Overview
To investigate the possibility of polarized charge injection at the interface between a ferromagnetic metal and an organic material, we fabricated planar light-emitting organic diodes (OLEDs) having as cathode and anode thin films of different ferromagnetic metals with unequal coercive forces.
As recombination luminescence arises from the singlet exciton, we expect, for bipolar injection of polarized charge carrier injection, a modulation of the electroluminescence (EL) intensity when the relative orientation of the electrode magnetization switches, under the influence of an external magnetic field, from parallel to antiparallel. Measurements on such OLEDs show that the EL intensity increases when the magnetization configuration changes from parallel to antiparallel. Measurements on OLEDs having one non-magnetic and one magnetic electrode, however, show that the EL intensity also changes, namely when the magnetic field reaches the coercive field of the magnetic layer.
This result suggests that the modulation of the EL signal in the OLED with two ferromagnetic electrodes does not arise from spin-polarized charge carriers injected from the ferromagnetic layers into the organic material, but rather due to an electric field dependence of the EL intensity combined with magnetic stray fields from the electrodes.
OLED with Ni anode and permalloy cathode, T = 300 K. (a) Hysteresis loop. (b) Corresponding OLED EL intensity measurement. The offset between the minima of the the two branches of the curve is a measure of the stray field of the ferromagnetic layer. (c) Theoretically expected hysteretic behavior of the electroluminescence.
Figure 2. Longitudinal magneto-optical Kerr measurement
of the hysteresis loop obtained from an OLED having a Ni anode
and a nonmagnetic cathode (Ca) (upper panel). Corresponding OLED EL
intensity measurement showing an intensity level change at the
point where the magnetic field crosses the coercive force of
the layers (lower panel).
