Matthew Groesbeck

Inverse spin Hall effect from pulsed spin current in organic semiconductors with tunable spin–orbit coupling

Exploration of spin currents in organic semiconductors (OSECs) induced by resonant microwave absorption in ferromagnetic substrates is appealing for potential spintronics applications. Owing to the inherently weak spin-orbit coupling (SOC) of OSECs, their inverse spin Hall effect (ISHE) response is very subtle; limited by the microwave power applicable under continuous-wave (cw) excitation. Here we introduce a novel approach for generating significant ISHE signals in OSECs using pulsed ferromagnetic resonance, where the ISHE is two to three orders of magnitude larger compared to cw excitation. This strong ISHE enables us to investigate a variety of OSECs ranging from π-conjugated polymers with strong SOC that contain intrachain platinum atoms, to weak SOC polymers, to C60 films, where the SOC is predominantly caused by the curvature of the molecules surface. The pulsed-ISHE technique offers a robust route for efficient injection and detection schemes of spin currents at room temperature, and paves the way for spin orbitronics in plastic materials.

Organic-based magnon spintronics

Magnonics concepts utilize spin-wave quanta (magnons) for information transmission, processing and storage. To convert information carried by magnons into an electric signal promises compatibility of magnonic devices with conventional electronic devices, that is, magnon spintronics1. Magnons in inorganic materials have been studied widely with respect to their generation2,3, transport4,5 and detection6. In contrast, resonant spin waves in the room-temperature organic-based ferrimagnet vanadium tetracyanoethylene (V(TCNE)x (x ≈ 2)), were detected only recently7. Herein we report room-temperature coherent magnon generation, transport and detection in films and devices based on V(TCNE)x using three different techniques, which include broadband ferromagnetic resonance (FMR), Brillouin light scattering (BLS) and spin pumping into a Pt adjacent layer. V(TCNE)x can be grown as neat films on a large variety of substrates, and it exhibits extremely low Gilbert damping comparable to that in yttrium iron garnet. Our studies establish an alternative use for organic-based magnets, which, because of their synthetic versatility, may substantially enrich the field of magnon spintronics.Generation, transport and detection of spin-wave quanta in vanadium tetracyanoethylene, an organic ferrimagnet with low Gilbert damping, are reported.

Studies of spin related processes in fullerene C60 devices

We have investigated spin related processes in fullerene C60 devices using several experimental techniques, which include magnetic field effect of photocurrent and electroluminescence in C60-based diodes; spin polarized carrier injection in C60-based spin-valves; and pure spin current generation in NiFe/C60/Pt trilayer devices. We found that the ‘curvature-related spin orbit coupling’ in C60 plays a dominant role in the obtained spin-related phenomena. The measured magneto-photocurrent and magneto-electroluminescence responses in C60 diodes are dominated by the difference in the g-values of hole and electron polarons in the fullerene molecules. We also obtained giant magneto-resistance of ∼10% at 10 K in C60 spin-valve devices, where spin polarized holes are injected into the C60 interlayer. In addition, using the technique of spin-pumping in NiFe/C60/Pt trilayer devices with various C60 interlayer thicknesses we determined the spin diffusion length in C60 films to be 13 ± 2 nm at room temperature.

Study of spin transport in polyfluorene films

Abstract. We have investigated spin transport in films of polyfluorene, a π-conjugated polymer, using the technique of “spin pumping” from a ferromagnet substrate, namely Ni80Fe20 in a NiFe/polyfluorene/Pt trilayer device at room temperature. Pure spin current (without carrier injection) is generated in the polymer film by microwave excitation of the NiFe magnetic moment at ferromagnetic resonance conditions. The induced spin current through the ferromagnet/polymer interface crosses the polymer layer and is detected by a Pt overlayer in the device, where it is converted into electric current via the inverse spin Hall effect. We have successfully determined the spin diffusion length, λS, in the polyfluorene film by varying the polymer thickness in the trilayer structure, and found λS  =  118  ±  9  nm. We also measured the charge-carrier mobility, μ in polyfluorene film using the time-of-flight technique and found it to be affected by dispersive transport. From the obtained λS and μ values, we estimated the spin relaxation time in polyfluorene to be ∼5  μs at room temperature.