Hard X-ray photoelectron spectroscopy on buried, off-stoichiometric CoxMnyGez (x : z = 2 : 0.38) Heusler thin films

摘要

Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometricCo2-based Heusler alloy shows a intense dependency of the tunnelmagnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratiosof up to 1995% at 4.2 K for 1. This work reports on the electronic structure ofnon-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-rayphotoelectron spectroscopy combined with first-principles calculations. Thehigh-resolution measurements of the valence band of the non-stoichiometricCoxMnyGez films close to the Fermi energy indicate a shift of the spectralweight compared to bulk Co2MnGe. This is in agreement with the changes in thedensity of states predicted by the calculations. Furthermore it is shown thatthe co-sputtering of Co2MnGe together with additional Mn is an appropriatetechnique to adjust the stoichiometry of the CoxMnyGez film composition. Theresulting changes of the electronic structure within the valence band willallow to tune the magnetoresistive characteristics of CoxMnyGez based tunneljunctions as verified by the calculations and photoemission experiments.