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B6: Spin-dependent tunneling in oxidic heterostructures

Completely oxidic, planar tunneling magnetoresistance (TMR) structures will be investigated experimentally and theoretically. The project focuses on the influence of the material composition, especially the interfaces, on the magnetic and magnetotransport properties. First of all, ZnO:TM/MgZnO/ZnO:TM (TM: transition metals, Co, Mn) structures should be used. Motivated by own theoretical predictions, the ferromagnetic coupling in ZrO₂:TM and HfO₂:TM at temperatures higher than 300 K should be proven experimentally.

Preliminary investigations present ferromagnetic ZnO:(Mn, Co) without any indices of impurity phases. Due to improvements in the field of homoepitaxial ZnO (on ZnO substrates), it is possible to achieve an up to now not reached material- and interface quality of epitaxial grown thin films, which should be used to establish enhanced TMR-structures of the type ZnO:TM/oxidic barrier/ZnO:TM. Beside ZnO:TM, novel materials like transition metal doped ZrO₂ and HfO₂ should be investigated experimentally and implemented in TMR-structures. Our own theoretical predictions reveal that ZrO₂:TM as well as HfO₂:TM show ferromagnetic coupling with Curie temperatures above 300 K; the experimental proof of this fact should be done. However, as well as in the case of ZnO, the homoepitaxial approach on yttria stabilized ZrO₂ substrates (YSZ) should be realised. The experimental structures should be described parameter-free on the basis of ab initio calculations based on density functional theory (DFT) under implementation of correlation corrections. The theory of tunneling magnetoresistance includes, derived from works about bulk properties, the calculation of thin films of the implemented materials, the calculation of spin-flip-scattering cross sections, the conductance at coherent transport based on the Landauer-theory and finally the current-voltage characteristic by means of Keldysh-nonequilibrium-greenfunctions.

Principal Investigators

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