Studies of spin transport in fullerene films

The electron spin degree of freedom has been utilized in spintronic devices, where information is carried out by the spin sense.¹ In the past decade, organic semiconductors have attracted increasing attention in the field of spintronics due to their inherent long spin lifetime.^2,3 The organic semiconductors are mainly composed of light atoms such as carbon and hydrogen and are thus expected to have weak spin-orbit coupling (SOC). Because fullerene molecules are composed of carbon atoms with ∼99% naturally abundant ¹²C atoms having spinless nuclei, the hyperfine interaction (HFI) is also expected to be weak in these molecules. The weak SOC and HFI should lead to a long spin relaxation time in the fullerenes. Spin related processes in the fullerenes have been recently studied in various optoelectronic and spintronic devices^4–8 such as organic light emitting diodes (OLED),^9,10 spin valves,^4,11,12 spin-pumping devices,¹³ and devices based on spin optical conversion.⁸ However, no direct comparison of the spin related properties among the various fullerene molecules has been done so far.

The spin diffusion length in organic semiconductors has been traditionally measured by electrical means using spin-valve devices,² i.e., La_2/3Sr_1/3MnO₃ (LSMO)/organic/Co. In such devices, spin polarized electric current is injected into the organic layer from one ferromagnetic (FM) electrode (LSMO) and detected by the other FM electrode (Co). Indeed, giant magnetoresistance (GMR) has been obtained in such organic spin valves, which originates from the spin scattering rate change when the magnetizations of the two FM electrodes vary between parallel and antiparallel configurations. Furthermore, C₆₀ molecules based spin valves have been successfully fabricated using a variety of FM electrodes. Also, it has been shown that GMR occurs in C₆₀ based spin valves in a wide range of temperatures.^4,5,12 However, the obtained spin diffusion length, $λS60$⁠, in such devices has been in the range of several to hundreds of nanometers.^4,5 One possible reason for the obtained broad $λS60$ distribution is that the spin polarized electrical current injection in C₆₀ spin valves is limited by the conductivity mismatch between the FM electrodes and the C₆₀ interlayer. Consequently, it crucially depends on the FM/C₆₀ spinterface states that may help the injection of spin aligned carriers into the C₆₀ interlayer.^14–18 

Recently, however, thanks to the discovery of spin pumping,¹⁹ pure spin current transport without the need of a charge carrier may be achieved at FM/organic interfaces.^20,21 The pure spin current is generated in this case by microwave (MW) induced magnons at the FM/organic layer that are at ferromagnetic resonance (FMR) conditions. At these conditions, the FM layer absorbs the resonant microwaves and induces pure spin current in the organic layer through the FM/organic interface via the spin-pumping process. Subsequently, the induced spin current in the organic layer may be detected either via the inverse spin-Hall effect (ISHE) in the same layer or by reaching another interface with heavy atom metals having a large SOC, say Pt, where it is detected electrically again by the ISHE.^21,22 Upon varying the organic layer thickness, the latter method may be effectively used to determine the spin diffusion length in the organic interlayer, since the induced spin current is attenuated as it reaches the Pt overlayer. The advantage of this method is that the well-known FM/organic conductivity mismatch and spinterface problems are overcome, since no spin polarized electric current is involved.

Most notably, it has been shown that the SOC plays a key role in the attenuation of spin polarized electric current in the fullerene molecules, which cannot be attributed to the very weak intrinsic SOC and HFI in these compounds.^5,10 One explanation of this finding may be based on theoretical models claiming that the SOC strength, $ξ$⁠, in fullerene molecules should be larger than thought before because it is composed of three terms: intrinsic, curvature-induced, and Rashba:^23,24 $ξ=ξin+ξcurv+ξRashba$⁠. The Rashba SOC, $ξRashba$⁠, is related to the inversion symmetry breaking and should be negligible in the fullerene films. Since the other terms are small, the SOC strength in the fullerene mainly comes from $ξcurv$⁠, which originates from the curvature related hybridization between the π and σ electrons.¹⁰ 

