Spin-flip induced magnetoresistance in positionally disordered organic   solids
  A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin flips on hopping transport by considering the effect of spin dynamics on an effective density of hopping sites. Faster spin-flip transitions open up `spin-blocked' pathways to become viable conduction channels and hence produces magnetoresistance. The magnetoresistance can be found analytically in several regimes, including when the spin-flip time is slower than the hopping time. The ratio of hopping time to the hyperfine precession time is a crucial quantity in determining the shape of magnetoresistance curves. Studies of magnetoresistance in known systems with controllable positional disorder would provide a stringent test of this model. 
N. J. Harmon and M. E. Flatt\'e
cond-mat.dis-nn