  We study gadolinium thin films as a model system for ferromagnets with
negative thermal expansion. Ultrashort laser pulses heat up the electronic
subsystem and we follow the transient strain via ultrafast X-ray diffraction.
In terms of a simple Grueneisen approach the strain is decomposed into two
contributions proportional to the thermal energy of spin and phonon subsystems.
Our analysis reveals that upon femtosecond laser excitation phonons and spins
can be driven out of thermal equilibrium for several nanoseconds.
