The resonance frequencies of a cobalt (100 nm)/Permalloy (50 nm) exchange spring bilayer have been measured using a pulsed inductive microwave magnetometer. Two field dependent frequencies are detected which vary in relative intensity as a function of applied magnetic field. The two frequencies are identified as the optic and acoustic spin wave modes of coupled ferromagnetic films. Only over a limited range of fields are both frequencies observable. In low fields the acoustic mode is dominant, and in higher fields the optic mode gives the largest intensity. These intensity variations are explained in terms of off-resonant driving of cobalt modes by the uniform mode in Permalloy and a field dependent interfacial pinning. (c) 2005 American Institute of Physics.