  The exchange interaction in ferromagnetic ultra-thin films is a critical
parameter in magnetization-based storage and logic devices, yet the accurate
measurement of it remains a challenge. While a variety of approaches are
currently used to determine the exchange parameter, each has its limitations,
and good agreement among them has not been achieved. To date, neutron
scattering, magnetometry, Brillouin light scattering, spin-torque ferromagnetic
resonance spectroscopy, and Kerr microscopy have all been used to determine the
exchange parameter. Here we present a novel method that exploits the wavevector
selectivity of Brillouin light scattering to measure the spin wave dispersion
in both the backward volume and Damon-Eshbach orientations. The exchange,
saturation magnetization, and magnetic thickness are then determined by a
simultaneous fit of both dispersion branches with general spin wave theory
without any prior knowledge of the thickness of a magnetic "dead layer". In
this work, we demonstrate the strength of this technique for ultrathin metallic
films, typical of those commonly used in industrial applications for magnetic
random-access memory.
