Perturbation of magnetostatic modes observed by FMRFM

Magnetostatic modes of Yttrium Iron Garnet (YIG) films are investigated by ferromagnetic resonance force microscopy (FMRFM). A thin film ``probe'' magnet at the tip of a compliant cantilever introduces a local inhomogeneity in the internal field of the YIG sample. This influences the shape of the sample's magnetostatic modes, thereby measurably perturbing the strength of the force coupled to the cantilever. We present a theoretical model that explains these observations; it shows that tip-induced variation of the internal field creates either a local ``potential barrier'' or ``potential well'' for the magnetostatic waves. The data and model together indicate that local magnetic imaging of ferromagnets is possible, even in the presence of long-range spin coupling, through the induction of localized magnetostatic modes predicted to arise from sufficiently strong tip fields.