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.