Enhancement of Spin Transport Properties in Angled-Channel Graphene Spin Valves via Hybrid Spin Drift-Diffusion.

Manipulation of spin information in semiconductors has been the topic of both experimental and theoretical studies. In this paper, the theoretical compact models for the spin-relaxation length (SRL) in nondegenerately doped silicon and gallium arsenide are presented. These models account for the impact of impurity doping and phonons on mediating spin relaxation. In addition, the models are exhaustively calibrated with the published experimental data. It is shown that the SRL at room temperature in Si drops from 5 to 1 μm as the doping increases from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . However, the SRL in GaAs is independent of doping for nondegenerate doping levels. While the rolloff of the SRL with temperature in Si depends upon the doping concentration, the rolloff of the SRL for nondegenerately doped GaAs is doping independent and proceeds as T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , where T is the operating temperature. The models of the SRL in conjunction with spin drift-diffusion equation are used to study the steady-state spin transport in both the conventional and nonlocal semiconducting spin-valve geometries. The presence of an electric field leads to a clear enhancement of spin injection and transport efficiency in the conventional spin-valve geometry. The degradation in the spin injection and transport efficiency with the channel length is much steeper in nonlocal spin valves as compared to that in the conventional spin valves.

Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes

Single crystals of organic semiconductors with perfect crystal structure and minimal density of defects can exhibit high mobility and low spin scattering compared with their amorphous or polycrystalline counterparts. Therefore, these materials are promising candidates as the spin transport media to obtain long spin relaxation times and spin diffusion lengths in spintronic devices. However, the investigation of spin injection and transport properties in organic single crystals is hindered by the inability to construct devices such as single-crystalline organic spin valves (OSVs). Herein, thin and large organic single crystals of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) were grown on a liquid substrate and transferred to a target substrate carrying ferromagnetic electrodes to construct single-crystalline OSVs. The magnetoresistance (MR) responses of the single crystals were investigated to study their spin injection and transport properties. MR value as high as 17% was probed with an intermediate layer thickness of 269 nm. More importantly, spin transport was still observed in a single crystal of a thickness up to 457 nm, which was much larger than that of polycrystalline thin film. Our research provides a general methodology for constructing single-crystalline OSVs and paves the way to probe the intrinsic spin transport properties of organic semiconductors based on single crystals.

The nonlocal spin valve (NLSV) enables unambiguous study of spin transport, owing to its ability to isolate pure spin currents. A key principle of NLSV operation is that the ``spin signal'' is invariant under application of in-plane magnetic fields (above the ferromagnetic contact saturation field). Yet, for certain ferromagnet/normal metal pairings in NLSVs, an unexpected field enhancement of the spin signal occurs, presenting a challenge that has, thus far, been difficult to resolve with existing models. By correlating the extracted spin transport parameters with material, temperature, and field dependencies, in this work we identify field quenching of magnetic impurity scattering as the origin of this effect, confirmed by excellent agreement between our results and field-dependent Kondo theory. In addition to addressing this long-standing mystery, our findings highlight a potential systematic underestimation of spin transport parameters. By identifying signature field and temperature dependencies, we provide here a relatively simple means to isolate and quantify this additional relaxation mechanism.

The unique physical properties and low temperature solution processability of organic semiconductors have enabled many applications such as light emitting diodes, flexible logic and solar cells, they are unexploited in their potential for use in solid state devices for spintronics and spin-based information processing. Organic semiconductors composed of mainly light elements appeal to the field of spintronics due to their long spin lifetime originating from their weak spin-orbit coupling. The significant progress in improving carrier mobility of organic semiconductors in the past decade may lead to organic spin transport materials with both long spin diffusion length and spin lifetime which is important for spintronics applications. This dissertation explores the spin transport in organic semiconductors using a variety of experimental techniques from all electrical spin injection and detection to ferromagnetic resonance spin pumping and ISHE spin detection. Non-local spin valves and novel all electrical spin transport device architectures based on high mobility conjugated polymers were studied systematically. The intrinsic roadblocks for electrical spin injection-based measurements were identified as the current spreading effect (electrical cross-talk between the injector and detector electrodes) and the hopping conduction in organic semiconductors which makes all electrical nonlocal spin injection and detection measurements extremely challenging if not impossible for organic semiconductors. In addition, spin current transmission in the out of plane direction of organic semiconductors was studied by tri-layer spin pumping technique where the spin transport properties of organic semiconductors are correlated with their molecular structure and charge transport properties. Spin pumping, a charge-free spin injection method together with ISHE spin detection successfully overcome the impedance mismatch problem and the intrinsic roadblocks imposed by electrical spin injection-based techniques and enabled lateral spin current transport in organic semiconductors to be detected electrically. The lateral spin diffusion length of up to a micrometre was observed in doped conjugated polymers in agreement with theoretical calculations based on exchange mediated spin diffusion model and parameters obtained from first principle. Moreover, this non-local spin transport device structure provides a platform for studying spin transport in a wide range of organic semiconductors where the spin current propagates along the high mobility direction and could potentially be used as building blocks for high performance flexible spintronics devices.

