  We study the zero-bias magnetoresistance MR of an interacting quantum dot
connected to two ferromagnetic leads and capacitively coupled to a gate voltage
source Vg. We investigate the effects of the spin-activity of the contacts
between the dot and the leads by introducing an effective exchange field in an
Anderson model. This spin-activity makes easier negative MR effects, and can
even lead to a giant MR effect with a sign tunable with Vg. Assuming a twofold
orbital degeneracy, our approach allows to interpret in an interacting picture
the MR(Vg) measured by S. Sahoo et al. [Nature Phys. 2, 99 (2005)] in single
wall carbon nanotubes with ferromagnetic contacts. If this experiment is
repeated on a larger Vg-range, we expect that the MR(Vg) oscillations are not
regular like in the presently available data, due to Coulomb interactions.
