Nonlocal Spin Transport Research Articles

Nonlocal spin transport based on a half-metallic ferromagnet

Nonlocal spin transport based on a half-metallic ferromagnet

Low temperature spin relaxation length exceeding 3 μm in highly conductive copper channels

Despite extensive studies of spin transport in metallic structures, it remains a challenge to achieve spin relaxation length well above 1 μm in metals even at low temperatures. We explore nonlocal spin transport in Cu channels with a cross section of 0.5 × 0.5 μm2, which exhibit superior values of electrical conductivity and residual resistivity ratio (RRR). Based on structures fabricated in a single batch, we found an average spin relaxation length of λCu=3.2±0.7μm and an average spin relaxation time of τs = 120 ± 50 ps at 30 K. Substantial variations of λCu, RRR, and resistivity ρCu are found among the structures and the three quantities correlate well to one another. The most conductive Cu channel in the batch yields λCu=5.3±0.8μm and τs=250±80ps. These superior values exceed expectations for metals and can be attributed to reduced spin relaxation from grain boundaries and surfaces.

Micromagnetic study of spin transport in easy-plane antiferromagnetic insulators

Magnon eigenmodes in easy-plane antiferromagnetic insulators are linearly polarized and are not expected to carry any net spin angular momentum. Motivated by recent nonlocal spin transport experiments in the easy-plane phase of hematite, we perform a series of micromagnetic simulations in a nonlocal geometry at finite temperatures. We show that by tuning an external magnetic field, we can control the magnon eigenmodes and the polarization of the spin transport signal in these systems. We argue that a coherent beating oscillation between two orthogonal linearly polarized magnon eigenmodes is the mechanism responsible for finite spin transport in easy-plane antiferromagnetic insulators. The sign of the detected spin signal is also naturally explained by the proposed coherent beating mechanism. Our finding opens a path for on-demand control of the spin signal in a large class of easy-plane antiferromagnetic insulators.

A puzzling insensitivity of magnon spin diffusion to the presence of 180-degree domain walls

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl0.5Fe1.5O4 (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180° domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion – nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180° domain wall.

Nonlocal Spin Valves Based on Graphene/Fe3GeTe2 van der Waals Heterostructures.

With recent advances in two-dimensional (2D) ferromagnets with enhanced Curie temperatures, it is possible to develop all-2D spintronic devices with high-quality interfaces using 2D ferromagnets. In this study, we have successfully fabricated nonlocal spin valves with Fe3GeTe2 (FGT) as the spin source and detector and multilayer graphene as the spin transport channel. The nonlocal spin transport signal was found to strongly depend on temperature and disappear at a temperature below the Curie temperature of the FGT flakes, which stemmed from the temperature-dependent ferromagnetism of FGT. The spin injection efficiency was estimated to be about 1%, close to that of conventional nonlocal spin valves with transparent contacts between ferromagnetic electrodes and the graphene channel. In addition, the spin transport signal was found to depend on the direction of the magnetic field and the magnitude of the current, which was due to the strong perpendicular magnetic anisotropy of FGT and the thermal effect, respectively. Our results provide opportunities to extend the applications of van der Waals heterostructures in spintronic devices.

Terahertz Spectral Signatures of Ultrafast Spin Transports in Ferromagnetic Heusler Alloy

