Session A6: Focus Session: Spin-Dependent Physics in Carbon-Based Materials I

Spin injection into Pt-polymers with large spin-orbit coupling

Organic spintronics has entered a new era of devices that integrate organic light-emitting diodes (OLED) in organic spin valve (OSV) geometry (dubbed bipolar organic spin valve, or spin-OLED), for actively manipulating the device electroluminescence via the spin alignment of two ferromagnetic electrodes (\textit{Science} \textbf{337}, 204-209, 2012; \textit{Appl. Phys. Lett}. 103, 042411, 2013). Organic semiconductors that contain heavy metal elements have been widely used as phosphorescent dopants in white-OLEDs. However such active materials are detrimental for OSV operation due to their large spin-orbit coupling (SOC) that may limit the spin diffusion length and thus spin-OLED based on organics with large SOC is a challenge. We report the successful fabrication of OSVs based on pi-conjugated polymers which contain intrachain Platinum atoms (dubbed Pt-polymers). Spin injection into the Pt-polymers is investigated by the giant magnetoresistance (GMR) effect as a function of bias voltage, temperature and polymer layer thickness. From the GMR bias voltage dependence we infer that the ``impendence mismatch'' between ferromagnetic electrodes and Pt-polymer may be suppressed due to the large SOC.   

Interface control of spin transport in magnetic tunnel junctions with MgO\textbackslash Cu-Phthalicyanine hybrid barrier

In this work, systematic investigation of interface electronic properties in Fe(001)\textbackslash MgO(001)\textbackslash Cu-Phthalocyanine (CuPc) and Fe(001)\textbackslash CuPc was carried out by using spin polarized metastable He de-excitation spectroscopy (SP-MDS) technique. The electronic structure related to the absorption geometry of CuPc on the Fe (001) and MgO(001) was carefully explored. Differences in the spin resolved density of states were observed as a function of CuPc thickness. The clear evidence of spin-polarized organic spinterface appears even at room temperature in ultra-thin (< 2 nm) CuPc films on the epitaxially grown Fe(001)\textbackslash MgO(001) bilayer. These findings have significant implications for understanding of spin injection from a ferromagnetic layer into an organic semiconductor (OSC), and highlight the importance of adsorption geometry and interfacial exchange coupling in the process of spin injection. This is demonstrated in measurements of the spin transport of Fe\textbackslash MgO(001)\textbackslash CuPc\textbackslash Co tunnel junctions. For the MgO\textbackslash CuPC hybrid barrier, high magnetoresistance value ($> 100$\%) and rather small value ($\sim$ 10\%) were measured at 77 K and 300 K, respectively. Our results provide significant new insights into the phenomenon of spin injection into an OSC and the operation of molecular spintronic devices.   

Spin Filtering in Graphene Magnetic Tunnel Junctions

We present experimental measurements of spin filtering across ferromagnet-graphene-ferromagnet tunnel junctions. These junctions are predicted to yield nearly 100{\%} spin-polarized charge currents [1,2] and were previously shown to sustain spin-polarized tunnel currents at room temperature [3]. In this work, high-quality multi-layer graphene was synthesized directly on crystalline (111) close-packed ferromagnetic thin films by chemical vapor deposition. All deposition and patterning steps employed standard, wafer-scale photolithography, deposition and ion milling techniques. The charge transport and spin transport across the junctions were measured in a four-probe geometry as a function of applied magnetic field and temperature ranging from 5K to 500K. The signature of minority-pass spin filtering with a low-resistance anti-parallel state is evident throughout the temperature range studied. [1] Karpan et al., Phys. Rev. Lett. 99, 176602, 2007 [2] Yazyev and Pasquarello, Phys. Rev. B. 80, 035408, 2009 [3] Cobas et al., Nano Letters 12, 3000, 2012.   

