Temperature dependent sign change of the organic magnetoresistance effect

A sign change of the organic magnetoresistance effect is observed as a function of temperature. There is a large difference in the IV behavior when the sign of the magnetoresistance (MR) is positive compared to when the sign of the MR is negative, pointing to the possibility that the sign change of the MR is due to a change in the charge transport mechanism. The positive and negative MRs show different characteristic field widths B-0 in the MR versus magnetic field curves. Also, the traces with positive MR show a clear temperature dependence of B-0 while no systematic dependence on temperature is seen in the traces with negative MR. This behavior can be qualitatively explained by the recently proposed bipolaron model. (C) 2008 American Institute of Physics.