Monte Carlo study of organic magnetoresistance: Effect of exchange interaction

We present a 3D multiparticle Monte Carlo simulation approach to model hopping transport in disordered organic materials and study the effect of organic magnetoresistance (OMAR). The model is based on the bipolaron mechanism within a framework of strongly correlated disordered systems, incorporating both random hyperfine fields and an external magnetic field. The model includes the effects of exchange and short-range Coulomb interactions on magnetoresistance. Our results demonstrate that the shape of the magnetoresistance curve closely matches the experimentally observed Lorentzian profile. In particular, we identify a critical dependence of the OMAR sign on the interplay between the exchange interaction, Coulomb repulsion, and charge carrier concentration at room temperature. A transition from a positive to a negative OMAR is observed, correlated with the phase change from a paramagnetic phase at zero magnetic field to the formation of ferromagnetic ordering clusters under a finite magnetic field (B = 100 mT). These results highlight the essential role of carrier density in controlling the sign of OMAR, providing new insights into spin-dependent transport in organic semiconductors.