  Magnetic nanoparticles (MNPs) exhibiting superparamagnetic properties might
generate large magnetic dipole-dipole interaction with electron spins in
organic semiconductors (OSECs). This concept could be considered analogous to
the effect of hyperfine interaction (HFI). In order to investigate this model,
Fe3O4 MNPs are used as a dopant for generating random hyperfine-like magnetic
fields in a HFI-dominant {\pi}-conjugated polymer host,
poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MeH-PPV). The
magnetoconductance (MC) response in organic light emitting diodes made by
MeH-PPV/MNP blends is used to estimate the effective hyperfine field in the
blends. Firstly, we find that the shape of the MC response essentially remains
the same regardless of the MNP concentration, which is attributed to the
similar functionality between the nuclear spins and the MNPs. Secondly, the
width of MC increases with increasing MNP concentration. Magneto-optical Kerr
effect experiments and micromagnetic simulation indicate that the additional
increase of the MC width is associated with the strength of the magnetization
of the blend. Finally, the MC broadening has the same temperature dependent
trend as the magnetization of the MNPs where the unique effect of the MNPs in
their superparamagnetic and ferromagnetic regimes on the MC response is
observed. Magneto-photoinduced absorption (MPA) spectroscopy confirms that the
MC broadening is not due to defects introduced by the MNPs, but is a result of
unique superparamagnetic behavior. Our study yields a new pathway for tuning
OSECs' magnetic functionality, which is essential to organic optoelectronic
devices and magnetic sensor applications.
