Preparation and investigation of interfaces of Heusler thin films
Zusammenfassung der Projektergebnisse
At the moment the highest TMR-values are obtained by employing epitaxial MgO tunneling barriers. In these junctions the magnitude of the tunneling magnetoresistance (TMR) effect is based on the long decay length of one specific state of the conventional ferromagnetic electrodes in the epitaxial MgO tunneling barrier (coherent tunneling). In this case no large total spin polarization is required in order to obtain a large TMR. If instead an amorphous or polycrystalline AlOx barrier is used, the basic prerequisite for a state selection is not fulfilled, e. g. the symmetry at the barrier is broken and kparallel is not conserved. In this case the semiclassical Jullière model can be applied, which relates the TMR with the total spin polarization of the ferromagnetic electrodes. However, the interface spin polarization can differ from the bulk value in general. In the framework of spin polarization investigations of Heusler compounds by the measurement of the magnetoresistance (TMR) of tunneling junctions with AlOx barrier special emphasis was put on the role of the interfaces. It was demonstrated how an unsuitable morphology can limit the TMR. The barrier morphology could be improved by inserting a Mg layer at the Heusler/barrier interface. Evidence was given that this very thin Mg layer is acting as a seed layer for improving the Al morphology and not as a barrier for coherent MgO tunneling. Thus, the Jullière model could be used for evaluating a relatively large spin polarization of 67% for the B2 ordered Heusler compound Co2Cr0.6Fe0.4Al, which is close to theoretical predictions. With increasing temperature usually a pronounced reduction of the TMR of Heusler junctions is observed. This is surprising considering the high magnetic ordering temperatures of the compounds. A self-evident explanation of the strong temperature dependence of the magnetoresistance of devices based on Heusler thin films was investigated: The possible reduction of the magnetic interface moments compared to the bulk properties of the samples. The magnetic surface and bulk moments of the ferromagnetic Heusler compounds Co2Cr0.6Fe0.4Al and Co2CrAl were comparatively investigated by x-ray magnetic circular dichroism measurements (collaboration with Prof. H.-J. Elmers, P5). We provided evidence that the magnetism of the film interface region is fully developed with an interface magnetic ordering temperature which can be even higher than in the bulk.
Projektbezogene Publikationen (Auswahl)
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(2009). Interface and bulk magnetism of Co 2 Cr 0.6 Fe 0.4 Al and Co 2 CrAl thin films. Applied Physics Letters, 95(17), 172504
M. Jourdan, E. Arbelo Jorge, C. Herbort, M. Kallmayer, P. Klaer, H.-J. Elmers
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(2009). Morphology induced magnetoresistance enhancement of tunneling junctions with the Heusler electrode Co 2 Cr 0.6 Fe 0.4 Al. Applied Physics Letters, 94(14), 142504
C. Herbort, E. Arbelo Jorge, M. Jourdan
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(2009): Morphology and magnetoresistance of Co 2 Cr 0.6 Fe 0.4 Al-based tunnelling junctions. In: J. Phys. D: Appl. Phys. 42 (8), S. 84006
C. Herbort, E. Arbelo, M. Jourdan
