Hard X-ray photoelectron spectroscopy on buried, off-stoichiometric Co x Mn y Ge z (x:z=2:0.38) Heusler thin films
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Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric Co2-based Heusler alloy shows an intense dependency of the tunnel magnetoresistance (TMR) on the Mn composition, demonstrating high TMR ratios of above 1000 % at 4.2 K (Yamamoto et al. J. Phys. Condens. Matter 22:164212, 2010). This work reports on the electronic structure of nonstoichiometric Co x Mn y Ge z 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 nonstoichiometric Co x Mn y Ge z 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 Co x Mn y Ge z film composition. The resulting changes of the electronic structure within the valence band will allow one to tune the magnetoresistive characteristics of Co x Mn y Ge z -based tunnel junctions as verified by the calculations and photoemission experiments.
KeywordsFermi Energy Electron Spin Polarization Magnetic Tunnel Junction Valence Band Spectrum Heusler Compound
The authors thank E. Ikenaga (BL47XU) and S. Ueda (BL15XU) for help with the experiments. This work was funded by the Deutsche Forschungs Gemeinschaft DFG (TP 1.2-A and 1.3-A of the Research Unit ASPIMATT) and the Japan Science and Technology Agency JST (DFG-JST project: FE633/6-1). The work at Hokkaido University was partly supported by a Grant-in-Aid for Scientific Research (A) (Grant No. 20246054) from the MEXT, Japan, and by the Strategic International Cooperative Program of JST. The synchrotron radiation HAXPES measurements were performed at BL47XU with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Long-term Proposal 2008B0017, 2009A0017) and at BL15XU with the approval of NIMS (Nanonet Support Proposal 2008B4903). The HAXPES experiment at BL15XU was partially supported by the Nanotechnology Network Project MEXT (Japan).
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