  Spin-dependent coherent quantum transport through carbon nanotubes (CNT) is
studied theoretically within a tight-binding model and the Green's function
partitioning technique. End-contacted metal/nanotube/metal systems are modelled
and next studied in the magnetic context, i.e. either with ferromagnetic
electrodes or at external magnetic fields. The former case shows that quite a
substantial giant magnetoresistance (GMR) effect occurs ($\pm 20%$) for
disorder-free CNTs. Anderson-disorder averaged GMR, in turn, is positive and
reduced down to several percent in the vicinity of the charge neutrality point.
At parallel magnetic fields, characteristic Aharonov-Bohm-type oscillations are
revealed with pronounced features due to a combined effect of:
length-to-perimeter ratio, unintentional electrode-induced doping, Zeeman
splitting, and energy-level broadening. In particular, a CNT is predicted to
lose its ability to serve as a magneto-electrical switch when its length and
perimeter become comparable. In case of perpendicular geometry, there are
conductance oscillations approaching asymptotically the upper theoretical limit
to the conductance, $4 e^2/h$. Moreover in the ballistic transport regime,
initially the conductance increases only slightly with the magnetic field or
remains nearly constant because spin up- and spin down-contributions to the
total magnetoresistance partially compensate each other.