Many spintronic device concepts rely on the efficient electrical generation of a spin accumulation inside a semiconductor (SC) using the interface with a ferromagnetic metal (FM). The application of a reverse or forward bias voltage to an FM/n-type SC hybrid contact leads to the generation of spin-polarized electrons in the SC via spin injection or extraction, respectively. Frequently employed lateral transport structures include the non-local and the three-terminal (3T) geometries. However, the results obtained in the 3T geometry are often found to be inconsistent with the expectations derived from the well-established detection of spin signals in the non-local spin valve (NLSV) configuration [1,2]. Reports on spin transport in the technologically more relevant two-terminal arrangement of the local spin valve (LSV) are scarce due to the difficulty to fulll the specific requirements on the device parameters [3]. Here, we demonstrate spin transport for a FM/SC system with a particularly favorable I-V characteristic in both the NLSV and LSV configurations. In addition, our results obtained in the 3T configuration are consistent with the spin-transport characteristics in NLSV and LSV structures.

While very recent, the introduction of 2D materials in Magnetic Tunnel Junctions (MTJs) has already shown some promising properties (atomic thickness control, diffusion barrier, spin filtering…)[1]. Graphene and the 2D insulator h-BN have been the first 2D materials to show strong impact on spin transport in MTJs. It was shown that strong spin filtering occurred with the creation of an insulating spin channel in metallic graphene and metallic channel in insulating h-BN[2][3]. The recent advent of the wide Transition Metal Dichalcogenides (TMD) family of 2D semiconductors (MoS2, WS2…) opened new opportunities for further tailoring of spintronics properties. Preliminary results highlighted that maintaining interfaces and materials quality (such as preventing ferromagnets oxidation) remains a crucial issue.Here, we will present results on band filtering in WS2 based spin valves. We will detail a protocol to fabricate spin valves based on CVD grown WS2, with step-by-step characterizations in support (Raman spectroscopy, photoluminescence, AFM measurements…) which aims at preserving interfaces spin properties by avoiding oxidations and degradations. The WS2 layers are integrated in Co/Al2O3/WS2/Co magnetic tunnel junctions acting as hybrid spin valve structures (Figure 1). We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface.The fabrication process is further validated by the measurements of magnetoresistance spin signals above state of the art for 2D semiconductors based MTJs. For monolayer based junctions, the spin polarization extracted from the WS2/Co interface is measured to be positive. Interestingly, for bi- and tri-layers WS2, the observed spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction (Figure 2). This unusual thickness dependent property will be explained in light of the peculiar thickness band-structure evolution of WS2, with Density Functional Theory calculations in support. This allows us to unravel a model of band filtering enabling spin selection and sign inversion through a selective K-Q transition in the band structure.Our work thus demonstrates the potential of WS2 for K-Q band filtering and its tailoring in MTJs. As this band structure evolution is common to many other TMDs, our work also opens the way to the integration of members of this very large 2D materials family, in order to reveal their spin transport properties in Magnetic Tunnel Junctions [4]. **

Spin transport in graphene devices has been investigated in both local and non-local spin valve geometries. In the nonlocal measurement, spin transport and spin precession in single layer and bilayer graphene have both been achieved with transparent Co contacts. Gate controllable non-local spin signal was also demonstrated in this system. For the local graphite spin valve device, we report MR up to 12% for devices with tunneling contacts. We observe a correlation between the nonlinearity of the I-V curve and the presence of local MR and conclude that tunnel barriers can be employed to surmount the conductance mismatch problem in this system. These studies indicate that the improvement of tunnel barriers on graphene, especially to inhibit the formation of pinholes, is an important step to achieve more efficient spin injection into graphene.

