How to Create a Spin Current

High temperature stability in few atomic layer MoS2 based thin film heterostructures: structural, static and dynamic magnetization properties

Layered transition metal dichalcogenides (TMDs) have shown commendable properties for spintronic applications. From the device perspective, the structural quality of the TMD as well as its interface with the adjacent ferromagnetic (FM) layer is of paramount importance. Here, we present the spin-dynamic behaviour in the widely studied TMDs, i.e., MoS₂ using Co₆₀Fe₂₀B₂₀ (CoFeB), i.e., in MoS₂(1-4 layers)/CoFeB(4-15 nm) heterostructures, both in the as-grown state and in the in situ annealed state (400 °C in a vacuum). Raman spectroscopy revealed systematic variation in the separation (δ) between the characteristic Raman shifts corresponding to the E_2g and A_1gvis-à-vis the number of layers (n_L) of MoS₂. The analysis of the ferromagnetic resonance (FMR) spectroscopy measurements performed on these heterostructures revealed the spin pumping from CoFeB to the MoS₂ layer as evidenced by the ∼49% (∼51%) enhancement in the effective damping parameter with respect to the damping parameter of bare as-deposited (annealed) CoFeB films. This enhancement is attributed to the spin-pumping owing to the high spin-orbit coupling of monolayer MoS₂. The latter is also confirmed by density functional theory calculations. By finding the effective spin mixing conductance of the MoS₂/CoFeB interface, the effective spin current density in the MoS₂ layer is estimated to increase from ∼0.3 to 0.7 MA m^-2 with CoFeB thickness for both the as-deposited and annealed heterostructures. Furthermore, the δ vs. n_L curve of the as-deposited heterostructure did not show any significant change upon annealing, which demonstrated that the spin transport and magnetic properties of these heterostructures remained unaffected even after annealing at a high temperature of 400 °C. Hence, this establishes the high thermal stability of the sputter grown MoS₂/CoFeB heterostructures. Thus, this study highlights the important role of MoS₂ as an efficient spin current-generating source for spin-orbit torque based magnetic memory applications, given the high-temperature stability and high-quality monolayers of MoS₂ and its excellent performance with CoFeB thin films.

Graphene Spin Valves for Spin Logic Devices

An alternative to charge-based electronics identifies the spin degree of freedom for information communication and processing. The long spin-diffusion length in graphene at room temperature demonstrates its ability for highly scalable spintronics. The development of the graphene spin valve (SV) has inspired spin devices in graphene including spin field-effect transistors and spin majority logic gates. A comprehensive picture of spin transport in graphene SVs is required for further development of spin logic. This review examines the advances in graphene SVs and their role in the development of spin logic devices. Different transport and scattering mechanisms in charge and spin are discussed. Furthermore, the on/off switching energy between graphene SVs and charge-based FETs is compared to highlight their prospects for low-power devices. The challenges and perspectives that need to be addressed for the future development of spin logic devices are then outlined.

Enhanced Magnetic Order and Reversed Magnetization Induced by Strong Antiferromagnetic Coupling at Hybrid Ferromagnetic-Organic Heterojunctions

Organic-molecular magnets based on a metal-organic framework with chemically tuned electronic and magnetic properties have been attracting tremendous attention due to their promising applications in molecular magnetic sensors, magnetic particle medicines, molecular spintronics, etc. Here, we investigated the magnetic behavior of a heterojunction comprising a ferromagnetic nickel (Ni) film and an organic semiconductor (OSC) 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) layer. Through the magneto-optical Kerr effect (MOKE), a photoemission electron microscopy (PEEM), X-ray magnetic circular dichroism (XMCD), and X-ray photoelectron spectroscopy (XPS), we found that the adsorption of F4-TCNQ on Cu(100)/Ni not only reverses the in-plane magnetization direction originally exhibited by the Ni layer but also results in enhanced magnetic ordering. Furthermore, the cyano group (CN) in adsorbed F4-TCNQ was found spin-polarized along with conspicuous charge transfer with Ni. The density functional theory (DFT) calculations suggest that the experimentally found spin polarization originates from hybridization between the CN group's π orbitals and Ni's d band. These findings signify that the hybrid states at the organic-ferromagnet interface play a key role in tailoring the magnetic behavior of interfaces. For the case of the F4-TCNQ and Ni heterojunction reported here, interface coupling is an antiferromagnetic one.

