P1
P1 Synthesis of new materials with high spin polarization and high Curie temperatures (Felser)
This project focuses on the synthesis and characterization of new materials with high spin polarization. In the past, two major problems arose when Heusler compounds were used as electrodes for magnetoelectronic devices. The first problem was the discrepancy between the predicted spin polarization of 100 % and the considerably lower values found in experiments. The second problem was the temperature stability and the reduced spin polarization at room temperature.
The project will continue to search for new materials that exhibit the same extraordinary halfmetallic electronic structure such as that proposed for Co2Cr0.6Fe0.4Al (27.8 valence electrons). In particular, substitutional series with the formula Co2Y1-xY'xZ (Y, Y´ = 3dtransition metals, Z = main group element), and having a valence electron concentration of approximately 27.8 ± 0.5, will be prepared and optimized. The starting point - to overcome the decrease of the spin polarization with increasing temperature - is to find a material where the Fermi energy is located in the middle of the minority band gap. Besides varying the 3d element at the Y position, this will also be established by varying the main group element Z. Compounds with 4d and 5d transition metals will be used to investigate an increasing influence of the spin-orbit interaction. The spin-orbit interaction separates the states having different total angular momenta. This leads to separated states with well-defined total and spin angular momenta at the Fermi energy. The investigation of X2YZ compounds with the 4d and 5d element at the X position is planned. The project will investigate the crystal growth of new materials in detail. In particular, this includes the structural phase transitions (L21 ⇔ B2) and the growth of single crystals. The order-disorder transition has a major impact on the physical properties of the Heusler compounds because they lose their half-metallic character when they are in the B2 structure. The growth of single crystals will be established by different growing techniques. The single crystals will be prepared with different well-defined surfaces so that their properties can be investigated using spectroscopic methods.
Project P 1 provides samples for further investigation in Projects P 5 (Elmers), P 7 (Felser / Schönhense), P 9 (Hamrle / Hillebrands), P 10 (Cinchetti / Aeschlimann) and targets for thin film growth in Project P 2 (Jakob), P 11 (Jourdan), and P 12 (Ando).
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