Synthesis of new materials with high spin polarization and high Curie temperatures
Final Report Abstract
This project is designing and synthesising new Heusler compounds, based on electronic structure calculations. In the past the search of new Co2-containing Heusler compounds as electrodes for tunnel junctions or giant magneto resistance effects were in the focus of the project. The interest in Heusler compounds for spintronic applications has started with the discovery of a large magneto resistance effect in Co2Cr0.6Fe0.4Al in powder samples, followed by the discovery of the highest Curie temperature for a half metallic ferromagnet of 1120K in Co2FeSi. Important contributions were made also in context of adjusting the Fermi energy in the middle of the gap and the application of spectroscopy such as Mößbauer and spin echo NMR or the improvement of the crystalline order in bulk materials and thin films. The growth of large single crystals is now established. Czochralski grown single crystals reveal areas of 2–3 mm width and 5–7 mm length. Single crystals of Co2FeSi by the optical floating zone technique were grown in collaboration with the group of Bernd Büchner, Dresden. All crystals show good atomic ordering, resulting in outstanding electrical behavior with low residual resistivity and high residual-resistivity-ratio. However, a careful NMR investigation shows that the floating zone crystal is perfectly ordered and this perfect L21 order is reflected in the best transport properties measured up to now for Heusler compounds.The design and production of half metallic Heusler compounds Co2MnSi, Co2FeSi, Co2MnAl and Co2FeSi0.5Al0.5 is now well established, especially because of their half metallicity they are used in magnetic tunnel junctions as well as in CPP-GMR devices. With Mn3-xGa a first example of a tetragonal Heusler compound was provided. In comparison with the cubic Co2-Heusler compounds, which show in-plane magnetisation in thin films, spintronic devices with perpendicular magnetocrystalline anisotropy can satisfy the thermal stability requirement and have no limit of cell aspect ratio. Furthermore, many new Co2-based Heusler compounds were successfully synthesized and investigated using various techniques to analyze their suitability as new materials for spintronic applications. Among others, the project investigated Co2YZ (with Y = Sc, Ti, V, Cr, Mn, or Fe and Z = Al, Ga, Si, Ge, or Sn), Co2Y1-xY'xZ (with Y, Y' = Sc, Ti, Cr, Mn, or Fe and Z = Al, Ga, Si, Ge, or Sn), Co2YZ1-xZ'x (with Y = Ti, Mn, or Fe and Z, Z' = Al, Ga, Si, Ge, or Sn), and Co2-xFe1+x. As examples for Heusler compounds with 4d electrons Rh2MnGe, Rh2MnAl, Ru2VAl, Ru2VSi, Ru2TiSi, Ru2MnSi, Ru2MnAl, and Ru2FeSi were investigated. As examples of quaternary Heusler compounds the solid solution series CoMn1+xV1-xAl and CoFe1+xTi1-xAl were investigated.
Publications
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Mn3Ga, a compensated ferrimagnet with high Curie temperature and low magnetic moment for spin torque transfer applications. Appl. Phys. Lett. 90 (2007) 152504
B. Balke, G. H. Fecher, J. Winterlik, C. Felser
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Spintronics: A Challenge for Materials Science and Solid-State Chemistry. Angewandte Chemie, Interna. Ed. 46 (2007) 668
C. Felser, G. H. Fecher, B. Balke
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Highly ordered, half-metallic Co2FeSi single crystals. Appl. Phys. Lett. 95 (2009) 161903
C. G. F. Blum, C. A. Jenkins, J. Barth, C. Felser, S. Wurmehl, G. Friemel, C. Hess, G. Behr, B. Büchner, A. Reller, S. Riegg, S. Ebbinghaus, T. Ellis, P. J. Jacobs, J. T. Kohlhepp, H. J. M. Swagten
