Wachstum und Transporteigenschaften dünner Co-basierter Heusler-Filme

For the advancement of spinelectronicsmuch importance is attached to Heusler compounds. Especially compounds with the stoichiometry Co2YZ are supposed to exhibit a large asymmetry between majority and minority electrons at the Fermi edge. Ideally, only majority states are present. This property leads to high magnetoresistive effects. However, the experimental results available at present fall behind the expectations. In particular, a strong reduction of the spin asymmetry with increasing temperature is problematic. For this reason,rnthe investigation of further representatives of this material class as well as optimization of their deposition is required. Therefore, during the course of this work thin Heusler films with the composition Co2Cr0.6Fe0.4Al and Co2Mn1−xFexSi were fabricated. At first, this was accomplished by sputter deposition, which is the standard technique for the preparation of thin Heuslerrnfilms. It resulted also here in samples with high structural order. On the other hand, these films exhibit only a reduced magnetic moment. To improve this situation, a laser ablation system was constructed. The resulting film deposition under ultra-high vacuum led to a clear improvement especially of the magnetic properties. In addition to the improved deposition conditions, this method allowed the flexible variation of the film stoichiometry as well. This possibility was successfully demonstrated in this work by deposition of epitaxial Co2Mn1−xFexSi films. The availableness of these high quality quaternary alloys allowed the systematic investigation of their electronic properties. Band structure calculations predict that the substitution of Mn by Fe lead to a shift of the Fermi energy over the minority energy gap, whereas the density of states remains nearly unchanged. This prediction could by tested by electronic transport measurements. Especially the normal Hall effect, which was measured at these samples, shows a transition from a hole-like charge transport in Co2MnSi to an electron-like transport in Co2FeSi. This is in accordance with corresponding band structure calculations as well as with comparative XMCD experiments. Furthermore, the behavior of the anomalous Hall effect was studied. Here it could be seen, that the effect is influenced by two mechanisms: On the one hand an intrinsic contribution, caused by the topology of the Fermi surface and on the other hand by temperature dependent impurity scattering. These two effects have an opposing influence on the anomalous Hall effect. This can lead to a sign reversal of the anomalous contribution. This behavior has been predicted just recently and was here systematically investigated for the first time for Heusler compounds.