In the present work, we have measured the spin diffusion lengths of several fullerene molecules including C₆₀, C₇₀, and C₈₄ by the method of spin-pumping pure spin current transport, excluding the inaccuracy related to the FM/fullerene conductivity mismatch and spinterface states. We found that the spin diffusion length in the fullerenes actually increases from C₆₀ to C₈₄, in agreement with the decrease in the molecular curvature, which is consistent with the theory.^23,24

Figure 1(a) shows the molecular structure of the fullerene C₆₀, C₇₀, and C₈₄, and the corresponding surface morphologies in fullerene films with ∼30 nm capped on the Pt layer, which were measured by atomic force microscopy (AFM). We note that the effective molecular curvature decreases from C₆₀ to C₈₄. In addition, all the fullerene films show extremely low roughness on a large scale, which indicates comparable deposition conditions for the subsequent NiFe overlayer, needed to complete the trilayer device structure. Figure 1(b) shows the schematics of our spin current transport measurement geometry. The Ni₈₀Fe₂₀ /fullerene/Pt trilayer (protected by a SiO₂ layer) was placed on a coplanar microwave guide. Magnon excitations in the NiFe were generated by microwave absorption at FMR conditions. The microwave-generated spin waves transfer spin angular momentum to the fullerene film via spin pumping at the NiFe/fullerene interface, inducing pure spin current in the fullerene layer. The spin current is attenuated while being transported through the fullerene film, reaching the fullerene/Pt interface at the other side of the trilayer device. The attenuated spin current is then detected as an electric current in the Pt film via the ISHE, measured parallel to the interface.

The metallic NiFe and Pt layers and the SiO₂ protection layer in the device were deposited by electron beam evaporation. All depositions were done in high vacuum at a base pressure of 1 × 10⁻⁷ mbar. The C₆₀/C₇₀/C₈₄ powders (American Dye Source) were thermally evaporated at a slow evaporation rate of ∼0.05 nm/s on top of the Pt film. The fullerene film thickness was determined by profilometry (KLA Tencor) measurements. The device size was typically 0.7 × 3 mm² for the Pt layer and 0.7 × 1 mm² for the NiFe layer designed to be at the center of the Pt layer. The FM/C₆₀ interface was characterized by cross-section view scanning electron microscopy (SEM) using a FEI Helios NanoLab 650 setup [see Fig. 1(c)]. The sharp interfaces seen in the SEM picture indicate relatively low fullerene surface roughness.

The microwave (MW) radiation for the spin pumping into the fullerene layer was generated by an Agilent N5173B amplifier at a frequency ranging from 9 kHz to 20 GHz.²² The MW guide consisted of a 250 μm wide transmission line with the radio frequency magnetic field perpendicular to the DC magnetic field. For the FMR measurements, we used a DC magnetic field, H, a 10 dBm attenuator for C₆₀ and C₇₀, and a 20 dBm attenuator for the C₈₄ device, and the MW was detected by a zero-bias Schottky detector (Krytar 201B). The ISHE voltage generated in the Pt overlayer was detected by the phase sensitive technique, where the MW intensity was modulated at 17 kHz. The electric current direction in the Pt layer is perpendicular to both spin current and spin polarization directions, $E→ISHE∝J→S×σ→$⁠, where $J→S$ and $σ→$ are the spin current and spin polarization, respectively [see Fig. 1(b)]. For each device with a certain fullerene film thickness, we measured two nominally same devices in order to estimate the experimental error bars for the ISHE measurements.

Figures 2(a) and 2(c) show the FMR(H) response spectra of the NiFe/Fullerene/Pt trilayer for the three fullerene films of thickness ∼65 nm, measured by sweeping the DC magnetic field, H, subjected to MW irradiation at a fixed frequency, f = 3 GHz. The resonance fields (H_R = 8.1 mT) and linewidths are comparable for all three fullerene-based trilayers; this shows that the fullerene interlayer does not influence the NiFe magnetization properties. The obtained H_R can be fit using the Kittel equation²⁵ $f=γHR(HR+4πMS)$⁠, where $γ$ is the gyromagnetic ratio, and $MS=1.1×106A/m$ is the saturation magnetization of the NiFe film. The relatively large NiFe obtained magnetization value indicates high quality FM films grown onto the fullerene films, having relatively flat surfaces. The very similar resonance fields, combined with the AFM measurements shown in Fig. 1(a), demonstrate that the NiFe films qualities do not significantly change upon deposition on the various fullerene films used here. We note that the different signal to noise (S/N) levels of the FMR responses are due to different attenuations for the MW detections, as noted above.

