  Spin wave spectra of perpendicularly magnetized disks with trilayers
consisting of a 100 nm permalloy (Py) layer sandwiched by two Cu layers of 30
nm, are measured individually with a Magnetic Resonance Force Microscope
(MRFM). It is demonstrated by 3D micromagnetic simulations that in disks having
sub-micron size diameters, the lowest energy spin wave mode of the saturated
state is not spatially uniform but rather is localized at the center of the
Py/Cu interface in the region of a minimum demagnetizing field.
