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Changes of the electronic and magnetic structure near the martensitic phase transition of Ni-Mn-In Heusler alloys doped with Co are investigated by experiment and theory. The non-stoichiometric Ni48Co5Mn35In12 epitaxial film undergoes a transition from a weakly magnetic martensitic phase below Tm = 350 K to a ferromagnetic austenitic phase above Tm. Element-specific magnetic moments and the unoccupied density of states function is investigated using X-ray magnetic circular dichroism. We find an antiparallel alignment of Mn and Ni/Co magnetic moments in both phases. The electronic structure is calculated using the SPR-KKR Green's function approach considering experimental results on the atomic disorder in the sample. The good agreement of experimental and theoretically simulated X-ray absorption spectra in both phases confirms that a band Jahn-Teller like lifting of a degenerate d-state is the origin of the energetic preference of the martensitic phase at low temperatures.
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