  We theoretically investigate the role of spin fluctuations in charge
transport through a magnetic junction. Motivated by recent experiments that
measure a nonlinear dependence of the current on electrical bias, we develop a
systematic understanding of the interplay of charge and spin dynamics in
nanoscale magnetic junctions. Our model captures two distinct features arising
from these fluctuations: magnon-assisted transport and the effect of
spin-transfer torque on the magnetoconductance. The latter stems from magnetic
misalignment in the junction induced by spin-current fluctuations. As the
temperature is lowered, inelastic quantum scattering takes over thermal
fluctuations, exhibiting signatures that make it readily distinguishable from
magnon-assisted transport.