AbstractAlthough the Co‐based Heusler compounds are predicted to be half‐metals, their sub‐picosecond demagnetization dynamics upon laser excitation show a transition‐metal‐like behavior. Any possible role of ultrafast nonlocal spin transport on the ultrafast demagnetization of half‐metallic Heusler compounds has only been inferred indirectly by time‐resolved magneto‐optical Kerr effect. Here, an ultrafast optically driven spin current traveling from Co2FeSi half metal into an adjacent Pt layer by using terahertz (THz) emission time‐domain spectroscopy (TDS) is demonstrated. By varying the magnetic field and excitation symmetry, the sizable THz generation from Co2FeSi/Pt stack arises from optically induced spin transport across the Co2FeSi/Pt interface in conjunction with spin‐to‐charge current conversion, via the inverse spin Hall effect (ISHE). The chemical ordering, interface roughness and magnetization of the samples become vital to engineer the THz emission properties. The amplitude of THz emission is comparable to that of CoFeB/Pt stack, thereby indicating Co2FeSi as an efficient ultrafast light‐driven spin current injector. Finally, by varying the pumping energy, the contributions are distinguished to the ultrafast spin current from thermalization and optical excitation of majority spins in Heusler alloy. This work illustrates THz emission TDS as a powerful tool to investigate the ultrafast spintronic properties of Heusler alloy/normal‐metal bilayers.

Development of an optically gated Fe/n-GaAs spin-polarized transistor

Efficient modulation of electrically injected spin signals that is suitable for modern-day transistor functionality is yet to be established. In this work, we demonstrate in detail the fabrication of a $\mathrm{Fe}/n\text{-GaAs}$ spin injection device and the experimental setup for an optical gating of the nonlocal spin transport signal. In situ scanning electron microscopy interface imaging reveals more uniform current distribution at the $\mathrm{Fe}/n\text{-GaAs}$ injector interface at bias voltages higher than the Schottky barrier height. Three- and four-terminal Hanle measurements confirm successful spin injection into $n$-GaAs, with strong interfacial spin dephasing at high magnetic fields. A time-resolved pump-probe Kerr rotation setup was used to illuminate circularly polarized light in the region of the pure spin current in $\mathrm{Fe}/n\text{-GaAs}$ lateral spin injection devices, where ($0.4\ifmmode\pm\else\textpm\fi{}0.3)%$ modulation of the nonlocal signal depending on the light helicity was observed at 30 K.

Probing laser-induced spin-current generation in synthetic ferrimagnets using spin waves

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future opto-magnetic data storage applications, the role of non-local spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring THz standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current, but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the sub-ps timescale. We present modelling efforts using the $s$-$d$ model, which effectively reproduce our results and allow us to explain the behavior in terms of the temporal profile of the spin current.

Nonlocality of Electrically-Induced Spin Accumulation in Chiral Metals

Spin accumulation induced by an electric field in chiral electron system is investigated based on a linear response theory. It is shown that the spin response function has a spatially uniform component due to the chiral angular momentum generation effect, resulting in a nonlocal spin generation. The result suggests a scenario for nonlocal long-range spin transport in chiral metals reported experimentally recently.

Charge-Induced Artifacts in Nonlocal Spin-Transport Measurements: How to Prevent Spurious Voltage Signals

A lively discussion has emerged in both the valleytronics and spintronics communities, concerning the correct interpretation of nonlocal transport experiments. The authors contribute to this ongoing debate their overview of all mechanisms that can create charge-induced nonlocal voltages, which could be misattributed to spin- or valley-related effects. A detailed description of a further measurement artifact, apparently not discussed in the literature, is also included. Fortunately, special instrumentation can significantly reduce most of these measurement artifacts, and thus can strongly diminish the risk of misinterpretation.

Progress in ultrafast spintronics research

Ultrafast spintronics is one of the most promising fields for information processing technology innovation. When compared with traditional electronic devices, new spin-based devices have advantages such as low power consumption, high speed, and nonvolatility. Exploring novel physical phenomena and mechanisms relating to electronic spins significantly promotes the development of next-generation spintronic devices. In this review, we outline a sequence of advances in the fields of ultrafast magnetics and spintronics. First, a quick overview of spintronics and the discovery of ultrafast demagnetization induced by a femtosecond laser is provided. The proposed physical mechanisms and theories of ultrafast spin dynamics are then summarized, including the phenomenological three-temperature model, computations based on the time-dependent density functional theory, the Elliott-Yafet spin-flip mechanism, and the nonlocal superdiffusive spin transport model. Following that, the most recent experimental and theoretical studies on all-optical switching of ferrimagnetic and ferromagnetic metals are thoroughly reviewed. Furthermore, the discovery of hot-electron spin transport induced by ultrashort lasers as well as its influence on ultrafast magnetization dynamics, ultrafast spin transfer torque, spin injection into semiconductors, and spin accumulation and dissipation in hot-electron transport are described. The advancements in the optimization and manipulation of spintronics-based terahertz emitters are reviewed and analyzed. Finally, we discuss the current challenges in ultrafast spintronics research as well as possible future studies.

