Max Planck Institute for Chemical Physics of Solids
Electronic Structure - G. H. Fecher
Theory and Experiment
Hard X-ray photoelectron spectroscopy (HAXPES) is a well-adaptable non-destructive technique for the analysis of electronic bulk states, buried films and interfaces. This technique provides important information for the design of new Heusler compounds.
HAXPES is a photoemission technique that employs hard X-ray (photon energy > 3000 eV). The excitation by hard X-ray results in the emission of electrons with high kinetic energies and, consequently, in very large probing depth. The probe of real bulk states allows a more reliable comparison between experimental spectra and theoretical predictions in Heusler compounds.
HAXPES is a versatile technique
HAXPES is able to determine the symmetry of the states composing the valence band. Circularly polarized hard X-rays allows the investigation of magnetic properties in multilayer structures. Even structural properties can be investigated by using the angular distribution of the high kinetic energy photoelectrons.
Our group has extensively investigated the electronic properties of Heusler compounds for spintronics, magneto shape memories and thermoelectric applications. We have studied the changes in the electronic structure upon chemical substitution or induced phase transitions. Many properties of Heusler compounds are essentially connected to the valence band, which is formed by bonding and hybridized states. The measurement of valence band states by HAXPES is used as validation and feedback for the calculation and tailoring of the Heusler properties. Using HAXPES, we are able to investigate the interfaces of multilayer structures, which is important for devices applications (e.g. magneto tunneling junctions). More recently we have investigated also new materials as topological insulator and new phases induced by ionic liquid electrolyte.
XAS & XMCD
X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) are powerful techniques to probe the electronic and magnetic structure of materials and are used extensively in our group to investigate Heusler compounds, strongly correlated electron systems and other oxide materials.
Heusler compounds are promising for many magnetism-related applications such as spintronics, magnetocalorics and hard magnets. In particular, the Mn-containing thin film compounds are especially compelling, where the presence of Mn on two inequivalent sublattices leads to a wide variety of interesting properties (e.g. non-collinear magnetism, large exchange bias) and potential device applications (e.g. STT-RAM). Bulk Heusler alloys are also being investigated for magnetocalorics and rare-earth free hard magnets. The element specificity of L-edge XAS and XMCD grants the opportunity to probe the magnetic properties of these materials, including the individual magnetic sublattices.
We have also begun to investigate strongly correlated electron systems and other oxide materials, where the orbital polarization in novel states of matter induced by liquid electrolyte gating can be probed. Finally, we have recently used K-edge XAS to probe the valance states in a series of Mn-containing bulk Heusler compounds.