  We report here on the electronic structure of electron-doped half-metallic
ferromagnetic perovskites such Sr$_{2-x}$La$_x$FeMoO$_6$ ($x$=0-0.6) as
obtained from high-resolved valence-band photoemission spectroscopy (PES). By
comparing the PES spectra with band structure calculations, a distinctive peak
at the Fermi level (E$_F$) with predominantly (Fe+Mo) t$_{2g}^\downarrow$
character has been evidenced for all samples, irrespectively of the $x$ values
investigated. Moreover, we show that the electron doping due to the La
substitution provides selectively delocalized carriers to the
t$_{2g}^\downarrow$ metallic spin channel. Consequently, a gradual rising of
the density of states at the E$_F$ has been observed as a function of the La
doping. By changing the incoming photon energy we have shown that electron
doping mainly rises the density of states of Mo parentage. These findings
provide fundamental clues for understanding the origin of ferromagnetism in
these oxides and shall be of relevance for tailoring oxides having still higher
T$_C$.
