Rugang Geng

Curvature-enhanced Spin-orbit Coupling and Spinterface Effect in Fullerene-based Spin Valves.

We investigated curvature-enhanced spin-orbit coupling (SOC) and spinterface effect in carbon-based organic spin valves (OSVs) using buckyball C60 and C70 molecules. Since the naturally abundant 12C has spinless nuclear, the materials have negligible hyperfine interaction (HFI) and the same intrinsic SOC, but different curvature SOC due to their distinct curvatures. We fitted the thickness dependence of magnetoresistance (MR) in OSVs at various temperatures using the modified Jullière equation. We found that the spin diffusion length in the C70 film is above 120 nm, clearly longer than that in C60 film at all temperatures. The effective SOC ratio of the C70 film to the C60 film was estimated to be about 0.8. This was confirmed by the magneto-electroluminescence (MEL) measurement in fullerene-based light emitting diodes (LED). Next, the effective spin polarization in C70-based OSVs is smaller than that in C60-based OSVs implying that they have different spinterface effect. First principle calculation study shows that the spin polarization of the dz2 orbital electrons of Co atoms contacted with C60 is larger causing better effective spin polarization at the interface.

Tunable magneto-conductance and magneto-electroluminescence in polymer light-emitting electrochemical planar devices

We report studies of magneto-conductance (MC) and magneto-electroluminescence (MEL) in polymer light-emitting electrochemical planar devices using “super-yellow” poly-(phenylene vinylene). We observed consistent negative MC while MEL becomes positive when electroluminescence quantum efficiency (ELQE) increases. At an optimal ELQE, the MC has a much narrower width than the MEL, indicating that the MC and MEL do not share a common origin. However, MC reverses and has the same width as MEL when exposed to a threshold laser power. We show that the e-h pair model can explain the positive MEL and MC while the negative MC can be explained by the bipolaron model.

Large magnetoelectric effect in organic ferroelectric copolymer-based multiferroic tunnel junctions

We report electrically controlled interfacial spin polarization, or the magnetoelectric effect in multiferroic tunnel junctions by employing organic ferroelectric copolymers, poly(vinylindene fluoride-trifluoroethylene) (P(VDF-TrFE)), as a tunneling barrier. First, we show that the ferroelectric domains and spontaneous ferroelectric polarization of the P(VDF-TrFE) films can be formed in a thin interlayer. Next, we demonstrate that the tunneling magnetoresistance in the unpolarized multiferroic tunnel junction severely quenches from 21% at 20 K to 0.7% at 296 K. Remarkably, we find that the interfacial spin polarization of the device, dubbed spinterface, can be gradually tuned by controlling the ferroelectric polarization with an applied electric field. Specifically, the tunneling electromagnetoresistance can reach around 1000% while the tunneling electroresistance reaches about 30% at 200 K. We speculate that the interface might act as a polarization-dependent spin filter causing the large spinterface eff...

Thermal Conductance of Poly(3-methylthiophene) Brushes

A wide variety of recent work has demonstrated that the thermal conductivity of polymers can be improved dramatically through the alignment of polymer chains in the direction of heat transfer. Most of the polymeric samples exhibit high conductivity in either the axial direction of a fiber or in the in-plane direction of a thin film, while the most useful direction for thermal management is often the cross-plane direction of a film. Here we show poly(3-methylthiophene) brushes grafted from phosphonic acid monolayers using surface initiated polymerization can exhibit through-plane thermal conductivity greater than 2 W/(m K), a 6-fold increase compared to spin-coated poly(3-hexylthiophene) samples. The thickness of these films (10-40 nm) is somewhat less than that required in most applications, but the method demonstrates a route toward higher thermal conductivity in covalently grafted, aligned polymer films.

Enhanced room-temperature spin Seebeck effect in a YIG/C60/Pt layered heterostructure

We report on large enhancement of the longitudinal spin Seebeck effect (LSSE) in the Y3Fe5O12 (YIG)/Pt system at room temperature due to the addition of a thin layer of organic semiconductor (C60) in between the YIG and the Pt. LSSE measurements show that the LSSE voltage increases significantly, from the initial value of 150 nV for the YIG/Pt structure to 240 nV for the YIG/C60(5nm)/Pt structure. Radio-frequency transverse susceptibility experiments reveal a significant decrease in the surface perpendicular magnetic anisotropy (PMA) of the YIG film when C60 is deposited on it. These results suggest that the LSSE enhancement may be attributed to increased spin mixing conductance, the decreased PMA, and the large spin diffusion length of C60.