Absence of spin transport in amorphous YIG evidenced by nonlocal spin transport experiments

It remains highly controversial whether long-distance spin transport or only Joule heating without spin transport exists in the disordered magnetic amorphous ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (a-YIG). Here, we carefully check and analyze the origin of the observed electrical signals in the Py/a-YIG/Pt-based nonlocal spin-transport experiments with both horizontal and vertical geometries, based on the ferromagnetic resonance (FMR)-induced spin pumping and inverse spin Hall effect. In all lateral nonlocal devices with a distance from 2.0 \ensuremath{\mu}m to 200 nm and vertical structures with a $\ensuremath{\ge}\text{40-nm-thick}$ a-YIG spacer layer, the spin-current-induced direct voltage peak signal is absent under FMR conditions, indicating that a-YIG does not possess long-distance spin transport. Furthermore, we demonstrate that a weak spin-torque FMR voltage signal, which gradually reduces to zero with increasing a-YIG thickness in the vertical geometry, is attributed to the leaked spin rectification voltage generated in the Py layer due to the electric leakage between the top Pt layer and the bottom Py layer through the pinhole defects in the thin middle a-YIG layer. Additionally, we find an improvement of the interfacial spin-mixing conductance due to spin accumulation in the interface of a-YIG/Py, which can significantly enhance spin-orbit torque efficiency in the ferromagnet/heavy metal systems with a disordered magnetic a-YIG buffer layer.

Nonlocal magnon transport in a magnetic domain wall waveguide

Magnetic domain walls (DWs) function as a waveguide for low-energy magnons. In this paper we develop the theory for the nonlocal transport of these bound magnons through a ferromagnetic insulator (FMI) that are injected and detected electrically in adjacent normal metal leads by spin-flip scatting processes and the (inverse) spin-Hall effect. Our setup requires a twofold degeneracy of the magnetic ground state, which we realize by an easy axis and hard axis anisotropy, in the FMI. This is readily provided by a broad range of materials. The DW is a topologically protected feature of the system and we obtain the nonlocal spin transport through it, and thereby we provide a framework for reconfigureable magnonic devices.

Magnon-polaron anomaly in nonlocal spin transport through antiferromagnetic insulators

We present a nonlocal spin transport theory for the coupled dynamics of magnetization and lattice vibrations in antiferromagnetic insulators. We find that magnon-polaron formation, i.e., coherently hybridized magnon and acoustic phonon modes, not only leads to anomalous features in the nonlocal spin current but also renormalizes the spin-flop transition field of the antiferromagnets. A length scale for the magnon-polaron formation below which the spin current is not affected by the lattice is also extracted from this nonlocal setup.

Pure Spin Currents Driven by Colossal Spin-Orbit Coupling on Two-Dimensional Surface Conducting SrTiO3.

Spin accumulation is generated by passing a charge current through a ferromagnetic layer and sensed by other ferromagnetic layers downstream. Pure spin currents can also be generated in which spin currents flow and are detected as a nonlocal resistance in which the charge current is diverted away from the voltage measurement point. Here, we report nonlocal spin-transport on two-dimensional surface-conducting SrTiO3 (STO) without a ferromagnetic spin-injector via the spin Hall effect (and inverse spin Hall effect). By applying magnetic fields to the Hall bars at different angles to the nonlocal spin-diffusion, we demonstrate an anisotropic spin-signal that is consistent with a Hanle precession of a pure spin current. We extract key transport parameters for surface-conducting STO, including: a spin Hall angle of γ ≈ (0.25 ± 0.05), a spin lifetime of τ ∼ 49 ps, and a spin diffusion length of λs ≈ (1.23 ± 0.7) μm at 2 K.

