Spin-dependent transport in Fe-doped carbon nanotubes

We report on a first principles calculation of spin-dependent quantum transport in Fe-doped single-wall carbon nanotube (CNT) junctions. For junctions of a pristine $(9,0)$ CNT in contact with Fe-doped $(9,0)$ CNT leads, the total current in parallel configuration of the moment is larger than that of antiparallel configuration under lower bias voltage. For higher bias voltages the opposite happens. A tunnel magnetoresistance ration as large as 40% is found at zero bias, it decays with bias, and eventually goes to negative values at larger bias. Similar results are obtained for junctions with pristine $(10,0)$, $(8,0)$ CNT in contact with Fe-doped $(10,0)$ or $(8,0)$ CNT leads. The spin-dependent transport features can be understood by analyzing microscopic details of the transmission coefficients.