  The quaternary full Heusler compound Mn(2-x)Co(x)VAl with x = 1 is predicted
to be a half-metallic antiferromagnet. Thin films of the quaternary compounds
with x = 0...2 were prepared by DC and RF magnetron co-sputtering on heated MgO
(001) substrates. The magnetic structure was examined by x-ray magnetic
circular dichroism and the chemical disorder was characterized by x-ray
diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn2VAl (x = 0).
For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to
Mn. The observed reduced magnetic moments are interpreted with the help of band
structure calculations in the coherent potential approximation. Mn2VAl is very
sensitive to disorder involving Mn, because nearest-neighbor Mn atoms couple
anti-ferromagnetically. Co2VAl has B2 order and has reduced magnetization. In
the cases with x >= 0.9 conventional ferromagnetism was observed, closely
related to the atomic disorder in these compounds.
