Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B42: Organic SpintronicsFocus
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Sponsoring Units: GMAG DMP DCOMP FIAP Chair: Val Vardeny, University of Utah Room: 389 |
Monday, March 13, 2017 11:15AM - 11:27AM |
B42.00001: Studies of magnetically induced circular polarization phosphorescence in Pt-rich pi-conjugated polymers at high fields C. Zhang, D. Sun, R. McLaughlin, E. D. Semenov, S. McGill, E. Ehrenfreund, Z. V. Vardeny We have measured magnetic-field induced circular polarization (FICPO) phosphorescence in various Pt-rich pi-conjugated polymers, in which the spin-orbit coupling can be tuned by changing the intrachain Pt atom concentration through incorporation of different organic ligand spacers. FICPO response, which includes the difference between right ($\sigma +)$ and left ($\sigma $-) circularly polarized phosphorescence emission bands, increases linearly at low field $B$\textless 5 T, but starts to saturate at $B$\textasciitilde 17 T. We determined the effective g-factor from the Zeeman splitted $\sigma +$ and $\sigma $- emission bands, and found that its deviation from the free electron value (g$=$2) scales with the strength of spin-orbit coupling in the Pt-polymer chains. Surprisingly, we also found that at intermediate $B \quad \sigma +$ emission intensity at the higher photon energy is stronger than that of the $\sigma $- emission, indicating $B$-dependent radiative decay rates in the triplet spin sublevels. [Preview Abstract] |
Monday, March 13, 2017 11:27AM - 11:39AM |
B42.00002: Theoretical proposal for a magnetic resonance study of charge transport in organic semiconductors Vagharsh Mkhitaryan Charge transport in disordered organic semiconductors occurs via carrier incoherent hops in a band of localized states. In the framework of continuous-time random walk the carrier on-site waiting time distribution (WTD) is one of the basic characteristics of diffusion. Besides, WTD is fundamentally related to the density of states (DOS) of localized states, which is a key feature of a material determining the optoelectric properties. However, reliable first-principle calculations of DOS in organic materials are not yet available and experimental characterization of DOS and WTD is desirable. We theoretically study the spin dynamics of hopping carriers and propose measurement schemes directly probing WTD, based on the zero-field spin relaxation and the primary (Hahn) spin echo. The proposed schemes are possible because, as we demonstrate, the long-time behavior of the zero-field relaxation and the primary echo is determined by WTD, both for the hyperfine coupling dominated and the spin-orbit coupling dominated spin dynamics. We also examine the dispersive charge transport, which is a non-Markovian sub-diffusive process characterized by non-stationarity. We show that the proposed schemes unambiguously capture the effects of non-stationarity, e.g., the aging behavior of random walks. [Preview Abstract] |
Monday, March 13, 2017 11:39AM - 11:51AM |
B42.00003: Quantitative inverse spin Hall effect detection under control of the ferromagnetic resonant driving field amplitude Marzieh Kavand, Chuang Zhang, Dali Sun, Hans Mallisa, Valy Vardeny, Christoph Boehme Charge-free spin current from a ferromagnetic (FM) into a non-magnetic material is possible under FM resonance (FMR), and this can be observed through spin- to charge-current conversion using the inverse spin Hall effect (ISHE) [1,2]. As the magnitude of the ISHE scales linearly with the power associated with the FMR driving field amplitude~$B_{\mathrm{1}}$, quantitative ISHE measurements require precise control of~$B_{\mathrm{1}}$. This study demonstrates in~\textit{situ}~monitoring of~$B_{\mathrm{1}}$~by electron paramagnetic resonantly induced transient nutation of paramagnetic molecules (1:1complex of $\alpha $, $\gamma $-bisdiphenylene- $\beta $-phenylallyl and benzene, BDPA) placed in proximity of an NiFe/Pt-based ISHE device. Concurrent to an ISHE experiment,~$B_{\mathrm{1}}$~is obtained from the inductively measured BDPA Rabi-nutation frequency. High reproducibility of the ISHE measurement is achieved using this approach combined with the renormalization of the ISHE voltage to~$B_{\mathrm{1}}^{\mathrm{2}}$, with an accuracy limited only by the inhomogeneity of the FMR driving field [3]. [1] K. Ando, at al.,~\textit{J. Appl. Phys.}~\textbf{109}, 103913 (2011). [2] D. Sun et al.,~\textit{Nat. Mater.~}\textbf{15~}863-869$~($2016). [3] M. Kavand et al., arXiv:1610.2759v1. [Preview Abstract] |
Monday, March 13, 2017 11:51AM - 12:03PM |