Self-assembly of monolayers on SFMO for fabrication of molecular magnetic tunnel junctions

Half-metallic oxides, such as La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) and Sr$_2$FeMoO$_6$ (SFMO), are highly spin polarized and air stable, making them attractive as spin injectors for organic and molecular spintronics. Recently, it was demonstrated that self-assembled monolayers (SAMs) of alkylphosphonic acids can be grafted onto LSMO while maintaining LSMO`s spin polarization. However, due to its relatively low Curie temperature, the magnetoresistance of devices based on LSMO is severely curtailed at room temperature. In contrast, SFMO has a $T_C > 400$ K. As a first step in incorporating this material in a molecular magnetic tunnel junction, we show that it also supports alkylphosphonic SAMs. Epitaxial SFMO films are grown on STO via off-axis sputtering and have a room temperature magnetic moment per formula unit of about 1.2 $\mu_B$ and a Fe:Mo stoichiometry ratio of 0.9:1.0, determined by RBS. The quality and structure of SAMS grafted on these films is interrogated through methods including contact angle measurements, AFM, and FTIR spectroscopy. Progress towards fully realized spin polarized tunnel junctions and implications for room temperature molecular spintronics will be discussed.   

Quantum Fluctuations of Local Magnetoresistance in Organic Spin Valves

Aside from interfacial effects, the performance of organic spin valves is limited by the spin memory loss in course of electron transport between the magnetized electrodes. One of the most prominent mechanisms of this loss is the spin precession in the random hyperfine fields of nuclei. We assume that the electron transport is due to incoherent multi-step tunneling. Then the precession takes place while electron ``waits'' for the subsequent tunneling step. While the spatial coherence of electron is lost after a single step, the spin evolution remains absolutely coherent all the way between the electrodes. As a result, the {\em amplitudes} of subsequent spin rotation interfere with each other. We demonstrate that this interference leads to a wide spread in the {\em local} values of tunnel magnetoresistance (TMR). Moreover, if {\em on average} the TMR is positive, the portion of the surface area where the TMR is negative is appreciable. We calculate analytically and numerically the distribution of local TMR as a function of the spin-valve thickness.   

Noise spectroscopy of magnetic tunnel junctions with organic barriers

Understanding the details of spin and charge transport through organic barriers remains one of the main challenges in organic spintronics. Here we present low frequency noise studies in magnetic tunnel junctions with thin (2-5nm) organic PTCDA barriers in the tunnelling regime, investigated at temperatures of under 1K up to 300K. Shot noise measurements show a superpoissonian contribution at low biases giving rise to a Fano factor of around 1.5-2. We tentatively link the enhanced shot noise with electron bunching induced by inelastic interaction with collective low frequency vibration modes of the molecules. On the other hand, the bias dependence of 1/f noise studied up to 350mV reveals reproducible anomalies which could be linked with excitations induced by inelastic tunnelling, due to individual vibrational modes of higher frequency of the PTCDA molecules. The dependence of the shot and 1/f noise with the magnetic alignment of the electrodes will also be discussed   

Disorder induced spin coherence in polyfluorene thin film semiconductors

Charge carrier spins in polymeric organic semiconductors significantly influence magneto-optoelectronic properties of these materials [1]. In particular, spin relaxation times influence magnetoresistance and electroluminescence. We have studied the role of structural and electronic disorder in polaron spin-relaxation times. As a model polymer, we used polyfluorene, which can exist in two distinct morphologies: an amorphous (glassy) and an ordered (beta) phase [2]. The phases can be controlled in thin films by preparation parameters and verified by photoluminescence spectroscopy. We conducted pulsed electrically detected magnetic resonance (pEDMR) measurements to determine spin-dephasing times by transient current measurements under bipolar charge carrier injection conditions and a forward bias. The measurements showed that, contrary to intuition, spin-dephasing times increase with material disorder. We attribute this behavior to a reduction in hyperfine field strength for carriers in the glassy phase due to increased structural disorder in the hydrogenated side chains, leading to longer spin coherence times.\\[4pt] [1] C. Boehme, J.M. Lupton, Nature Nano. 8, 612 (2013).\\[0pt] [2] A. Khan et al. Phys. Rev. B 69, 085201 (2004).   

