In general the material class under investigations - Heusler compounds with X2YZ with L21 and XYZ with C1b structure type – show the same broad variety of properties comparable to the perovskites, including topological insulators, Kondo behaviour, non-centrosymmetric superconductivity and conventional, tunable magnetic properties, non-collinear magnetism, semiconductivity, magnetoresistance effects, Li-ion-conductivity and other physical properties. Different to the perovskites only a few groups mainly in Germany and Japan are working on this class of materials. However many companies such as Hitachi, IBM, Toshiba, Toyota etc. have research activities based on this material class.
Most Heusler compounds with C1b symmetry are ternary semiconductors and structurally and electronically strongly related to binary semiconductors. The band gap can be tuned from 4 eV to zero and more than 50 compounds show a band inversion. But in addition to the known applications for binary semiconductors, multifunctional properties are possible in this class of materials. For example, similar to the quantum spin Hall system CdTe/HgTe, quantum well structures with band inversion can be predicted for different Heusler compounds combinations such as LuPdSb/LuPtBi. Many of these ternary zero-band semiconductors also contain rare earth elements, which can induce a secondary property ranging from super-conductivity ( e.g. LaPtBi) to antiferromagnetism (e.g. GdPtBi) and heavy fermion behavior (e.g. YbPtBi).
Heusler compounds are well established in the field of spintronics, especially the half metallic ferromagnetic compounds with high Curie temperatures such as Co2FeSi and Co2MnSi. These compounds show a high spin polarization and Curie temperatures up to 1120K. Additionally, Heusler compounds show a lower damping than Permaloy, the material that is widely used for the investigation of domain wall motion. Recently, tetragonal Heusler compounds such as Mn3Ga are in the focus of spintronics research for spin torque applications. To switch the spin by a small current a material is needed with a low magnetic moment, a high Curie temperature, a low Gilbert damping and a high spin polarization. Mn3Ga fulfils all these conditions. Recently, we have found several new compounds with comparable properties. All potential candidates were initially designed by first principle calculation, synthesized as a bulk material and later integrated into nano-devices. The new material groups Mn2YZ and Cr2YZ open also new directions for new multiferroic behaviour, and the path to rare earth free hard magnets, which are nearly unexplored. The recent highlights, a giant exchange bias in Mn2PtGa and a spin gapless semiconductivity, are published in Phys. Rev. Lett. 2013.
Besides material synthesis and standard characterization techniques, we use photoelectron spectroscopy (PES) to verify the predicted electronic structures. High-energy photoemission and Conversion Electron Mössbauer Spectroscopy (CEMS) complement our primary methodological foci. Both of these methods enable the determination of electronic properties of materials, specific for surfaces, interfaces of devices and bulk materials. High-energy photoemission is a very new method, which allows us to investigate the electronic structure of buried interfaces and devices. Our planned spin-resolved high-energy photoemission (together with R. Claessen at PETRAIII, Hamburg and K. Kobayashi, SPring8, Japan) is outstanding worldwide. Our recent highlight is the in operandi measured valence band of electrolyte gated VO2 thin films prepared in collaboration with the lab of S. Parkin (IBM, Almaden, USA).