B42.00004: Spin-Orbit Induced Spin Relaxation in Organic Semiconductors Stephen McMillan, Nicholas Harmon, Michael Flatt\'e Weak spin-orbit coupling suggest long spin relaxation times in organic semiconductors. The correlations between slow carrier transport and these mechanisms of spin relaxation can yield complex behavior, including large magnetic-field effects on spin and charge dynamics. We use a continuous time random walk approach to investigate the effect of spin-orbit coupling on spin relaxation in organic materials with non-interacting carriers that incoherently hop from place to place. The simulation has been adapted from earlier work in this area [1] to emphasize the quantum nature of the relaxation. Transition rates for spin conserving and spin flipping hops are calculated as functions of the randomly assigned spatial orientation of the molecular sites. In a 3D cubic lattice with nearest-neighbor hopping we observe deviations of 10\% when compared to published analytic results [2]. The disparity is due to a correlation between spin flipping hops and spin conserving hops. The time that carriers spend at a given site is determined by the sum of the conserving and flipping rates. Correlations between the two types of rates affect the time between two transport events altering the relaxation time.1. N. J. Harmon and M. E. Flatt\'e, PRB 90, 115203 (2014).2. Z.G. Yu, PRB 85, 115201 (2012). [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B42.00005: Tuning spin-polarized transport in organic semiconductors Richard Mattana, Marta Galbiati, Sophie Delprat, Sergio Tatay, Cyrile Deranlot, Pierre Seneor, Frederic Petroff Molecular spintronics is an emerging research field at the frontier between organic chemistry and the spintronics. Compared to traditional inorganic materials molecules are flexible and can be easily tailored by chemical synthesis. Due to their theoretically expected very long spin lifetime, they were first only seen as the ultimate media for spintronics devices. It was recently that new spintronics tailoring could arise from the chemical versatility brought by molecules. The hybridization between a ferromagnet and molecules induces a spin dependent broadening and energy shifting of the molecular orbitals leading to an induced spin polarization on the first molecular layer. This spin dependent hybridization can be used to tailor the spin dependent transport in organic spintronics devices. We have studied vertical Co/Alq3/Co organic spin valves. The negative magnetoresistance observed is the signature of different coupling strengths at the top and bottom interfaces. We have then inserted an inorganic tunnel barrier at the bottom interface in order to suppress the spin-dependent hybridization. In this case we restore a positive magnetoresistance. This demonstrates that at the bottom Co/Alq3 interface a stronger coupling occurs which induces an inversion of the spin polarization. [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B42.00006: Resistive Switching and Voltage Induced Modulation of Tunneling Magnetoresistance in Nanosized Perpendicular Organic Spin Valves Georg Schmidt, Robert Goeckeritz, Nico Homonnay, Alexander Mueller, Bodo Fuhrmann Resistive switching has already been reported in organic spin valves (OSV), however, its origin is still unclear. We have fabricated nanosized OSV based on La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/Co. These devices show fully reversible resistive switching of up to five orders of magnitude. The magnetoresistance (MR) is modulated during the switching process from negative (-70\%) to positive values (+23\%). The results are reminiscent of experiments claiming magnetoelectric coupling in LSMO based tunneling structures using ferroelectric barriers. By analyzing the I/V characteristics of the devices we can show that transport is dominated by tunneling through pinholes. The resistive switching is caused by voltage induced creation and motion of oxygen vacancies at the LSMO surface, however, the resulting tunnel barrier is complemented by a second adjacent barrier in the organic semiconductor. Our model shows that the barrier in the organic material is constant, causing the initial MR while the barrier in the LMSO can be modulated by the voltage resulting in the resistive switching and the modulation of the MR as the coupling to the states in the LSMO changes. A switching caused by LSMO only is also supported by the fact that replacing ALQ$_3$ by H$_2$PC yields almost identical results. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 1:03PM |
