Anomalous organic magnetoresistance from competing   carrier-spin-dependent interactions with localized electronic and nuclear   spins
  We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial room-temperature magnetoresistance ($\sim 20$\%). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current. 
Y. Wang, N. J. Harmon, K. Sahin-Tiras, M. Wohlgenannt and M. E.   Flatt\'e
cond-mat.mtrl-sci cond-mat.dis-nn