Effect of spin-orbit coupling on magnetoresistance in organic semiconductors

Abstract

We study the recently discovered organic magnetoresistance ͑OMAR͒ effect in a pair of materials that have similar chemical structures except that one contains a heavy atom to enhance spin-orbit coupling. We use photoluminescence spectroscopy to estimate the spin-orbit coupling strength. In the material with weak spinorbit coupling the characteristic magnetic field scale is comparable to the hyperfine coupling strength. In the material with strong spin-orbit coupling we find that the OMAR is strongly reduced in size and the OMAR traces clearly exhibit a second, higher field scale which we identify with the spin-orbit coupling strength. We model our results using the standard spin-dynamics Hamiltonian.

Temperature dependent sign change of the organic magnetoresistance effect

“…This is an interesting observation since in almost all previously measured devices in literature, B 0 has not shown any temperature dependence. Curiously, Sheng et al observed that the large B 0 feature in Ir͑PPy͒ 3 decreases in width as the temperature decreases, 11 opposite to the dependence we observe; however, they did not speculate on the origin of this effect. Therefore, it seems that the width of large B 0 features are more temperature sensitive than that of small B 0 features.…”

“…One exception is the molecule Ir͑PPy͒ 3 which has shown a B 0 as large as 100 mT. 11 This has been explained by the large spin-orbit coupling present in the molecule. However, spinorbit coupling cannot explain the large difference in widths between the +MR and −MR features since the features coexist in the same material and the spin-orbit coupling is independent of temperature.…”

Modeling of positive and negative organic magnetoresistance in organic light-emitting diodes

“…38 This function was first proposed by Mermer et al 10 and was shown by Sheng et al to be a solution to the Hamiltonians for both hyperfine 11 and spin-orbit 20 interactions, and as such may be a generic expression for a spin interaction in the presence of a magnetic field in these systems. We have already demonstrated that in thick devices the OMR has a positive contribution from two processes which, depending on the relative spin states of the polaron and triplet, are due to the weak electrostatic trapping or spin blocking of polarons at triplets and the interaction between strongly interacting polarons and triplets.…”

Recent progress in organic spintronics

“…However, its significance should not be dismissed so easily. In fact, several recent works suggests that SOI is important for spin relaxation in organic materials [3][4][5][6][7].…”