  In this work we compute the torque field present in a ferromagnet in contact
with a metallic nanowire when a skyrmion is present. If the nanowire is narrow
enough the current is carried by a single conduction band. In this regime the
classical torque model breaks down and we show that a skyrmion driven by spin
transfer torque moves in a different direction than predicted by the classical
model. However, the amount of charge current required to move a skyrmion with a
certain velocity in the single band regime is similar to a classical model of
torque where it is implicitly assumed current transport by many conduction
bands. The single band regime is more efficient creating spin current from
charge current because of the perfect polarization of the single band but is
less efficient creating torque from spin current. Nevertheless, it is possible
to take profit of the single band regime to move skyrmions even with no net
charge or spin current flowing between the device contacts. We have also been
able to recover the classical limit considering an ensemble of only a few
electronic states. In this limit we have discovered that electron diffusion
needs to be considered even in ballistic nanowires due the effect of the
skyrmion structure on the electron current.

Mesoscale and Nanoscale Physics