  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.
