Giant magnetoresistance in ferromagnet/organic semiconductor/ferromagnet heterojunctions

We report the spin injection and transport in ferromagnet/organic semiconductor/ferromagnet (FM/OSC/FM) heterojunctions using rubrene $({\text{C}}_{42}{\text{H}}_{28})$ as an organic semiconductor spacer. For completeness of our study, both tunneling magnetoresistance (TMR) and giant magnetoresistance (GMR) were studied by varying the thickness of the rubrene layer (5--30 nm). A thorough study of the device characteristics reveals spin-polarized carrier injection into and subsequent transport through the OSC layer. When the thickness of the rubrene layers are beyond the tunneling limit, the device currents are limited by carrier injection and bulk transport. The carrier injection is well described with phonon-assisted field emission. The behavior of GMR in response to bias field and temperature shows significant differences from that of TMR.