Origin and stability of highly spin polarized surface resonances
Theoretical Condensed Matter Physics
Final Report Abstract
The design and control of specific electronic properties of metallic thin films is a major requirement for the development of powerful spin based electronics (spintronics). Due to their compositional tunability Heusler compounds represent a prime example for such an optimization of electronic states, which is usually based on band structure calculations. The half metallic ferromagnet Co2MnSi represents a typical example for this class of materials. Our UV-photoemission spectroscopy of epitaxial Co2MnSi thin films previously demonstrated a large spin polarization close to 100%. This large spin polarization was measured in a range of binding energies much broader than expected by bulk band structure calculations, which is consistent with photoemission calculations, including surface effects and predicting a highly spin polarized surface resonance at the Fermi energy. Within this project we provided strong evidence for the validity of this theory by comparing the results of SX-ARPES experiments with corresponding calculations. We were able to conclude that indeed a strongly spin polarized surface resonance just above the Fermi energy is present at Co 2MnSi(001) free surfaces and dominates the photoemission intensity distribution. HAXPES investigations of Co2MnSi(001)/metal interfaces allowed us to study the influence of different metals on the surface resonance derived interface states, which is highly relevant for spintronics devices such as spin valves (GMR). A HAXPES feature close to the Fermi energy, which was identified as characteristic for the surface resonance of Co2MnSi(001) free surfaces, was observed at interfaces with epitaxial Ag(001) as well. On the other hand, it is diminished for interfaces with polycrystalline Cu and vanishes completely for interfaces with polycrystalline Al as well as with epitaxial Cr. To relate these photoemission spectroscopy based insight with transport investigations relevant for spintronics devices, which rely on a large spin polarization of ferromagnetic Heusler electrodes, we performed investigations of the unidirectional spin Hall magnetoresistance (USMR) of Co2MnSi/(Ag, Cu, or Cr)(0:5 nm)/Pt (or Ta) multilayers. Consistent with the HAXPES results discussed above, the insertion of a thin Ag layer clearly increases the transport spin polarization of Co 2MnSi(001) compared to all other metallic interfaces. An alternative concept for obtaining a large transport spin polarization at interfaces is based on spin polarized surface states as predicted for topological insulators. We investigated this possibility by UV- momentum microscopy (ARPES) of the Half Heusler compound YPtBi. Indeed, we were able to identify a Dirac cone like surface state, which in principle is able to generate a surface current with a large spin polarization.
Publications
- Dirac cone and pseudogapped density of states in the topological half-Heusler compound YPtBi. Phys. Rev. B, 161108 (2016)
A. Kronenberg, J. Braun, J. Minár, H.-J. Elmers, D. Kutnyakhov, A. V. Zaporozhchenko, R. Wallauer, S. Chernov, K. Medjanik, G. Schönhense, M. Kläui, S. Chadov, H. Ebert, and M. Jourdan
(See online at https://doi.org/10.1103/PhysRevB.94.161108) - Signature of a highly spin polarized resonance state at Co2MnSi(001)/Ag(001) interfaces. J. Phys. D: Appl. Phys. 51, 135307 (2018)
C. Lidig, Jan Minár, J. Braun, H. Ebert, A. Gloskovskii, A. Kronenberg, M. Kläui and M. Jourdan
(See online at https://doi.org/10.1088/1361-6463/aab1cf) - Interface spin polarization of the Heusler compound Co2MnSi probed by unidirectional spin Hall magnetoresistance. Phys. Rev. Appl.
C. Lidig, J. Cramer, L. Weißhoff, T.R. Thomas, T. Kessler, M. Kläui, M. Jourdan
(See online at https://doi.org/10.1103/PhysRevApplied.11.044039) - Surface resonance of the Heusler half metal Co2MnSi probed by SX-ARPES
C. Lidig, J. Minár, J. Braun, H. Ebert, A. Gloskovskii, J. A. Krieger, V. Strocov, M. Kläui, and M. Jourdan
(See online at https://doi.org/10.1103/PhysRevB.99.174432)
