For spintronic devices ferromagnetic materials with high spin polarization and Curie temperatures above room temperature are required. Promising candidates are transition metal oxides such as the doped manganites, magnetite (Fe₃O₄), or the double perovskites (A₂BB'O₆ with A= Sr, Ba, Ca and BB' = CrW, CrRe, FeMo, FeRe). In addition, transition metal doped oxide semiconductors such as Mn or Co doped ZnO or TiO₂ have attracted considerable interest. We have grown epitaxial thin films and complex heterostructures of various ferromagnetic oxides using UHV pulsed laser deposition. By process optimization a layer-by-layer growth mode could be established resulting in high quality epitaxial films. With respect to device applications heterostructures consisting of different materials are required. Here, strain accomodation, interdiffusion, oxygen stoichiometry and proper choice of termination layers are important factors in the engineering and optimization of interface properties. The heterostructures have been used for both the clarification of the underlying physics in half-metallic transition metal oxides and the realization of simple device structures. Among our various activities we discuss (i) the exchange mechanism and effect of doping and strain in the double perovskites [1], (ii) the tunnel magnetoresistance in magnetite based magnetic tunnel junctions, and (iii) the exchange mechanism and magnetotransport properties of Mn and Co doped ZnO [2].

This work is supported by the DFG within SPP 1157 (project GR 1132/13).

[1] P. Majewski et al., Phys. Rev. B 72, 132402 (2005), Appl. Phys. Lett. 87, 202503 (2005). [2] K.-W. Nielsen et al., Superlattices and Microstructures 37, 327 (2005).

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1. Magnetoresistance and magnetic properties of the double perovskites