  Femtosecond laser excitations in half-metal (HM) compounds are theoretically
predicted to induce an exotic picosecond spin dynamics. In particular,
conversely to what is observed in conventional metals and semiconductors, the
thermalization process in HMs leads to a long living partially thermalized
configuration characterized by three Fermi--Dirac distributions for the
minority, majority conduction and majority valence electrons respectively.
Remarkably, these distributions have the same temperature but different
chemical potentials. This unusual thermodynamics state causes a persistent
non-equilibrium spin polarization only well above the Fermi energy. Femtosecond
spin dynamics experiments performed on Fe$_3$O$_4$ by time-, spin-, and
angle-resolved photoelectron spectroscopy confirm our model. Furthermore, the
spin polarization response proves to be very robust and it can be adopted to
selectively test the bulk HM character in a wide range of compounds.