Figures 2(d)–2(f) show the H-response of the corresponding ISHE voltages, $VISHE(H)$⁠, measured at the Cu contacts attached to the Pt layer in the three fullerene devices. It is clear that $VISHE$ increases with an increase in the fullerene size from C₆₀ to C₈₄, in which the molecular effective curvature decreases, indicating that the spin current in the related fullerene film that survives the spin polarization attenuation increases from C₆₀ to C₈₄. This indicates that the smaller the attenuation of the spin polarization, the smaller is the molecular curvature, which, according to the theory, is consistent with a smaller SOC.^23,24

Figures 3(a)–3(c) show the $VISHE$(H) responses in the NiFe/fullerene/Pt trilayers with the three fullerene molecules (C₆₀, C₇₀, or C₈₄) measured at various thicknesses, d. It is clear that $VISHE$ decays with d for all three fullerene-based trilayers. The decay with d may be fit by an exponential function,²¹ namely, $VISHE∝e−d/λs$⁠, where $λs$ is the spin diffusion length in the fullerene film [see Fig. 3(d)]. From the fits, we obtain the value of the room temperature spin diffusion lengths in the C₆₀, C₇₀, and C₈₄ films. These are $λS60$ = 13 ± 2 nm, $λS70$ = 18 ± 3 nm, and $λS84$ = 25 ± 3 nm, respectively.

The spin-orbit coupling in conventional organic semiconductors dominates the spin relaxation time, $τS$⁠, which is linked to the spin diffusion length via the carrier mobilities μ:²⁶ $λS=τSμe/kBT$⁠. The reported carrier mobilities of the fullerene films range from 0.02 to 0.65 cm²/V s for C₆₀, from 0.003 to 0.066 cm²/V s for C₇₀, and 0.003 cm²/V s for C84, respectively.^27,28 We can conclude that the obtained variation in the spin diffusion lengths does not originate from the variation of the carrier mobilities in the different fullerene films. We, therefore, infer that the spin relaxation time dominates the spin diffusion length in the fullerenes. Thus, the measured increase of the spin diffusion length from C₆₀ to C₈₄ is due to a decrease of the corresponding spin relaxation time, $τS$⁠, with the fullerene molecular size. The spin relaxation mechanism in the fullerene films is mainly attributed to the SOC, since the HFI is negligibly small in these compounds.³ Under this assumption, the spin diffusion length is nearly proportional to the square root of the SOC strength. Our results are therefore in agreement with the assumption that the SOC in the fullerenes decreases upon increasing the molecular size or equivalently decreasing the effective molecular curvature.^29,30 The π-electrons alone do not possess a substantial SOC, but the molecular curvature in the fullerene induces hybridization between the π and the σ electrons, and this, in turn, leads to an enhanced SOC in the fullerene molecules.

In summary, we have systematically investigated the spin diffusion length in films based on three different fullerene molecules, namely, C₆₀, C₇₀, and C₈₄, using a pure spin current transport method. In this technique, magnons in the NiFe film are excited by microwave absorption at resonance conditions and subsequently transfer spin angular momentum to the fullerene film via the FM/fullerene interface; this spin-pumping method does not require charge injection into the fullerene semiconductor. The induced spin current in the fullerene layer is attenuated and converted by the ISHE into charge current in the Pt overlayer at the fullerene/Pt interface. We found that the room temperature spin diffusion length in the fullerenes is of the order of ∼10 nm and increases with a smaller molecular curvature. We attribute the mechanism of spin attenuation in the fullerene films to the curvature-induced SOC that stems from the hybridization between the π and the σ electrons of the carbon atoms on the fullerene molecule surface.

We thank M. Groesbeck and C. Zhang for help with the experiments. This work was supported by the National Science Foundation (NSF) (Grant No. DMR-1701427).