  The temperature evolution of GdFeCo electrons following optical heating plays
a key role in all optical switching of GdFeCo and is primarily governed by the
strength of coupling between electrons and phonons. Typically, the strength of
electron-phonon coupling in a metal is deduced by monitoring changes in
reflectance following optical heating and then analyzing the transient
reflectance with a simple two-temperature thermal model. In a magnetic metal,
the change in reflectance cannot be assumed to depend only the electron and
phonon temperatures because a metal's reflectance also depends on the
magnetization. To deduce the electron-phonon coupling constant in GdFeCo, we
analyze thermal transport in Au and GdFeCo bilayers following optical heating
of the GdFeCo electrons. We use the reflectance of the Au layer to monitor the
temperature evolution of the Au phonons. By interpreting the response of the
bilayer to heating with a thermal model, we determine the electron-phonon
coupling constant in GdFeCo to be 6 x 10^17 W/(m^3-K) corresponding to an
electron-phonon relaxation time in GdFeCo of ~150 fs.
