Marzieh Kavand

Room-temperature coupling between electrical current and nuclear spins in OLEDs

Organic semiconductors go out for a spin Magnetism is a commonly observed phenomenon in the macroscopic world, but its origins lie in the quirky quantum-mechanical property of electrons and certain nuclei known as spin. Recent research has sought to leverage and expand the role of spin in the operation of electronic devices. Malissa et al. used a highly sensitive spectroscopic technique to probe, and ultimately manipulate, the subtle effects of spin interactions on the current that flows through organic light-emitting diodes (OLEDs) (see the Perspective by Bobbert). They pinpointed coupling between the spins of the current carriers and the hydrogen nuclei in the hydrocarbon-based material making up the device. Science, this issue p. 1487; see also p. 1450 Magnetic resonance spectroscopy enables detection and manipulation of subtle spin interactions in organic semiconductors. [Also see Perspective by Bobbert] The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.

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.

Separating hyperfine from spin-orbit interactions in organic semiconductors by multi-octave magnetic resonance using coplanar waveguide microresonators

Separating the influence of hyperfine from spin-orbit interactions in spin-dependent carrier recombination and dissociation processes necessitates magnetic resonance spectroscopy over a wide range of frequencies. We have designed compact and versatile coplanar waveguide resonators for continuous-wave electrically detected magnetic resonance and tested these on organic light-emitting diodes. By exploiting both the fundamental and higher-harmonic modes of the resonators, we cover almost five octaves in resonance frequency within a single setup. The measurements with a common π-conjugated polymer as the active material reveal small but non-negligible effects of spin-orbit interactions, which give rise to a broadening of the magnetic resonance spectrum with increasing frequency.

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.

Quantitative inverse spin Hall effect detection via precise control of the driving-field amplitude

Spin transport in thin-film materials can be studied by ferromagnetic resonantly (FMR) driven spin pumping of a charge-free spin current which induces an electromotive force through the inverse spin Hall effect (ISHE). For quantitative ISHE experiments, precise control of the FMR driving field amplitude B1 is crucial. This study exploits in situ monitoring of B1 by utilization of electron paramagnetic resonantly (EPR) induced transient nutation of paramagnetic molecules (a 1:1 complex of α,γ-bisdiphenylene-β-phenylallyl and benzene, BDPA) placed as B1 probe in proximity of a NiFe/Pt-based ISHE device. Concurrent to an ISHE experiment, B1 is obtained from the inductively measured BDPA Rabi-nutation frequency. Higher reproducibility is achieved by renormalization of the ISHE voltage to B 1 with an accuracy that is determined by the homogeneity of the FMR driving field and thus by the applied microwave resonator and ISHE device setup.

Characterization of Tetracyanopyridine (TCNPy)-Based Magnets: V[TCNPy]2⋅z (CH2Cl2) (Tc=111 K) and V[TCNPy]3⋅z (CH2Cl2) (Tc=90 K)

The reaction of 2,3,5,6-tetracyanopyridine (TCNPy) with V(CO)6 in CH2 Cl2 forms new organic-based magnets of V[TCNPy]x ⋅z (CH2 Cl2 ) (x=2, 3) composition. Analysis of the IR spectra suggests that the TCNPy is reduced and coordinated to V(II) sites through the nitriles. V[TCNPy]x order as ferrimagnets with 111 and 90 K Tc values for V[TCNPy]2 and V[TCNPy]3 , respectively. Their respective remanent magnetizations and coercive fields are 1260 and 250 emuOe mol(-1) and 9 and 6 Oe at 5 K, and they exhibit some spin-glass behavior.

The Tetracyanopyridinide Dimer Dianion, σ‐[TCNPy]22−

The reaction of 2,3,5,6-tetracyanopyridine (TCNPy) and Cr(C6 H6 )2 forms diamagnetic σ-[TCNPy]2 (2-) possessing a 1.572(3) Å intrafragment sp(3) -sp(3) bond. This is in contrast to the structurally related 1,2,4,5-tetracyanobenzene and 1,2,4,5-tetracyanopyrazine that form π-dimer dianions possessing long, multicenter bonds.

Hexacyanobutadienide-Based Frustrated and Weak Ferrimagnets: M(HCBD)2·zCH2Cl2 (M = V, Fe)

Hexacyanobutadiene (HCBD) and M(CO)x (M = V, x = 6; Fe, x = 5) react in CH2Cl2 to form new organic-based magnets of M[HCBD]2·z(CH2Cl2) composition. Analysis of the IR spectrum [M = V: ν(CN) 2193 and 2116 cm(-1) (fwhh ∼400 cm(-1)); Fe: 2196 and 2145 (fwhh ∼150 cm(-1))] suggests that HCBD is reduced to the radical anion, [HCBD](•-), and the broadness suggests multiple and variable nitriles sites are coordinated to the V(II), leading to a complex mixture of magnetic couplings and behaviors that deviate from paramagnetic behavior below ∼150 K, and a frustrated magnet with Tc ≈ 9 K is observed for V[HCBD]2, the first cyanocarbon-based frustrated magnet. Fe[HCBD]2 behaves as a weak ferromagnet (canted antiferromagnet) with some spin glass behavior with a 10 K Tc.

Spintronic detection of interfacial magnetic switching in a paramagnetic thin film of tris(8-hydroxyquinoline)iron(III)

Dali Sun,1,* Christopher M. Kareis,2 Kipp J. van Schooten,1 Wei Jiang,3 Gene Siegel,3 Marzieh Kavand,1 Royce A. Davidson,2 William W. Shum,2 Chuang Zhang,1 Haoliang Liu,1 Ashutosh Tiwari,3 Christoph Boehme,1 Feng Liu,3 Peter W. Stephens,4 Joel S. Miller,2 and Z. Valy Vardeny1,† 1Department of Physics & Astronomy, University of Utah, Salt Lake City, Utah 84112, USA 2Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA 3Department of Material Science & Engineering, University of Utah, Salt Lake City, Utah 84112, USA 4Department of Physics & Astronomy, Stony Brook University, Stony Brook, New York 11794, USA (Received 26 May 2016; revised manuscript received 11 November 2016; published 17 February 2017)