Complex Band Structures of Spintronics Materials

The launch of spintronics research was sparked by the discovery of the effect of Giant magnetoresistance in 1988. Exploiting the spin of electrons it led to new device principles for information processing, transmission and storage [7]. The pioneers of the field, the Frenchman Albert Fert and the German Peter Grünberg were honored with the Nobel Prize in 2007. It requires a detailed knowledge of the material properties to design new devices working on the basis of novel effects comprising the spin and charge degree of freedom. The spin dependent tunneling probability between two electrodes is determined by the properties of the states in the band gap of the insulator. These states can be described by a complex wave vector and for that they are forbidden in a bulk material. In contrast, they determine the electronic structure at interfaces and surfaces, in general in systems with broken translational invariance [2]. In experiments on various electrode materials with epitaxial MgO barriers one has observed oscillations of the transmission probability [8, 4]. First, we consider a onedimensional model to account for the oscillations observed. Secondly, the complex bandstructures of the insulators MgO and ZnO are compared.