Magnetic interface properties of new materials with high spin polarization
Final Report Abstract
This project investigated element-specific spin and orbital magnetic moments of polycrystalline bulk Heusler alloys that are predicted to be half-metallic with composition Co2YZ (Y = Ti, Cr, Mn, Fe and Z=Al, Ga, Si, Ge, Sn, Sb) using magnetic circular dichroism in x-ray absorption spectroscopy (XAS/XMCD). In addition to stoichiometric compounds we also investigated the composition series with partly replaced elements on the Y-site (Co2FexCr1-xSi, Co2MnxTi1-xSi and Co2MnxTi1-xGe) and on the Z-site (Co2MnGa1-xGex) promising a tailoring of the Fermi level with respect to the minority band gap. We compare experimental results with theoretical predictions elucidating the influence of local disorder in the experimental samples. Moreover, we demonstrate that a consideration of electron correlation in local density approximation theories is necessary for reproducing experimental results. Increased orbital magnetic moments in respect to theoretical predictions put forward the role of spin-orbit coupling for half-metallic properties. For the case of single crystalline thin films we developed a method of simultaneous measurement of bulk and surface sensitive magnetic properties including those at the crucial interface to a tunnelling barrier. Exploiting the comparison of bulk and interface information, film growth can be improved for specific applications. In order to directly determine the spin-resolved density-of-states function we present a calculation scheme for extracting this information from the XMCD spectra considering finalstate electron correlations. We investigate the electronic properties of epitaxial Co2FexMn1-xSi, Co2FeAl1-xSix, and Co2Cr0.6Fe0.4Al films on MgO(100) substrates and for the case of several bulk samples including Co2TiSb as a reference sample for normal metallic ferromagnetism. The experimental results, revealing the distribution of magnetic moments and the relative position of the Fermi energy as a function of the number of valence electrons, confirm the predicted possibility of tailoring the minority band gap using substitutional quaternary Heusler compounds. These findings may be of general importance for the understanding of the electronic structures in complex intermetallic compounds.
Publications
- (2006): Reduction of surface magnetism of Co2Cr0.6Fe0.4Al Heusler alloy films. In: Appl. Phys. Lett. 88 (7), S. 72506
M. Kallmayer, H. Schneider, G. Jakob, H. J. Elmers, K. Kroth, H. C. Kandpal, U. Stumm, S. Cramm
(See online at https://doi.org/10.1063/1.2175486) - (2009): Spin-resolved unoccupied density of states in epitaxial Heusler-alloy films. In: Phys. Rev. B 80 (2), S. 20406
M. Kallmayer, P. Klaer, H. Schneider, E. Arbelo Jorge, C. Herbort, G. Jakob, M. Jourdan, H. J. Elmers
(See online at https://doi.org/10.1103/PhysRevB.80.020406) - (2009): Tailoring the electronic structure of half-metallic Heusler alloys. In: Phys. Rev. B 80 (14), S. 144405
P. Klaer, M. Kallmayer, C. G. F. Blum, T. Graf, J. Barth, B. Balke, G. H. Fecher, C. Felser, H. J. Elmers
(See online at https://doi.org/10.1103/PhysRevB.80.144405)