We demonstrate all-electrical spin injection, transport, and detection in heavily n-type-doped Si nanowires using ferromagnetic Co/Al(2)O(3) tunnel barrier contacts. Analysis of both local and nonlocal spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency (up to ~30%) and long spin diffusion lengths (up to ~6 μm) are achieved. These values exceed those reported for spin transport devices based on comparably doped bulk Si. The spin valve signals are found to be strongly bias and temperature dependent and can invert sign with changes in the dc bias current. The influence of the nanowire morphology on field-dependent switching of the contacts is also discussed. Owing to their nanoscale geometry, ~5 orders of magnitude less current is required to achieve nonlocal spin valve voltages comparable to those attained in planar microscale spin transport devices, suggesting lower power consumption and the potential for applications of Si nanowires in nanospintronics.

Carbon-based spintronics refers mainly to the spin injection and transport in carbon materials including carbon nanotubes, graphene, fullerene, and organic materials. In the last decade, extraordinary development has been achieved for carbon-based spintronics, and the spin transport has been studied in both local and nonlocal spin valve devices. A series of theoretical and experimental studies have been done to reveal the spin relaxation mechanisms and spin transport properties in carbon materials, mostly for graphene and carbon nanotubes. In this article, we provide a brief review on spin injection and transport in graphene, carbon nanotubes, fullerene and organic thin films.

The non-local spin valve (NLSV) is an important tool in spintronics, primarily due to its ability to isolate pure spin currents1. As such, NLSVs have been instrumental in spin transport measurements of a wide range of materials2. Yet, even in relatively simple systems, questions remain on the impact of interfaces, defects and impurities3–5. Of particular interest is an increase in the “spin-signal”, ΔRNL, under large magnetic fields (up to 9 T) in all-metallic NLSVs6, in stark contrast to the key tenet that ΔRNL is invariant under a magnetic field. This effect, which appears only for certain combinations of FM and normal metals, has been a long-standing mystery and, thus far, has been a challenge to describe with existing models. Our earlier works on similar materials have shown that a low temperature upturn in spin-relaxation rate, 1/τs, originates from magnetic impurity (MI) scattering near the NLSV interfaces5,7, underlining the importance of the Kondo effect in determining spin-relaxation, particularly in Cu devices. In this study we demonstrate MI scattering as the origin of the field enhancement of ΔRNL, and we provide a theoretical framework. By correlating the magnitude of the field enhancement, δRNL, with the NLSV material pairings, we ascertain that δRNL is only present in those materials that can host MI moments (Figure 1). Next, focusing on Cu/Fe NLSVs, we extract spin-transport parameters from the field-enhanced ΔRNL. Under zero-field, we observe the same increase in 1/τs at low temperatures (Figure 2), due to Kondo scattering from MIs, in agreement with our previous work5. Importantly, we show that this mechanism is quenched by a magnetic field and we successfully model the data using a magnetic-field-modified Kondo expression8. This work not only highlights a systematic overestimation of 1/τs in materials containing MIs, including a clear impact on Hanle spin relaxation measurements, but also provides a simple means to quantify and suppress this scattering mechanism.

Review: 120 refs.

Spin transport and relaxation in graphene

A general form of the Hamiltonian for electrons confined to a curved one-dimensional (1D) channel with spin-orbit coupling (SOC) linear in momentum is rederived and is applied to a U-shaped channel. Discretizing the derived continuous 1D Hamiltonian to a tight-binding version, the Landauer--Keldysh formalism (LKF) for nonequilibrium transport can be applied. Spin transport through the U-channel based on the LKF is compared with previous quantum mechanical approaches. The role of a curvature-induced geometric potential which was previously neglected in the literature of the ring issue is also revisited. Transport regimes between nonadiabatic, corresponding to weak SOC or sharp turn, and adiabatic, corresponding to strong SOC or smooth turn, is discussed. Based on the LKF, interesting charge and spin transport properties are further revealed. For the charge transport, the interplay between the Rashba and the linear Dresselhaus (001) SOCs leads to an additional modulation to the local charge density in the half-ring part of the U-channel, which is shown to originate from the angle-dependent spin-orbit potential. For the spin transport, theoretically predicted eigenstates of the Rashba rings, Dresselhaus rings, and the persistent spin-helix state are numerically tested by the present quantum transport calculation.

