Isotope effect in the spin response of aluminum tris(8-hydroxyquinoline) based devices

Abstract

We studied the spin response of various magnetic field effects and magnetotransport in both protonated and deuterated aluminum tris(8-hydroxyquinoline) [Alq${}_{3}$]-based organic light emitting diodes and spin-valve devices. Both conductivity-detected magnetic resonance in diodes and magnetoresistance in spin valves show substantial isotope dependence pointing to the importance of the hyperfine interaction (HFI) in the spin response of spin \textonehalf{} charge polarons in Alq${}_{3}$. In addition, the low field ($B$ 20 mT) magnetoelectroluminescence (MEL) response is also isotope sensitive, showing that HFI-induced spin mixing of polaron-pairs spin sublevels dominates this response too. However, the magnetoconductance (MC) response was found to be much less sensitive to isotope exchange at low fields, in agreement with previous studies. The disparity between the isotope sensitivity of MC and MEL responses in Alq${}_{3}$ indicates that the HFI in the MC response is overwhelmed by an isotope independent spin mixing mechanism. We propose that collisions of spin \textonehalf{} carriers---with triplet species such as polaron pairs may be the main spin mixing mechanism in the low field MC response in Alq${}_{3}$ diodes.