Spin Current Transport Research Articles

Temperature dependence of spin transport behavior in Heusler alloy CPP-GMR

In this study, we investigate the effect of temperature on the performance of a read sensor by utilizing an atomistic model coupled with a spin transport model. Specifically, we study the temperature dependence of spin transport behavior and MR outputs in a Co2FeAl0.5Si0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {Co}_2\ ext {FeAl}_{0.5}\ ext {Si}_{0.5}$$\\end{document} (CFAS)(5nm)/Cu(5nm)/CFAS(5nm) trilayer with diffusive interfaces. Initially, the two-channel model of spin-dependent resistivity is used to calculate the temperature dependence of spin transport parameters which serves as essential input for the spin accumulation model. Our findings demonstrate that as the temperature increases, the spin transport parameters and magnetic properties decrease due to the influence of thermal fluctuation. At a critical temperature, where the ferromagnet transitions to a paramagnetic state, we observe zero spin polarization. Furthermore, at elevated temperatures, the spin accumulation deviates from the equilibrium value, leading to a reduction in the magnitude of spin current and spin transport parameters due to thermal effects. As a consequence, the MR ratio decreases from 65% to 20% with increasing temperature from 0 to 400 K. Our results are consistent with previous experimental measurements. This study allows to deeply understand the physical mechanism in the reader stack which can significantly benefit reader design.

Analysis of spin current in ferromagnetic graphene junctions

Magnetization transport in a ferromagnetic/normal/ferromagnetic graphene junction has been investigated. Inspired by a seminal paper [Phys. Rev. B 66, 014,407, 2002], the longitudinal and transverse components of spin current i.e. components parallel and perpendicular to the magnetization direction in the ferromagnetic layer have been studied. It has been shown that only when the reflected and transmitted waves from the interface between two normal/ferromagnetic layers have the same angle as that of the incident wave, there will be no torque associated with the transport of longitudinal spin current. In general, for an arbitrary direction of magnetization, all spin current components are coexisted and therefore the parallel and the perpendicular components of spin transfer torque are existed. Therefore it is not possible to draw a general conclusion that the torque associated with the transport of longitudinal spin current is always zero.

Highly Efficient Spin Current Transport Properties in Spintronic Devices Based on Topological Insulator

AbstractRecently, topological insulators (TIs) have regained extensive attention in spintronics due to their potential applications in new‐generation spintronic devices, following the discovery of the quantum Hall effect and quantum anomalous Hall effect, which introduce the topological concept. In this review, the exotic spin transport phenomena are explored in TIs. The review offers a concise overview of the fundamental principles of TIs, followed by an exploration of diverse fabrication methods for TI materials. Characterization techniques of the topological surface states are also presented. The review delves into the intriguing spin current transport phenomena, focusing on spin‐to‐charge and charge‐to‐spin conversions in TI/ferromagnet bilayers, respectively. The review culminates summarizing key insights and project future directions for research on spin transport phenomena in TIs, emphasizing practical implications.

Colossal anisotropic absorption of spin currents induced by chirality.

The chiral induced spin selectivity (CISS) effect, in which the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices. CISS implies that the spin preference of chiral structures persists upon injection of pure spin currents and can act as a spin analyzer without the need for a ferromagnet. Here, we report an anomalous spin current absorption in chiral metal oxides that manifests a colossal anisotropic nonlocal Gilbert damping with a maximum-to-minimum ratio of up to 1000%. A twofold symmetry of the damping is shown to result from differential spin transmission and backscattering that arise from chirality-induced spin splitting along the chiral axis. These studies reveal the rich interplay of chirality and spin dynamics and identify how chiral materials can be implemented to direct the transport of spin current.

Isotropic Spin Hall Effect in an Epitaxial Ferromagnet.

We report the observation of the isotropic spin Hall effect in a ferromagnet. We show that the spin Hall effect in an epitaxially grown Fe_{3}Si generates a sizable spin current with a spin direction that is noncollinear with the magnetization. Furthermore, we find that the spin Hall current is independent of the relative orientation between its spin direction and the magnetization; the spin Hall effect is isotropic. This observation demonstrates that the intrinsically generated transverse spin component is protected from dephasing, providing fundamental insights into the generation and transport of spin currents in ferromagnets.

Crossover from Positive to Negative Spin Hall Signal in a Ferromagnetic Metal Induced by the Magnetization Modulated Interface Effect

AbstractThe spin Hall effect and its inverse (SHE/ISHE), describing the charge‐to‐spin interconversion, are of critical importance to the fundamental physics involving spin‐orbit coupling and the application of spintronic devices. These phenomena in nonmagnetic materials are reasonably understood after extensive studies. In ferromagnetic metals, however, the fundamental issue of generation and transport of pure spin current, especially the interplay of charge‐to‐spin interconversion and magnetization, is as‐yet poorly understood. Here, direct experimental evidence for the sign change in ISHE by injecting a spin current from Y3Fe5O12 into a Fe film via a Cu spacer is reported. The sizable negative ISHE signal (VISHE) in nanometer‐thick Fe films reverses its sign with a comparable magnitude when Fe magnetization is varied from a longitudinal to transverse orientation with respect to spin current polarization. With decreasing Fe thickness, this negative longitudinal VISHE increases rapidly to a larger positive value in magnitude. The opposite spin Hall angles for longitudinally and transversely polarized spin currents are reproduced by first‐principles transport calculations and the sign change is attributed to the anisotropic contribution at the ferromagnetic interface. This work lays a firm foundation for manipulating spin‐to‐charge interconversion with an extra degree of freedom of magnetization orientation through ferromagnetic metals.

