Overview

The main focus of research in our working group is on inorganic functional and multifunctional materials. For many years we have followed one primary philosophy: that is, the rational design of novel materials based on electronic structure calculations. Recently our priority has been half-metallic ferromagnets for spintronics applications. For more information on our activities in this field, please see 'ASPIMATT'. Highlights include the discovery of a high magnetoresistive effect in Co₂Cr_0.6Fe_0.4Al in 2000 and the high Curie temperature of Co₂FeSi in 2005. Both systems are now widely studied. With the Heusler compound Co₂Cr_0.6Fe_0.4Al as one electrode, a record TMR (tunneling magnetoresistance) ratio at 4 K of 317% was found by Yamamoto’s group in Sapporo, Japan. However, although high TMR ratios at 4 K are easily achieved with Heusler electrodes, the temperature stability of the TMR was disappointing. Guided by our theoretical predictions for tailoring the Fermi energy, the Inomata group built a tunneling device from the Co₂FeAl_0.5Si_0.5. Heusler compound that showed a TMR ratio above 200% at room temperature. Further, we have shown the importance of electron-electron correlation in Heusler compounds.

The materials under investigation - Heusler compounds and C1_b compounds that are sometimes referred to as half Heusler compounds – show the same variety and tenability as has been shown in the perovskites, including magnetoresistive effects, superconductivity, semiconductivity, Li-ion conductivity, magnetic shape memory behavior and other properties. Therefore, they are very interesting as materials for advanced energy applications. One of our important ambitions is the combination of several functions in one compound for the design of multifunctional inorganic materials. Multifunctional materials are a major challenge for the future: The realization of such materials appears to be possible for the case of perovskites, as well as for the Heusler compounds through the elementary understanding of the relationship: crystal structure – electronic structure – function.

Innovative materials science – materials science bridging fundamental research and industrial applications – is another challenge for our team. In Mainz, we have intense collaboration with local companies like Prema, Naomi, Singulus and Sensitec (Spintronics), as well es with international companies like IBM, Schott (Solar cells) and Siemens (Spintronics). In collaboration with EMPA (Zürich), we are searching for and investigating new materials for thermoelectric applications.

Besides the synthesis of new materials and their characterization by standard methods (X-ray diffraction, magnetometry etc), we use several synchrotron radiation based methods. In particular, we perform photoelectron emission (PES) and absorption spectroscopy (XAS) to verify the predicted electronic and magnetic structure. We also have in-house expertise in both high-energy photo emission and Conversion Electron Mössbauer Spectroscopy (CEMS). Both of these methods enable the determination of the electronic properties of materials, specifically at the surfaces and interfaces of heterostructures as well as of bulk materials. The planned bulk-specific spin-resolved high-energy photoemission at PETRA III (Hasylab) is unique worldwide. Therefore, we have numerous national (G. Schönhense, Mainz; R. Claessen, Würzburg, W. Eberhardt, BESSY Berlin; W. Drube, Hasylab) and international collaborations (C. Fadley, ALS Berkeley; S. Suga, Osaka; K. Kobayashi, Spring8).

Reference:

Spintronics: a challenge for material science and solid state chemistry, C. Felser, G. Fecher, and B. Balke, Angewandte Chemie, Internal. Ed. 46 (2007) 668.