Hierarchical Superstructures of Ferromagnetic Semiconductor Nanoparticles: Synthesis an Exploration of Magnetoresistive Phenomena.

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Materials that combine semiconductor and magnetic properties are expected to enable a technological breakthrough in information technology. So-called spin-based electronics re-quire compounds capable of generating and processing spin-polarized currents. The recent scientific attention was largely focused on dilute magnetic semiconductors, respectively classical semiconductors doped with paramagnetic ions. Semiconductors with intrinsic ferromagnetic properties such as europium(II) oxide (EuO) have been studied much less, although such materials are valuable models for exploring and understanding fundamental phenomena such as magnetoresponsive properties. This special class of functional solids deliver examples for new effects in spintronics, magneto-electricity in general and also magneto-optics. Unlike other materials such as ZnO, TiO2, Fe3O4, etc., it is still largely un-known how the nanomorphology (size and shape) affects its properties. Because synthesis methods like sol-gel or precipitation methods suitable for the preparation of such other metal oxides fail for Eu in oxidation state +II, there is currently no method to fabricate tai-lored EuO nanostructures and especially defined nanoparticles. The aim of the current project is to fill this gap and to investigate the effects of particle morphology on electronic, optical, magnetic and magneto-electric properties. The concept described in the current proposal is based on the important observation that the indirect synthesis of EuO is possible by using sacrificial templates from another material. Our previous success in converting ZnO microcrystals to EuO while maintaining particle shape and size provides the base for transferring the concept to the nano-scale. EuO nanorods with adjustable aspect ratio will be prepared from ZnO nanocrystals by treatment with elemental europium vapor, and the optimal conditions for the reduction of Zn2+ to Zn0, the removal of Zn and the substitution by Eu will be examined. The investigation of the magnetic properties of the resulting EuO nanoparticles will provide new insights into shape-property correlations. Self-assembly with these building blocks will be explored aiming at the fabrication of ordered nanorod arrays on a substrate as novel particle-based materials with highly directional properties. The focus will be on studying the magneto-electric properties above, below and near the Curie temperature of EuO. The particle-based, periodic EuO nanorod superstructures may become candidates for magnetic random access memories (mRAMs). Other than conventional transistors, the state of resistivity could be maintained even without the constant supply of energy.