Brillouin light scattering ~BLS!. Longitudinal MOKE enabled us to compare the hysteresis loops of the specimens with different dot size and interdot separation, showing a marked influence of the demagnetizing field inside the dots. MFM images recorded at zero applied field showed that, depending on the interdot spacing, there is a prevalence of either four- or seven-domain patterns together with a minority of nonsolenoidal patterns that possess a net magnetic moment. BLS from thermally activated spin waves were then used to determine the intrinsic magnetic parameters of the permalloy films and to show evidence of the discretization of the spin-wave peak due to the wave vector resonance condition within each dot.