PUB - Publikationen an der Universität Bielefeld

A systematic analysis of the bias voltage and temperature dependence of the tunneling magnetoresistance (TMR) in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions with barrier thickness t(B) between 1.8 and 4.0 nm has been performed. The resistance measured at low temperature in the parallel state shows the expected exponential increase with increasing barrier thickness. The low-temperature TMR amplitude of about 300% is quite similar for all MgO thicknesses. This is in accordance with microstructural investigations by transmission electron microscopy, which do not give hints to a reduction in the barrier quality with increasing MgO thickness. Both the junction resistance and TMR decrease with increasing temperature and bias voltage. In general, the decrease is much stronger for thicker barriers, e. g., a decrease in the TMR by a factor of 13.4 from 293% at 15 K to 21.9% at 300 K was observed for t(B) = 4.0 nm compared to a reduction by only a factor of 1.6 for t(B) = 1.8 nm. This behavior can be described self-consistently for all barrier thicknesses within a model that extends the magnon-assisted tunneling model by adding an inelastic, unpolarized tunneling contribution. Furthermore we discuss our results in the framework of a recent model by Lu et al. [Phys. Rev. Lett. 102, 176801 (2009)] claiming that polarized hopping conductance becomes important for larger MgO thickness.