Half‐Metallic Heusler Alloy/GaN Heterostructure for Semiconductor Spintronics Devices

AbstractBecause spin‐orbit coupling in wurtzite semiconductors is relatively weak compared with that in zincblende ones, the III‐nitride semiconductor GaN is a promising material for high‐performance optical semiconductor spintronic devices such as spin lasers. For the purpose of reducing the operating power of spin lasers, it is necessary to demonstrate highly efficient electrical spin injection from a ferromagnetic material into GaN with a low‐resistance contact. Here, an epitaxial half‐metallic Heusler alloy Co2FeAlxSi1−x(CFAS)/GaN heterostructure is developed by inserting an ultrathin Co layer between the CFAS and GaN. The CFAS/n+‐GaN heterojunctions clearly show tunnel conduction with very small rectification and a low resistance‐area product of ≈3.8 kΩµm2, which is several orders of magnitude smaller than those reported in previous work, at room temperature. Nonlocal spin signals and a Hanle effect curve are observed at low temperatures using lateral spin‐valve devices with the CFAS/n+‐GaN contacts, suggesting pure spin current transport in bulk GaN. The spin transport is observed at temperatures as high as room temperature, with a high spin polarization of 0.2 at a low bias voltage less than 2.0 V. This study is expected to open a path to GaN‐based spintronic devices with highly spin‐polarized and low‐resistance contacts.

Study on magnetization dynamics modulated by spin-orbit-torque in a Ni81Fe19/NiO/Ni81Fe19 multilayered wire

As electrical control of magnetization dynamics opens the door to realize spintronic devices, understanding the microscopic mechanisms of spin current transport and its effect through the antiferromagnetic and interface is crucial. We studied magnetization dynamics modulated by the spin current originated from the spin-orbit-torque using the rectifying planar Hall effect (PHE). In Ni81Fe19/NiO/Ni81Fe19 heterostructure wire deposited on Pt/Ta cross-tie electrode, we measured the rectifying PHE as a function of external magnetic field angle and dc electric current. By measuring the electrical responses of the heterostructure system, we found that the magnetization dynamics can be modulated by the dc electric current flowing through the Pt/Ta electrode.

Effective spin injection into the organic semiconductor PTCDA evaluated by a normalization method

Studies of spin current injection, transport, and interface control have drawn attention recently for efficient organic spintronic devices. In this study, we apply both spin pumping (SP) and the longitudinal spin Seebeck effect (LSSE) to inject spin currents into a π-conjugated organic semiconductor, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and characterize injection and transport by measuring inverse spin Hall voltage VISHE in spin detectors. A normalization factor introduced to SP analysis eliminates a contribution provoked by deviation of spin sources and leads to a more accurate determination of the spin diffusion length in PTCDA. While SP with Permalloy as a spin source is effective in generating detectable VISHE, the LSSE from yttrium iron garnet shows no convincing sign of spin injection. In addition, spin-flip scattering induced by hybrid states undermining electrical spin injection is negligible in SP. These results are attributed to interfaces between spin sources and PTCDA, indicative of the importance of injection methods and material choices.

Spin-current in a Magnetic Semiconductor Tunnel Junction: Effect of External Bias Voltage

This paper investigates spin-current transport in a GaMnAs/GaAs/GaMnAs magnetic semiconductor tunnel junctions under applied bias voltages. The 30-band k.p approach is used to describe the materials within the heterostructure, incorporating both spin-orbit and exchange interactions. We use the transfer-matrix formalism to derive numerical solutions for the wave functions. At specific bias values, we calculate the polarization of the spin-current component along the z direction of the structure. We show oscillations of the two spin-current components perpendicular to the magnetization with equal polarization amplitude and characteristic period. The polarization amplitude varies around 10%, reflecting the typical polarization in such type of material. The oscillation period - which relates to the Larmor frequency for spin precession - increases with the bias voltage values.

