  Neutron diffraction and ab initio studies were carried out on Mn2V1-xCoxGa
(x=0, 0.25, 0.5, 0.75, 1) Heusler alloys which exhibits high TC fully
compensated ferrimagnetic characteristic for x=0.5. A combined analysis of
neutron diffraction and ab initio calculations revealed the crystal structure
and magnetic configuration which could not be determined from the X-ray
diffraction and magnetic measurements. As reported earlier, Rietveld refinement
of neutron diffraction data confirmed L21 structure for Mn2VGa and Xa structure
for Mn2CoGa. The alloys with x=0.25 and 0.5 possess L21 structure with Mn(C)-Co
disorder. As the Co concentration reaches 0.75, a structural transition has
been observed from disordered L21 to disordered Xa. Detailed ab initio studies
also confirmed this structural transition. The reason for the magnetic moment
compensation in Mn2(V1-xCox)Ga was identified to be different from that of the
earlier reported fully compensated ferrimagnet (MnCo)VGa. With the help of
neutron diffraction and ab initio studies, it is identified that the disordered
L21 structure with antiparallel coupling between the ferromagnetically aligned
magnetic moments of (Mn(A)-Mn(C)) and (V-Co) atom pairs enables the
compensation in Mn2V1-xCoxGa.