Suppression of the Hanle effect in organic spintronic devices

We study carrier spin transport under a transverse magnetic field in organic structures [1]. In organics, carriers are localized polarons and charge transport is via polaron hopping. Spin transport, however, can utilize the exchange coupling between localized polarons, which can be much faster than polaron hopping and rapidly increases with the carrier density. Consequently, a much stronger magnetic field is needed to modify spin polarization and observe the Hanle effect than estimated from the carrier mobility, which can help understand recent Hanle measurements in organic spin valves. The exchange-induced spin transport also greatly mitigates the conductivity mismatch between ferromagnets and organics, enabling spin injection into organics. [1] Z. G. Yu, Phys. Rev. Lett. 111, 016601 (2013).   

Simultaneous electrical and optical detection of magnetic resonance in MEH-PPV

While it is established that spin Pauli blockade controlled s=1/2-pair transitions are dominant spin-dependent transitions [1] at room temperature in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), there is controversy whether these pairs are unipolar or bipolar [1]. Spin-dependent processes can be observed with electrically (conductivity) and optically (photoluminescence) detected magnetic resonance. The former is sensitive to unipolar and bipolar processes [1] while the latter is sensitive only to bipolar charge carrier recombination [2]. Here, we present experiments on MEH-PPV organic light emitting diodes where the transient current and electroluminescence response to a pulsed magnetic resonance excitation of charge carriers is measured by detection of both observables on the same device at the same time. The measurements were made at various temperatures and injection (bias) conditions. Correlations between the dynamics of electrically and optically detected signals under these various conditions allows to discrimination between spin-dependent processes which affect one of the two observables only and those that affect both.\\[4pt] [1] C. Boehme, J.M. Lupton, Nature Nanotechnol.8 (9), 612-615 (2013).\\[0pt] [2] S.-Y. Lee et al. J. Am. Chem. Soc.133, 072019 (2011).   

Hyperfine spin interactions between polarons and nuclei in organic light emitting diodes: Magneto-EL measurements

Considerable attention in recent years has focused on the effects of applied magnetic fields on the conductance, photocurrent, electroluminescence (EL), and photoluminescence of nominally nonmagnetic organic semiconductor materials and devices. These magnetic field effects have proven useful in revealing the underlying physical mechanisms and relevant spin interactions that influence the electrical and optical properties in these organic systems (e.g., hyperfine coupling, exchange interactions, and spin-orbit coupling). Here we study the field-dependent properties of organic light-emitting diode (OLEDs) based on MTDATA/LiF/Bphen layered structures, in which exciplex recombination at the interface dominates the EL spectra. Small applied magnetic fields ($\sim$10 mT) are found to boost the net EL yield by up to 10\%, due to a suppression of the mixing between singlet and triplet polaron pairs which, in turn, arises from hyperfine spin coupling of the polarons to the underlying nuclei of the host molecules. We discuss the dependence of these field-induced effects on the LiF barrier thickness, device bias, and on the orientation of the applied magnetic field, as well as the mechanisms responsible.   

Electrical detection of nuclear spins in organic light-emitting diodes

We present pulsed combined electrically detected electron paramagnetic and nuclear magnetic resonance experiments on MEH-PPV OLEDs. Spin dynamics in these structures are governed by hyperfine interactions between charge carriers and the surrounding hydrogen nuclei, which are abundant in these materials. Hyperfine coupling has been observed by monitoring the device current during coherent spin excitation [1]. Electron spin echoes (ESEs) are detected by applying one additional readout pulse at the time of echo formation [2]. This allows for the application of high-resolution spectroscopy based on ESE detection, such as electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) available for electrical detection schemes. We conduct electrically detected ESEEM [3] and ENDOR [4] experiments and show how hyperfine interactions in MEH-PPV with and without deuterated polymer side groups can be observed by device current measurements.\\[4pt] [1] D. R. McCamey et al., Phys. Rev. Lett. 104, 017601 (2010).\\[0pt] [2] W. J. Baker et al., Phys. Rev. Lett. 108, 267601 (2012).\\[0pt] [3] M. Fehr et al., Phys. Rev. B 84, 193202 (2011).\\[0pt] [4] F. Hoehne et al., Phys. Rev. Lett. 106, 187601 (2011).   

