  The structure and magnetic properties in doped Heusler alloys of Mn2CoGa and
Mn2CoAl have been investigated by experiments and calculations. The main group
elements of Ga and Al are substituted by the magnetic or non-magnetic
transition metals, Co, Cu, V, and Ti in the alloy systems. Three kinds of local
ferromagnetic structures, Co-Mn-Co, Mn-Co-Mn and Mn-Co-V, have been found. They
embed in the native ferrimagnetic matrix and increase the magnetization with
different increments. The Co-Mn-Co ferromagnetic structure shows the largest
increment of 6.18{\mu}B /atom. In addition, interesting results for
non-magnetic Cu increasing the magnetization and the V atom having a large
ferromagnetic moment of about 1.0{\mu}B have been obtained. The exchange
interaction energy can be increased by the newly added Co and depleted by
supporting a ferromagnetic coupling in other substitution cases, and showing
the variation of the TC. Our calculation of electronic structure verifies the
strong d-d hybridization when the three ferromagnetic structures are achieved.
It has also been found that the covalent effect from the Ga and Al determines
the generation of the local ferromagnetic structure and the tolerance for
dopant content.
