  Doped Heusler compounds Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe
ratio $x$ were investigated experimentally and theoretically. The electronic
structure of the ordered, doped Heusler compound Co$_2$Cr$_{1-x}$Fe$_{x}$Al
($x=n/4, n=0,1,2,3,4)$ was calculated using different types of band structure
calculations. The ordered compounds turned out to be ferromagnetic with small
Al magnetic moment being aligned anti-parallel to the 3d transition metal
moments. All compounds show a gap around the Fermi-energy in the minority
bands. The pure compounds exhibit an indirect minority gap, whereas the
ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism
(MCD) in X-ray absorption spectra was measured at the $L_{2,3}$ edges of Co,
Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine
element specific magnetic moments. Calculations and measurements show an
increase of the magnetic moments with increasing iron content. The
experimentally observed reduction of the magnetic moment of Cr can be explained
by Co-Cr site-disorder. The presence of the gap in the minority bands of
Co$_2$CrAl can be attributed to the occurrence of pure Co$_2$ and mixed CrAl
(001)-planes in the $L2_1$ structure. It is retained in structures with
different order of the CrAl planes but vanishes in the $X$-structure with
alternating CoCr and CoAl planes.
