Study of Organic Magnetoresistance

Abstract

The objective of this study is to understand the mechanism of magnetoresistance in organic semiconductors. To achieve the goal, a new measurement scheme for organic magnetoresistance (OMR) was devised, which can minimize the errors caused by a long-term current drift and dielectric relaxation in quasi-single-layer organic light emitting diodes (OLED). The new measurement scheme allows to measure OMR with a very high precision even at low magnetic fields. Electrical, thermal, morphological, structural, and photophysical properties of the OLED devices were examined by using differential scanning calorimetry, UV fluorescence optical microscopy, X-ray diffraction, and photoluminescence measurements. The long-term current drift and dielectric relaxation ($\sim$383 s) were observed in a quasi-single-layer OLED device based on Alq$_{3}$. Using the new measurement scheme, the power law behavior of magnetoresistance in Alq$_{3}$ was observed in an entire range of magnetic field from 1 to $\sim$140 mT. The magnetoresistance was observable only above the threshold voltage and its sign was always negative. However, the sign change was observed upon annealing of the Alq$_{3}$ film. X-ray diffraction spectra showed that the as-deposited Alq$_{3}$ film was initially amorphous, but changed to a crystalline $\beta$-phase after annealing at 145 $^{°}$C and above. The concomitant sign change in the magnetoresistance from negative to positive was observed at room temperature after the annealing. The magnetoresistance was decomposed into positive and negative components, with the negative one showing a power law behavior and the positive one saturated with a magnetic field.