Spin transport in organic semiconductors: From spin pumping by ferromagnetic resonance to lateral spin-valves

Organic semiconductors (OSC) such as conjugated polymers/molecules are enabling flexible, large area optoelectronic devices, such as organic light-emitting diodes, transistors, and solar cells. Due to their exceptionally long spin lifetimes, these carbon-based materials could also have an important impact on spintronics, where carrier spins play a key role in transmitting, processing and storing information. Here, we combine three different approaches to study spin transport in OSC: spin pumping induced by ferromagnetic resonance (FMR), vertical and lateral spin-valves. i) We have recently demonstrated spin pumping by FMR in a ferromagnet (FM)/ conjugated polymer/nonmagnetic (NM) spin-sink trilayer to demonstrate the ability of polarons to carry pure spin currents over hundreds of nanometers with long spin relaxation times of up to a millisecond and to exhibit Hanle precession. Our trilayer architecture relies on spin pumping from a FM material for spin injection into the organic and on the inverse spin-Hall effect (ISHE) in a NM metal with strong spin-orbit coupling for detection of spins transmitted through the organic layer. This technique allows us to measure both spin and charge carrier transport through the organic in the same device. We have also studied spin pumping on different organic materials such as PBTTT, F4TCNQ doped PBTTT as well as P3HT with a thickness dependence in order to characterize the spin diffusion length of the organic layer. ii) We report the results of magnetoresistance (MR) measurements in vertical organic spin-valves where an organic material is contacted by two FM electrodes (Py) with the magnetic field oriented parallel to the layer stack. The organic layer is generally thick enough to avoid any coupling between the two FM electrodes. The MR is measured as a function of temperature, bias and orientation. iii) Finally, we report our attempts at spin injection into OSC using lateral organic spin-valves (LSV). We will present the fabrication method using electron beam lithography and evaporation/sputtering techniques. Typical LSV structures consist of two laterally separated ferromagnetic Co or other ferromagnetic electrodes (with and without tunnel barriers), connected by a patterned OSC ribbon of F4TCNQ doped PBTTT. In these structures, the charge and spin currents can be separated in order to rule out the MR effects of the electrodes and to avoid the presence of a background resistance coming from the contacts. We compared the devices where the FM layer is in direct contact with the organic to the one obtained using tunnel contacts such as FM/MgO and FM/Al₂O₃. These studies are very important in order to optimize the magnetic contacts to organic for future organic-based spintronic devices.