Structural Phase-Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin-Film Heterostructures

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (G_eff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co₂₀Fe₆₀B₂₀ (d)/SiO₂ (2 nm) thin-film heterostructures in the beta-tungsten (β-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the β-W/CoFeB interface. A stark variation of G_eff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

All-optical detection of interfacial spin transparency from spin pumping in β-Ta/CoFeB thin films

Generation and utilization of pure spin current have revolutionized energy-efficient spintronic devices. Spin pumping effect generates pure spin current, and for its increased efficiency, spin-mixing conductance and interfacial spin transparency are imperative. The plethora of reports available on generation of spin current with giant magnitude overlook the interfacial spin transparency. Here, we investigate spin pumping in β-Ta/CoFeB thin films by an all-optical time-resolved magneto-optical Kerr effect technique. From variation of Gilbert damping with Ta and CoFeB thicknesses, we extract the spin diffusion length of β-Ta and spin-mixing conductances. Consequently, interfacial spin transparency is derived as 0.50 ± 0.03 from the spin Hall magnetoresistance model for the β-Ta/CoFeB interface. Furthermore, invariance of Gilbert damping with Cu spacer layer thickness inserted between β-Ta and CoFeB layers confirms the absence of other interface effects including spin memory loss. This demonstrates a reliable and noninvasive way to determine interfacial spin transparency and signifies its role in generation of pure spin current by spin pumping effect.

Spin pumping with coherent elastic waves

We show that the resonant coupling of phonons and magnons can be exploited to generate spin currents at room temperature. Surface acoustic wave pulses with a frequency of 1.55 GHz and duration of 300 ns provide coherent elastic waves in a ferromagnetic thin-film-normal-metal (Co/Pt) bilayer. We use the inverse spin Hall voltage in the Pt as a measure for the spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect.

Design of the local spin polarization at the organic-ferromagnetic interface

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing π(p(z)) electrons onto a magnetic surface. The hybridization of the out-of-plane p(z) atomic-type orbitals with the d states of the metal leads to the inversion of the spin polarization at the organic site due to a p(z)-d Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

Quantum cavity for spin due to spin-orbit interaction at a metal boundary

A quantum cavity for spin is created using a tungsten crystal as substrate of high nuclear charge and breaking the structural inversion symmetry through deposition of a gold quantum film. Spin- and angle-resolved photoelectron spectroscopy shows directly that quantum-well states and the "matrioshka" or Russian nested doll Fermi surface of the gold film are spin polarized and spin-orbit split up to a thickness of at least nine atomic layers. Ferromagnetic materials or external magnetic fields are not required, and the quantum film does not need to possess a high atomic number as analogous results with silver show.

Topological change of the Fermi surface in low-density Rashba gases: application to superconductivity

In this Letter we show how, for small values of the Fermi energy compared to the spin-orbit splitting of Rashba type, a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states in the low charge density regime. We investigate its consequences on the onset of the superconducting instability. We show that the superconducting critical temperature is significantly tuned in this regime by the spin-orbit coupling. We suggest that materials with strong spin-orbit coupling are good candidates for enhanced superconductivity.

Self-assembled organic radicals on Au(111) surfaces: a combined ToF-SIMS, STM, and ESR study

Electron spin resonance (ESR), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and scanning tunneling microscopy (STM) have been used in parallel to characterize the deposition on gold surface of a series of nitronyl nitroxide radicals. These compounds have been specifically synthesized with methyl-thio linking groups suitable to interact with the gold surface to form self-assembled monolayers (SAMs), which can be considered relevant in the research for molecular-based spintronics devices, as suggested in recent papers. The degree of the expected ordering on the surface of these SAMs has been tuned by varying the chemical structure of synthesized radicals. ToF-SIMS has been used to support the evidence of the occurrence of the deposition process. STM has shown the different qualities of the obtained SAMs, with the degree of local order increasing as the degree of freedom of the molecules on the surface is decreased. Finally, ESR has confirmed that the deposition process does not affect the paramagnetic characteristics of radicals and that it affords a complete single-layered coverage of the surface. Further, the absence of angular dependence in the spectra indicates that the small regions of local ordering do not give rise to a long-range order and suggests a quite large mobility of the radical on the surface, probably due to the weak interaction with gold provided by the methyl-thio linking group.

Spintronics: A Challenge for Materials Science and Solid-State Chemistry

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.

Direct demonstration of decoupling of spin and charge currents in nanostructures

The notion of decoupling of spin and charge currents is one of the basic principles underlying the rapidly expanding field of spintronics. However, no direct demonstration of the phenomenon exists. We report a novel measurement in which a nonequilibrium spin population is created by a pointlike injection of current from a ferromagnet across a tunnel barrier into a one-dimensional spin channel and detected differentially by a pair of ferromagnetic electrodes placed symmetrically about the injection point. We demonstrate that the spin current is strictly isotropic about the injection point and, therefore, completely decoupled from the unidirectional charge current.

Unified description of bulk and interface-enhanced spin pumping

We describe a mechanism for generating nonequilibrium electron-spin accumulation in semiconductors or metals by rf magnetic field pumping. With a semiclassical model we show that a rotating applied magnetic field (or the processing magnetization inside a weak ferromagnet) generates a dc spin accumulation. For bulk systems this spin accumulation is in general given by a small fraction of h omega, where omega is the rotation or precession frequency. With the addition of a neighboring, field-free region, and allowing for the diffusion of spins across the interface, the spin accumulation is dramatically enhanced towards h omega near the interface. The interface-enhanced spin accumulation obtained within our bulk-oriented model is surprisingly similar to predictions based on interface-scattering theory [A. Brataas, Phys. Rev. B 66, 060404(R) (2002)].