Magneto-Electroluminescence Study of Fringe Field in "Magnetic" Organic Light-Emitting Diodes

Magneto-electroluminescence (MEL) represents the electroluminescence intensity change upon application of an external magnetic field. We show that the MEL field response in "magnetic" organic light-emitting diodes, where one electrode is ferromagnetic (FM), is a powerful technique for measuring the induced fringe field, B⃗_F, from the FM electrode in the organic layer. We found that the in-plane fringe field, B⃗_F∥, from 3 nm Co and Ni₈₀Fe₂₀ FM electrodes is proportional to the applied field, B⃗_∥. The fringe field of the 3 nm Ni₈₀Fe₂₀ film was also investigated for an applied out-of-plane magnetic field, B⃗_⊥. We found that the out-of-plane fringe field has two components: a component that is parallel or antiparallel to B⃗_⊥ and remains unchanged with the distance, d, from the FM electrode and the other component that is highly inhomogeneous, parallel to the surface, and steeply decreases with d. We show that the obtained B⃗_F is independent of the underlying mechanism for the MEL(B) response and thus may be considered universal.