Toward spin-resolved scanning tunneling microscopy using carbon nanotubes and half-metallic ferromagnets

A new concept for spin-resolved scanning tunneling microscopy (SR-STM) is presented. The probe, a carbon nanotube, is contacted with two oppositely magnetized ferromagnetic, electrodes. A maximum asymmetry of the two currents of about 16% is expected, which could be further improved when half-metallic ferromagnets are used. Devices were fabricated that consist of a multi-wall nanotube connecting two ferromagnetic electrodes. Measurements on several samples showed a magneto-resistance with a maximum-resistance change of 2.2% across a 1.4-μm gap, which corresponds to an estimated spin-scattering length of at least 400 nm. An effect of this magnitude measured over this long a distance is very encouraging with respect to the proposed SR-STM. However, the effect was only observed for temperatures below approximately 40 K. In addition, the disorder dependence of the electronic structure of the half-metallic ferromagnet NiMnSb was investigated. First-principles calculations showed that atomic disorder of only a few percent is sufficient to quench the half-metallic property.