B42.00007: Spin-dependent electronic processes in organic semiconductors Invited Speaker: Hans Malissa The development and improvement of organic electronics and spintronics concepts [1, 2] requires a detailed understanding of spin-dependent charge-carrier transitions, including transport and recombination processes, which govern the magneto-optoelectronic properties of these materials. In order to observe these processes, we developed various electrically detected magnetic resonance (EDMR) experiments, in particular pulsed EDMR experiments and we have applied these techniques to a range of polymer materials and devices. EDMR allows for the observation of spin-dependent electronic rates after the spin manifolds controlling these processes have been excited by short and powerful microwave pulses. Most polymers typically exhibit small, but non-zero spin-orbit coupling effect in their EDMR spectra due to the absence of heavy elements. They also display abundant hyperfine coupling between charge-carrier and adjacent nuclear spins, which are abundant in most organic materials [3]. While spectroscopically, the inhomogeneous broadening effects of EDMR lines by these two coupling types is indistinguishable (both display Gaussian lines due to the all abundant magnetic disorder), they can be separated when EDMR experiments are conducted over a wide range of different excitation frequencies, ranging from the radio-frequency domain where spin-orbit coupling is negligible and electronic transitions are predominantly governed by the weak random hyperfine fields, to the quasi-optical domain where differences and anisotropies in the charge-carrier g-factors emerge [4]. EDMR is uniquely suitable for experiments at very low excitation frequencies where spin polarization is negligible. This facilitates the study of spin collectivity in an ultra-strong coupling regime under strong radiofrequency drive [5, 6] as well as calibration-free magnetic field sensor applications that utilize magnetic resonance [2]. [1] Xiong et al., Nature 427, 821-824 (2004). [2] Baker et al., Nat. Commun. 3, 898 (2012). [3] Malissa et al., Science 345, 1487-1490 (2014). [4] Joshi et al., Appl. Phys. Lett. 109, 103303 (2016). [5] Roundy and Raikh, Phys. Rev. B 88, 125206 (2013). [6] Waters et al., Nat. Phys. 11, 910 (2015). [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B42.00008: Tailoring intermolecular and interfacial interactions in organic spin valves via selective deuteration of polyfluorene chains Alexandra Steffen, Nuradhika Herath, Jong Keum, Honhai Zhang, Kulun Hong, Jacek Jakowski, Jingsong Huang, Jim Browning, Steven Bennett, Kai Xiao, Christopher Rouleau, Ilia Ivanov, Valeria Lauter Organic spin valves (OSV) are in the focus of development of low power spintronic devices. Because of the very week spin-orbit coupling, the spin polarization of the carriers in organic semiconductors can be maintained for a very long time. The spin diffusion length is critical to the coherent transport of spins inside spintronics devices. Deuteration changes the spin diffusion length, but the underlying mechanisms are neither systematically investigated nor fully understood. Here we reveal the influence of different type polymers on the interfacial interactions and magnetization via Polarized Neutron Reflectometry, VSM and transport measurements. The investigated spin valve system is based on the tri-layer of LSMO/Polymer/Co, where PFO is selected as an n-type semiconductor and P3HT as p-type one. The combined effort of theoretical prescreening, distinct syntheses and deuteration of four PFO isotopes, and optimization of each individual component of the OSV fabrication allowed us to explore the influence of the PFO and P3HT with deuterium substitution of different chain parts on the magnetic structure of the spin valves. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B42.00009: Generation and electrical detection of spin collectivity in monolithic thin-film polymer devices Shirin Jamali, Gajadhar Joshi, Hans Malissa, John Lupton, Christoph Boehme Collectivity in paramagnetic spin ensembles, analogous to electric dipoles resonantly driven in the Dicke regime, occurs under magnetic resonance, when the driving field amplitude $B_{\mathrm{1\thinspace }}$reaches the order of the Zeeman field $B_{\mathrm{0}}$. Measurements of spin-dependent transition rates between weakly spin-coupled charge carrier pairs in organic semiconductors have shown that this spin-Dicke regime can be reached experimentally by lowering $B_{\mathrm{0}}$ to a minimum posed by random local hyperfine fields, requiring radio frequency (RF) driving fields, while maximizing $B_{\mathrm{1\thinspace }}$[2,3]. Here we show that this can be accomplished using a microscopic monolithic thin-film device consisting of a polymer diode on a \textasciitilde 1$\mu $m SiO$_{\mathrm{2}}$/SiN layer stack grown on a Cu thin-film wire for strong homogeneous RF excitation, which in turn is supported by a crystalline Si substrate. This setup allows for $B_{\mathrm{1}}$\textgreater 2mT, and thus the observation of spin-collectivity in super-yellow (SY) poly (p-phenylene vinylene) (PPV), a polymer with much stronger local hyperfine fields than the deuterated polymers previously used [2]. [1] R.C.Roundy, M. E. Raikh, \textit{Phys.rev. B} 88, 125206 (2013). [2] D. P. Waters et al. \textit{Nature Phys}. 