Effects of spin-spin interactions on magnetoresistance in disordered organic semiconductors

A recent theory of magnetoresistance in positionally disordered organic semiconductors is extended to include exchange and dipolar couplings between polarons. Through the use of spin-dependent perturbation theory in the percolative transport regime, analytic results are discovered when the hyperfine, exchange, and dipolar interactions have little time to operate between hopping events. We find an angle-of-field dependence of the magnetoresistance that agrees with previous experiments and numerical simulations. In addition, we report on magnetoresistive behavior that critically depends upon the amount of anisotropy in the dipolar interaction.