Research Highlights

Atomic-scale detection of magnetic impurity interactions in bulk semiconductors

The identification of materials systems to be used as dilute magnetic semiconductors has so far been following a trial-and-error approach, resulting in slow progress of this field. For rational materials design, understanding the magnetic interactions in a dilute magnetic semiconductor on a fundamental, atomic level is a prerequisite. We promote spin-polarized scanning tunneling microscopy as a powerful method to extract atomic-scale information about the magnetic interactions of impurities in semiconductors.

The results of large-scale ab initio simulations put us in position to devise a generally applicable experimental strategy that exploits passivation of the dangling bonds at semiconductor surfaces in order to preserve the bulklike behavior of subsurface impurities. The latter is not the case in state-of-the-art cross-sectional scanning tunneling microscopy experiments, e.g., on Mn-doped GaAs.
In advantage over conventional spectroscopic techniques, such as electron paramagnetic resonance or x-ray magnetic circular dichroism, the proposed strategy provides a visual explanation why ferromagnetism is more difficult to achieve in Si than in GaAs.

Read more: Phys. Rev. B 92, 100407(R) (2015)

Interplay of growth mode and thermally induced spin accumulation in Al/Heusler alloy contacts

The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between aluminium leads was investigated by means of ab initio calculations of the thermodynamic stability and electronic transport.

We have found that: (i)  the interface between Al and the closely lattice-matched Heusler alloys of type Co2TiZ (Z=Si or Ge) is stable under various growth conditions; and (ii) the conventional and spin-dependent Seebeck coefficients in such heterojunctions exhibit a strong dependence on both the spacer and the atomic composition of the Al/Heusler interface, in other words, on whether a Ti-Z or a Co-Co plane makes the contact between Al and the Heusler alloy. Our results show that it is in principle possible to tailor the spincaloric effects by a targeted growth control of the samples.

Read more: Phys. Rev. B 89, 184422 (2014)

Large Seebeck magnetic anisotropy in thin Co films embedded in Cu determined by ab initio investigations

The longitudinal thermopower of a Cu/Co/Cu trilayer system exhibits an oscillatory dependence on the thickness of the Co layer, a behavior related to the formation of quantum well states in the minority spin channel. In addition, it is found to be very sensitive to a switching between an in-plane and out-of-plane magnetisation, a setup schematically shown in the enclosed figure.

The resulting magnetothermopower (MTP) is much larger than anticipated from the conventional anisotropic magnetoresistance (AMR). Our calculations, performed within the framework of spin-polarised relativistic Korringa-Kohn-Rostoker Green's function method combined with the Landauer-Buttiker conductance formula, establish a direct connection between the magnitude of the MTP signal and the asymmetry of the AMR around the Fermi energy. An enhancement of MTP based on this understanding may offer the possibility of implementing an efficient spin read-out thermoelectric device based on a single ferromagnetic layer.

Read more: Phys. Rev. B 88, 104425 (2013)

Contact information

Prof. Dr. Peter Kratzer

Faculty of Physics
University of Duisburg-Essen
Lotharstr. 1
Room: MG 371
47048 Duisburg
Email
Phone: +49-(0)203-379-3313
Fax: +49-(0)203-379-4732

Secretary

Annemarie Tappert, M.A. 

Room: MG 324
Email
Phone: +49-(0)203-379-4731
Fax: +49-(0)203-379-4732