Injection of spin-polarized electrons from ferromagnetic Fe₃O₄ nanoparticles into CdSe/CdZnS colloidal nanoplatelets

We have studied the magnetic field dependence of the electroluminescence (EL) from organic light emitting devices (LEDs). The LEDs incorporate a layer of CdSe/CdZnS core/shell colloidal quantum wells also known as nanoplatelets as the electron-hole recombination site. The injected electrons transport through a layer of ferromagnetic Fe₃O₄nanoparticles and thus become spin polarized predominantly in their -1/2 spin state before they are captured by the nanoplatelets. The emitted EL is circularly polarized as σ₊ (LCP) and shows hysteretic behavior with maximum circular polarization of 4 % , coersive field of 0.16 tesla, and remanent polarization of 1 %. The polarization from a reference device that does not incorporate the Fe₃O₄ nanoparticle layer does not exhibit hysteresis. We interpret the observed results as due to the fact that the injected electrons become spin-polarized as they transport through the Fe₃O₄nanoparticle layer. The hysteresis disappears for T > 100 K.