11, 910 (2015). [3] S. L. Bayliss, et al. \textit{Nature Com}. 6, 8534 (2015). [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B42.00010: Spin transport in heavily doped PEDOT:PSS with tunable electrical conductivity Dali Sun, Yaxin Zhai, Matthew Groesbeck, Chuang Zhang, Kipp J. van Schooten, Oana Jurchescu, Reghu Menon, Christoph Boehme, Z. Valy Vardeny Among pi-conjugated conducting polymers, doped poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, or PEDOT:PSS, has been widely used in optoelectronic devices. Interestingly the PEDOT:PSS electrical conductivity can vary substantially between 10-3 S/cm to 103 S/cm depending on the doping concentration determined by the solvents and additives used. We report a comparative studies of the spin diffuse lengths, spin relaxation times and charge motilities in different PEDOT:PSS compounds with various conductivities using the techniques of spin-pumping, electron spin resonance and field effect transistor, respectively. In particular we contrast the charge transport and spin transport in these materials. [Preview Abstract] |
Monday, March 13, 2017 1:39PM - 1:51PM |
B42.00011: Counter-ion and dopant effects on charge carriers in intrinsically conductive polymer Jonathan Ogle, Mandefro Yehulie, Christoph Boehme, Luisa Whittaker-Brooks Recently, a significant amount of attention has been devoted to the optimization and applications of organic electronics. In particular, intrinsically conductive polymers have seen a strong continued interest for their use in thermoelectric and photovoltaic devices. With conductivities ranging from 10$^{\mathrm{-8}}$ to 10$^{\mathrm{3}}$ S cm$^{\mathrm{-1}}$, the conductive polymer poly(3,4-ethylenedioxythiophene) -PEDOT is one of the most studied solution-processable polymer material due to its unique optical and electronic properties. While charge carriers at lower conductivities have been identified as polarons, an understanding of the electronic structure of PEDOT as its conductivity increases is not well understood. We have investigated the effect that counter-ion exchange and doping has on the polaron concentration of PEDOT via electron paramagnetic resonance, ultraviolet photoelectron spectroscopy, and X-ray absorption fine structure spectroscopy studies. Such studies have allowed us to correlate charge carriers concentrations and the real and virtual electronic states in PEDOT as a function of various dopants. As discussed in our talk, we believe our findings could be extended to the understanding of other polymeric materials. [Preview Abstract] |
Monday, March 13, 2017 1:51PM - 2:03PM |
B42.00012: Magnetic Tunnel Junctions Based On Alkanethiol Self Assembled Monolayers Sophie Delprat, Benoit Quinard, Marta Galbiati, Michele Mattera, Samuel Manas-Valero, Alicia Forment-Aliaga, Sergio Tatay, Cyrile Deranlot, Sophie Collin, Karim Bouzehouane, Richard Mattana, Pierre Seneor, Frederic Petroff Molecular spintronics has opened novel and exciting functionalities for spintronics devices. Among them, it was shown that spin dependent hybridization at metal/molecule interfaces could lead to radical tailoring of spintronics properties[1]. In this direction Self-Assembled Monolayers (SAMs) appear to be a very promising candidate with their impressive molecular scale crafting properties. Despite all the promising possibilities, up to now less than a handful of experiments on SAMs as spin-dependent tunnel barriers have been reported[2] at low temperatures, but already showing potential[3]. Towards room temperature spin signal, we studied magnetic tunnel junctions based on alkanethiol and conventional ferromagnets such as Co,NiFe for which we developed a process to recover the ferromagnet from oxidiation[4]. We will present NiFe/SAMs/Co molecular magnetic tunnel junctions with magnetoresistance effects up to 10{\%} observed at 300K. [1] Galbiati et al., MRS Bull. 39, 602 (2014) [2] Wang et al., APL, 89, 153105 (2006)~; Petta et al.,~PRL, 93, 136601 (2004) [3] Galbiati et al., Adv. Mat. 24 6429, (2012) [4] Galbiati, Delprat et al. , AIP adv. 5, 057131 (2015) [Preview Abstract] |
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