  Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric
Co2-based Heusler alloy shows a intense dependency of the tunnel
magnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratios
of up to 1995% at 4.2 K for 1. This work reports on the electronic structure of
non-stoichiometric CoxMnyGez thin films with a fixed Co/Ge ratio of x : z = 2 :
0.38. The electronic structure was investigated by high energy, hard X-ray
photoelectron spectroscopy combined with first-principles calculations. The
high-resolution measurements of the valence band of the non-stoichiometric
CoxMnyGez films close to the Fermi energy indicate a shift of the spectral
weight compared to bulk Co2MnGe. This is in agreement with the changes in the
density of states predicted by the calculations. Furthermore it is shown that
the co-sputtering of Co2MnGe together with additional Mn is an appropriate
technique to adjust the stoichiometry of the CoxMnyGez film composition. The
resulting changes of the electronic structure within the valence band will
allow to tune the magnetoresistive characteristics of CoxMnyGez based tunnel
junctions as verified by the calculations and photoemission experiments.