Introduction Historical Overview: From Electron Transport in Magnetic Materials to Spintronics Albert Fert Spin Transport and Magnetism in Magnetic Metallic Multilayers Basics of Nano-Thin Film Magnetism Bretislav Heinrich Micromagnetism as a Prototype for Complexity. Anthony S. Arrott Giant Magnetoresistance: Experiment Jack Bass Giant Magnetoresistance: Theory Evgeny Y. Tsymbal, D.G. Pettifor, and Sadamichi Maekawa Spin Injection, Accumulation, and Relaxation in Metals Mark Johnson Spin Torque Effects: Experiment Maxim Tsoi Spin Torque in Magnetic Systems: Theory A. Manchon and Shufeng Zhang Hot Carrier Spin Transport Ron Jansen Spin Transport and Magnetism in Magnetic Tunnel Junctions Tunneling Magnetoresistance: Experiment (Non-MgO) Patrick R. LeClair and Jagadeesh S. Moodera Tunnel Magnetoresistance in MgO-Based Magnetic Tunnel JunctionsExperiment Shinji Yuasa Tunneling Magnetoresistance: Theory Kirill D. Belashchenko and Evgeny Y. Tsymbal Spin-Filter Tunneling Tiffany S. Santos and Jagadeesh S. Moodera Spin Torques in Magnetic Tunnel Junctions. Yoshishige Suzuki and Hitoshi Kubota Multiferroic Tunnel Junctions Manuel Bibes and Agnes Barthelemy Spin Transport and Magnetism in Semiconductors Spin Relaxation and Spin Dynamics in Semiconductors Jaroslav Fabian and M.W. Wu Electrical Spin Injection and Transport in Semiconductors Berend T. Jonker Spin-Polarized Ballistic Hot-Electron Injection and Detection in Hybrid Metal Semiconductor Devices Ian Appelbaum Magnetic Semiconductors: IIIV Semiconductors Carsten Timm Magnetism of Dilute Oxides J.M.D. Coey Tunneling Magnetoresistance and Spin Transfer with (Ga,Mn)As H. Jaffres and Jean Marie George Spin Transport in Organic Semiconductors Valentin Dediu, Luis E. Hueso, and Ilaria Bergenti Spin Transport in Ferromagnet/IIIV Semiconductor Heterostructures Paul A. Crowell and Scott A. Crooker Spin Polarization by Current Sergey D. Ganichev, Maxim Trushin, and John Schliemann Anomalous and Spin-Injection Hall Effects Jairo Sinova, Jorg Wunderlich, and Tomas Jungwirth Spin Transport and Magnetism at the Nanoscale Spin-Polarized Scanning Tunneling Microscopy Matthias Bode Point Contact Andreev Ref lection Spectroscopy Boris E. Nadgorny Ballistic Spin Transport. Bernard Doudin and N.T. Kemp Graphene Spintronics Csaba Jozsa and Bart J. van Wees Magnetism and Transport in Diluted Magnetic Semiconductor Quantum Dots Joaquin Fernandez Rossier and R. Aguado Spin Transport in Hybrid Nanostructures Saburo Takahashi and Sadamichi Maekawa Nonlocal Spin Valves in Metallic Nanostructures Yoshichika Otani and Takashi Kimura Molecular Spintronics Stefano Sanvito Applications Magnetoresistive Sensors Based on Magnetic Tunneling Junctions Gang Xiao Magnetoresistive Random Access Memory. Johan Akerman Emerging Spintronics Memories. Stuart Parkin, Masamitsu Hayashi, Luc Thomas, Xin Jiang, Rai Moriya, and William Gallagher GMR Spin-Valve Biosensors Drew A. Hall, Richard S. Gaster, and Shan X. Wang Semiconductor Spin-Lasers Rafal Oszwaldowski, Christian Gothgen, Jeongsu Lee, and Igor Zutic Spin Logic Devices Hanan Dery