Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. Competition between spin-dynamics due to these spatially uncorrelated fields and an applied magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Here, we discuss a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several percent, and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity values in the absence of an applied field. The fringe field effects are insensitive to the ferromagnetic film’s thickness (and therefore the fringe field magnitude) but sensitive to the magnetic domain’s correlation length. This points at fringe-field gradients as an important ingredient of this mechanism. We develop a model based on fringe-field induced polaron-pair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance.
Recent work has shown that weak applied magnetic fields of several tens of mT can lead to a change of several
percent in the (photo)conductivity of organic semiconductor devices. However, it remains to be determined
whether the applied magnetic field modifies the (photo)carrier density, their mobility or both. We use magneticfield-
dependent time-of-flight spectroscopy to disentangle these two possibilities. We find evidence that the
magnetic field leads to a decrease in the photocarrier time-of-flight. We also examine organic magnetoresistive
devices in the frequency domain to complete the characterization of the time-dependent field-effect response.
We report on the experimental observation of large magnetoresistance in polyfluorene organic light-emitting diodes (OLEDs). Very similar magnetic field effects (MFEs) of comparable magnitude were also observed in electroluminescence and photocurrent measurements. We provide a comprehensive overview of these three types of MFE. To the best of our knowledge, the mechanism causing these MFE is currently not known. Moreover, we show that these experiments do not allow determination whether the MFE acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in holeonly OLEDs and show that the MFE acts on the carrier mobility rather than carrier density.
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