Spin Wave Excitation, Detection, and Utilization in the Organic-Based Magnet, V(TCNE)x (TCNE = Tetracyanoethylene).

Spin waves, quantized as magnons, have low energy loss and magnetic damping, which are critical for devices based on spin-wave propagation needed for information processing devices. The organic-based magnet [V(TCNE)x ; TCNE = tetracyanoethylene; x ≈ 2] has shown an extremely low magnetic damping comparable to, for example, yttrium iron garnet (YIG). The excitation, detection, and utilization of coherent and non-coherent spin waves on various modes in V(TCNE)x is demonstrated and show that the angular momentum carried by microwave-excited coherent spin waves in a V(TCNE)x film can be transferred into an adjacent Pt layer via spin pumping and detected using the inverse spin Hall effect. The spin pumping efficiency can be tuned by choosing different excited spin wave modes in the V(TCNE)x film. In addition, it is shown that non-coherent spin waves in a V(TCNE)x film, excited thermally via the spin Seebeck effect, can also be used as spin pumping source that generates an electrical signal in Pt with a sign change in accordance with the magnetization switching of the V(TCNE)x . Combining coherent and non-coherent spin wave detection, the spin pumping efficiency can be thermally controlled, and new insight is gained for the spintronic applications of spin wave modes in organic-based magnets.

Separation of Spin and Charge Transport in Pristine π-Conjugated Polymers.

We have experimentally tested whether spin-transport and charge-transport in pristine π-conjugated polymer films at room temperature occur via the same electronic processes. We have obtained the spin diffusion coefficient of several π-conjugated polymer films from the spin diffusion length measured by the technique of inverse spin Hall effect and the spin relaxation time measured by pulsed electrically detected magnetic resonance spectroscopy. The charge diffusion coefficient was obtained from the time-of-flight mobility measurements on the same films. We found that the spin diffusion coefficient is larger than the charge diffusion coefficient by about 1-2 orders of magnitude and conclude that spin and charge transports in disordered polymer films occur through different electronic processes.

Spin current from sub-terahertz-generated antiferromagnetic magnons.

Spin dynamics in antiferromagnets has much shorter timescales than in ferromagnets, offering attractive properties for potential applications in ultrafast devices1-3. However, spin-current generation via antiferromagnetic resonance and simultaneous electrical detection by the inverse spin Hall effect in heavy metals have not yet been explicitly demonstrated4-6. Here we report sub-terahertz spin pumping in heterostructures of a uniaxial antiferromagnetic Cr2O3 crystal and a heavy metal (Pt or Ta in its ß phase). At 0.240 terahertz, the antiferromagnetic resonance in Cr2O3 occurs at about 2.7 tesla, which excites only right-handed magnons. In the spin-canting state, another resonance occurs at 10.5 tesla from the precession of induced magnetic moments. Both resonances generate pure spin currents in the heterostructures, which are detected by the heavy metal as peaks or dips in the open-circuit voltage. The pure-spin-current nature of the electrically detected signals is unambiguously confirmed by the reversal of the voltage polarity observed under two conditions: when switching the detector metal from Pt to Ta, reversing the sign of the spin Hall angle7-9, and when flipping the magnetic-field direction, reversing the magnon chirality4,5. The temperature dependence of the electrical signals at both resonances suggests that the spin current contains both coherent and incoherent magnon contributions, which is further confirmed by measurements of the spin Seebeck effect and is well described by a phenomenological theory. These findings reveal the unique characteristics of magnon excitations in antiferromagnets and their distinctive roles in spin-charge conversion in the high-frequency regime.

Surface-enhanced spin current to charge current conversion efficiency in CH3NH3PbBr3-based devices.

Hybrid organic-inorganic perovskites have shown great promise for spintronic applications due to their large spin-orbit coupling induced by the Pb and halogen atoms. Particularly, the large observed surface-induced Rashba splitting in CH3NH3PbBr3 indicates efficient spin-current-to-charge-current (StC) conversion, which, however, has not been demonstrated yet. In this work, the StC conversion efficiency in ferromagnet/CH3NH3PbBr3-based devices is studied using the pulsed spin-pumping technique measured by the inverse spin Hall effect. We found that the StC conversion efficiency is anomalous in that it increases at small perovskite layer thickness. This indicates the existence of a surface-dominated StC mechanism such as the inverse Rashba-Edelstein effect. By inserting a thin LiF layer between the ferromagnet and the perovskite film, the StC conversion efficiency is greatly suppressed, validating the existence of a Rashba surface in the CH3NH3PbBr3 film.

Sign reversal of magnetoresistance and inverse spin Hall effect in doped conducting polymers.

Conducting polymers, where pristine polymers are doped by active dopants, have been used in a variety of flexible optoelectronic device applications due to their tunable conductivity values. Charge transport in these materials has been intensively studied for over three decades. However, spin transport properties in these compounds have remained elusive. Here, we studied two polaron-dominated and trap-dominated spin transport processes in two types of PEDOT:PSS polymers that are lightly and heavily doped, respectively. Using pulsed spin-pumping and spin-injection techniques, we found the sign of inverse spin Hall effect and magnetoresistance obtained from the lightly doped PEDOT:PSS film can reverse its polarity as a function of temperature and applied bias, in contrast to that in the heavily doped PEDOT:PSS film. Our work provides an alternative approach for studying the spin transport in conducting polymer films.

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 spintronics 1 . Magnons in inorganic materials have been studied widely with respect to their generation2,3, transport4,5 and detection 6 . In contrast, resonant spin waves in the room-temperature organic-based ferrimagnet vanadium tetracyanoethylene (V(TCNE) x (x ≈ 2)), were detected only recently 7 . 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.

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.

Characterization of Tetracyanopyridine (TCNPy)-Based Magnets: V[TCNPy]2 ⋠z (CH2 Cl2 ) (Tc =111†K) and V[TCNPy]3 ⋠z (CH2 Cl2 ) (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]2 (2.).

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) Šintrafragment 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.

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 molecule's 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.