  Recent experimental measurements of magnetoresistance in dual spin valves [A.
Aziz et al., Phys. Rev. Lett. 103, 237203 (2009)] reveal some nonlinear
features of transport, which have not been observed in other systems. We
propose a phenomenological model describing current-dependent resistance (and
giant magnetoresistance) in double spin valves. The model is based on a
modified Valet-Fert approach, and takes into account the dependence of
bulk/interface resistance and bulk/interface spin asymmetry parameters for the
central magnetic layer on spin accumulation, and consequently on charge
current. Such a nonlinear model accounts for recent experimental observations.
