Magnetoresistance in Hybrid Organic Spin Valves at the Onset of Multiple-Step Tunneling

Abstract

By combining experiments with simple model calculations, we obtain new insight in spin transport through hybrid, $\mathrm{CoFeB}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(1.5\text{ }\text{ }\mathrm{nm})/\mathrm{\text{tris}}(8\mathrm{\text{-hydroxyquinoline}})\mathrm{\text{aluminium}}$ $({\mathrm{Alq}}_{3})/\mathrm{Co}$ spin valves. We have measured the characteristic changes in the $I\mathrm{\text{\ensuremath{-}}}V$ behavior as well as the intrinsic loss of magnetoresistance at the onset of multiple-step tunneling. In the regime of multiple-step tunneling, under the condition of low hopping rates, spin precession in the presence of hyperfine coupling is conjectured to be the relevant source of spin relaxation. A quantitative analysis leads to the prediction of a symmetric magnetoresistance around zero magnetic field in addition to the hysteretic magnetoresistance curves, which are indeed observed in our experiments.