Ni-Mn based Heusler alloys with composition related to the stoichiometry Ni2MnX, with X belonging to the IIIA-VA chemical species, are widely considered as a class of multifunctional magnetic materials. This magnetic phenomenology is related to the martensitic transformation undergoing from ferromagnetic cubic austenite. Depending upon the chemical composition, different martensitic structures are stabilized showing strong lattice distortions. Recently in Co-doped Ni-Mn-Ga ternary Heusler alloys have shown that during martensitic transformation it is observed a drastic drop of the magnetization. Preliminary neutron diffraction experiments on Co-based Ni-Mn-Ga alloy provided clear evidence of a long-range antiferromagnetic ordering in the tetragonal martensite. We suggest to extend the structural study of AF long-range state to others Co-doped Ni-Mn-Ga and Ni-Mn-Sn compositions.