Origin of the magnetic field enhancement of the spin signal in metallic nonlocal spin transport devices

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.

Effect of Fe atomic layers at the ferromagnet–semiconductor interface on temperature-dependent spin transport in semiconductors

Using artificially controlled ferromagnet (FM)–semiconductor (SC) interfaces, we study the decay of the nonlocal spin signals with increasing temperature in SC-based lateral spin-valve devices. When more than five atomic layers of Fe are inserted at the FM/SC interfaces, the temperature-dependent spin injection/detection efficiency (Pinj/det) can be interpreted in terms of the T32 law, meaning a model of the thermally excited spin waves in the FM electrodes. For the FM/SC interfaces with the insufficient insertion of Fe atomic layers, on the other hand, the decay of Pinj/det is more rapid than the T32 curve. Using magneto-optical Kerr effect measurements, we find that more than five atomic layers of Fe inserted between FM and SC enable us to enhance the ferromagnetic nature of the FM/SC heterointerfaces. Thus, the ferromagnetism in the ultra-thin FM layer just on top of SC is strongly related to the temperature-dependent nonlocal spin transport in SC-based lateral spin-valve devices. We propose that the sufficient ferromagnetism near the FM/SC interface is essential for high-performance FM–SC hybrid devices above room temperature.

Role of NiO in the nonlocal spin transport through thin NiO films on Y3Fe5O12

In spin transport experiments with spin currents propagating through antiferromagnetic (AFM) material, the antiferromagnet is treated as a mainly passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the N\'eel temperature and decreasing by lowering the temperature. In order to study the role of the AFM in local and nonlocal spin transport experiments, we send spin currents through NiO of various thickness placed on Y$_3$Fe$_5$O$_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below 100$\,$K which are unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence is similar as for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

Enhancement of YIG|Pt spin conductance by local Joule annealing

We report that Joule heating can be used to enhance the interfacial spin conductivity between a metal and an oxide. We observe that local annealing of the interface at about 550 K, when injecting large current densities (>1012A/m2) into a pristine 7 nm thick Pt nanostrip evaporated on top of yttrium iron garnet (YIG), can improve the effective spin transmission up to a factor of 3. This result is of particular interest when interfacing ultrathin garnet films to avoid strong chemical etching of the surface. The effect is confirmed by the following methods: spin Hall magnetoresistance, spin pumping, and non-local spin transport. We use it to study the influence of the YIG|Pt coupling on the non-linear spin transport properties. We find that the crossover current from a linear to a non-linear spin transport regime is independent of this coupling.

Antiferromagnetic magnon pseudospin: Dynamics and diffusive transport

We formulate a theoretical description of antiferromagnetic magnons and their transport in terms of an associated pseudospin. The need and strength of this formulation emerges from the antiferromagnetic eigenmodes being formed from superpositions of spin-up and -down magnons, depending on the material anisotropies. Consequently, a description analogous to that of spin-1/2 electrons is demonstrated while accounting for the bosonic nature of the antiferromagnetic eigenmodes. Introducing the concepts of a pseudospin chemical potential together with a pseudofield and relating magnon spin to pseudospin allows a consistent description of diffusive spin transport in antiferromagnetic insulators with any given anisotropies and interactions. Employing the formalism developed, we elucidate the general features of recent non-local spin transport experiments in antiferromagnetic insulators hosting magnons with different polarisations. The pseudospin formalism developed herein is valid for any pair of coupled bosons and is likely to be useful in other systems comprising interacting bosonic modes.