  Certain species of living creatures are known to orientate themselves in the
geomagnetic field. Given the small magnitude of approximately 48 {\mu}T, the
underlying quantum mechanical phenomena are expected to exhibit coherence times
approaching the millisecond regime. In this contribution, we show sensitivity
of organic light-emitting diodes (OLEDs) to magnetic fields far below Earth's
magnetic field, suggesting that coherence times of the spins of charge-carrier
pairs in these devices can be similarly long. By electron paramagnetic
resonance (EPR) experiments, a lower bound for the coherence time can be
assessed directly. Moreover, this technique offers the possibility to determine
the distribution of hyperfine fields within the organic semiconductor layer. We
extend this technique to a material system exhibiting both fluorescence and
phosphorescence, demonstrating stable anticorrelation between optically
detected magnetic resonance (ODMR) spectra in the singlet (fluorescence) and
triplet (phosphorescence) channel. The experiments demonstrate the extreme
sensitivity of OLEDs to both static as well as dynamic magnetic fields and
suggest that coherent spin precession processes of Coulombically bound electron
spin pairs may play a crucial role in the magnetoreceptive ability of living
creatures.