Tunable Magneto-conductance and Magneto-electroluminescence in Polymer Light-Emitting Electrochemical Planar Devices

We report first time studies of magneto-conductance (MC) and magneto-electroluminescence (MEL) in polymer light-emitting electrochemical \textit{planar devices} using ``super-yellow'' poly-(phenylene vynilene), SY-PPV. We observed consistent negative MC while MEL changes sign to positive when electroluminescence quantum efficiency increases (ELQE). At optimal ELQE, the MC has a much narrower width than MEL indicating that 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 depending on the applied voltage. In addition, MC reduces its magnitude when the device current increases at constant illumination power. We discuss the results in the context of the existing models. 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.   

Magneto-photocurrent in organic photovoltaic cells; the effect of short-lived charge transfer states

The spin degrees of freedom are responsible for the magnetic field effects in organic devices at low magnetic fields. The MFE is formed via a variety of spin-mixing mechanisms, such as the hyperfine (typical strength: B$_{\mathrm{hf}}$\textless 0.003 T), triplet-polaron or triplet-triplet (B$_{\mathrm{trip}}$\textless 0.1 T) interactions, that limit the response by their respective strength. We report on magneto-photocurrent (MPC) response of bulk hetero-junction organic photovoltaic cells in an extended field range B$=$0.00005 - 8 Tesla, and found that spin mixing mechanisms are still operative even at the highest fields. In fact, the response MPC(B) can be divided into three main regions, each with a different sign: sharp response that increases with B up to B$_{1}$ $\sim$ 0.04 T; broad response that decreases with B in the range from B$_{1}$ to B$_{2}$ $\sim$ 0.3-0.7 T; and even broader response that increases above B$_{2}$; this response does not saturate even at 8.5 T. We attribute the latter MPC component to short-lived charge transfer excitons (CTE) where spin-mixing is caused by the difference of the donor/acceptor g factors; a mechanism that is increasingly more effective at high magnetic field.   

Optical Detection of Injected Spin Aligned Carriers in Organic Semiconductors by Sagnac Interferometry

Conventionally, spin-aligned carrier injection into organic semiconductors has been investigated by Giant magneto-resistance (GMR), which is inherently difficult to interpret due to the large number of artifacts in organic spintronic devices. Optical detection of spin injection would allow for the direct characterization of spin-polarized carriers without the need for spin-analyzer layers, but has been considered difficult to achieve due to the small spin-orbit coupling in organic semiconductors. A Magneto-Optic Kerr Effect (MOKE) sensitive Sagnac Interferometer offers a robust, highly sensitive approach for detection of spin polarization in semiconductors with weak spin-orbit coupling. We have successfully constructed a Sagnac Interferometer having $\sim$50 nano-radian resolution for the change in polarization angle. Here we describe several experiments using the Sagnac to study spin injection in a variety of organic and inorganic spintronic junctions and devices, by measuring the Kerr rotation induced by the spin aligned carriers, to unambiguously demonstrate the injection of spin-polarized current from the ferromagnetic electrode.   

Testing the spin-dependent polaron-pair transition model for low–temperature PEDOT:PSS through electrically detected spin-Rabi beat detuning

Poly[styrenesulfonate] doped poly[3,4-ethylenedioxythiophene] (PEDOT:PSS), which is a well-known organic metal at room temperature, exhibits a very distinct spin-dependent transition at low temperatures ($<$70K). We have studied this with pulsed electrically detected magnetic resonance spectroscopy which revealed a two-spin s=1/2 pair recombination process with very weakly spin-spin coupled pairs that are exposed to very weak hyperfine fields. This is in contrast to strong hyperfine fields reported for similar mechanisms in other materials [1]. In absence of hyperfine fields, the detuning behavior of spin-Rabi oscillation controlled electronic transition rates as predicted by Rajevac et al. [2] can be tested. This theory predicts that the Rabi-beat frequency approaches twice the detuning (= difference between excitation frequency and the Larmor frequency of the spins), in contrast to the Rabi nutation frequency, which approaches the detuning frequency. Our electrically detected spin-Rabi beat oscillation measurements as a function of the detuning experimentally confirm these predictions with very high precision.\\[0pt] [1] W. J. Baker et al., Phys. Rev. Lett. 108, 267601 (2012).\\[0pt] [2] V. Rajevac et al., Phys. Rev. B 74, 245206 (2006).