Rational design of new materials for spintronics: Co₂FeZ (Z=Al, Ga, Si, Ge)

Spintronic is a multidisciplinary field and a new research area. New materials must be found for satisfying the different types of demands. The search for stable half-metallic ferromagnets and ferromagnetic semiconductors with Curie temperatures higher than room temperature is still a challenge for solid state scientists. A general understanding of how structures are related to properties is a necessary prerequisite for material design. Computational simulations are an important tool for a rational design of new materials. The new developments in this new field are reported from the point of view of material scientists. The development of magnetic Heusler compounds specifically designed as material for spintronic applications has made tremendous progress in the very recent past. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% in magnetic tunnel junctions. High Curie temperatures were found in Co₂-based Heusler compounds with values up to 1120 K in Co₂FeSi. The latest results at the time of writing are a tunnelling magnet resistance (TMR) device made from the Co₂FeAl_0.5Si_0.5 Heusler compound and working at room temperature with a (TMR) effect higher than 200%. Good interfaces and a well-ordered compound are the precondition to realize the predicted half-metallic properties. The series Co₂FeAl_{1- x} Si _x is found to exhibit half-metallic ferromagnetism over a broad range, and it is shown that electron doping stabilizes the gap in the minority states for x=0.5. This might be a reason for the exceptional temperature behaviour of Co₂FeAl_0.5Si_0.5 TMR devices. Using x-ray diffraction (XRD), it was shown conclusively that Co₂FeAl crystallizes in the B2 structure whereas Co₂FeSi crystallizes in the L2₁ structure. For the compounds Co₂FeGa or Co₂FeGe, with Curie temperatures expected higher than 1000 K, the standard XRD technique using laboratory sources cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L2₁ structure which makes these two compounds suitable new candidates as materials in magnetic tunnel junctions.