Role of Spin–Orbit Coupling on Ultrafast Spin Dynamics in Nonmagnet/Ferromagnet Heterostructures

AbstractSpin–orbit coupling (SOC), the interaction between spin and orbital angular momentum of electrons, is imperative to control magnetic properties of nonmagnet (NM)/ferromagnet (FM) heterostructures and design energy‐efficient and faster spin‐based devices. Here, femtosecond pulsed laser‐induced time‐resolved magneto‐optical Kerr effect magnetometry is employed to investigate magnetization dynamics in different NM/Co20Fe60B20 heterostructures, where the NM layer varies as Cu, Ta, W, Pt, Ta/Ru/Ta, and Si/SiO2 (no underlayer) that differ in SOC strength. It is observed that there is a systematic variation in ultrafast demagnetization time (τm), fast remagnetization time (τr), and Gilbert damping parameter (α) with the SOC strength of the underlayer and an inverse relationship between α and τm, τr is established due to the dominant contribution of spin current transport in ultrafast demagnetization and fast remagnetization processes. The spin pumping formalism estimates the effective spin‐mixing conductance (Geff) for different interfaces, which signifies that the high SOC strength of underlayers results in high Geff indicating more efficient transport of spin current through it. This study suggests that the SOC strength of the NM underlayer plays a significant role in controlling the ultrafast demagnetization process through interfacial spin current transport in a NM/FM heterostructure which can be beneficial for the development of ultrafast spintronics devices.

Tunable Magnonic Chern Bands and Chiral Spin Currents in Magnetic Multilayers.

Realization of novel topological phases in magnonic band structures represents a new opportunity for the development of spintronics and magnonics with low power consumption. In this work, we show that in antiparallelly aligned magnetic multilayers, the long-range, chiral dipolar interaction between propagating magnons generates bulk bands with nonzero Chern integers and magnonic surface states carrying chiral spin currents. The surface states are highly localized and can be easily toggled between nontrivial and trivial phases through an external magnetic field. The realization of chiral surface spin currents in this dipolarly coupled heterostructure represents a magnonic implementation of the coupled wire model that has been extensively explored in electronic systems. Our work presents an easy-to-implement system for realizing topological magnonic surface states and low-dissipation spin current transport in a tunable manner.

Piezoelectric Strain-Controlled Magnon Spin Current Transport in an Antiferromagnet.

As the core of spintronics, the transport of spin aims at a low-dissipation data process. The pure spin current transmission carried by magnons in antiferromagnetic insulators is natively endowed with superiority such as long-distance propagation and ultrafast speed. However, the traditional control of magnon transport in an antiferromagnet via a magnetic field or temperature variation adds critical inconvenience to practical applications. Controlling magnon transport by electric methods is a promising way to overcome such embarrassment and to promote the development of energy-efficient antiferromagnetic logic. Here, the experimental realization of an electric field-induced piezoelectric strain-controlled magnon spin current transmission through the antiferromagnetic insulator in the Y3Fe5O12/Cr2O3/Pt trilayer is reported. An efficient and nonvolatile manipulation of magnon propagation/blocking is achieved by changing the relative direction between the Néel vector and spin polarization, which is tuned by ferroelastic strain from the piezoelectric substrate. The piezoelectric strain-controlled antiferromagnetic magnon transport opens an avenue for the exploitation of antiferromagnet-based spin/magnon transistors with ultrahigh energy efficiency.

Spin superfluidity on the spherical surface

Spintronics on flat surfaces has been investigated over the years, and the scenario is relatively well-known; however, there is a lack of information when considering non-flat geometries. In this paper, we are interested in the spin current on the spherical surface of a ferromagnetic insulator. We are mainly concerned with the superfluid spin current transport when spin current is injected at the north pole and propagates to the south pole. We evaluate the spin transport over the spherical surface using the magnetoelectric circuit theory and the semiclassical magnetization formalism. Including the magnetization damping by the Landau–Lifshitz–Gilbert equation, we obtain a spin current that propagates over distances larger than the magnon ballistic decay length, evidence of spin superfluidity. In addition, provided that the ejected spin current is always radially aligned, along with the dissipationless nature, a spherical shell could be used as an efficient oriented spin valve.

Spin Current Sensing for Selective Detection of Explosive Molecules.

Spin current based sensing methods offer a new approach to the development of selective detection devices for explosive molecules. Employing a combination of bias voltages and transverse electric fields to vary the chemiresistive properties of a zigzag graphene nanoribbon, dual-input dual-output sensors of this kind offer major advantages: tuning the electrical properties of a single nanoribbon is equivalent to deploying a sensor array, and measuring two outputs (spin-up and spin-down currents, total current and spin current difference, etc.) offers improved selectivity. Ab initio modeling suggests that the magnetic properties of the analyte, charge transfer effects, current transmission pathways, and analyte molecule size all influence sensor signatures. Analysis of the sensing cause-effect physics relies upon the calculation of energy averaged bond currents, which visualize the global spin current transport. Principal component analysis of the proposed sensing scheme suggests that it can distinguish between common background gases, nitroaromatic explosives, and nitramine explosives and will offer far better selectivity than carbon nanotube based explosive sensing devices.

Nonvolatile modulation of spin transport in PMN-PT/LiFe5O8/Pt multiferroic heterostructures

Beside playing a keystone role in spin-field-effect transistor, electrical control of spin transport is also of fundamental and practical importance for many other spintronic devices due to the advantages of energy efficiency and versatility. In this work, we demonstrate a significant electrical modulation of spin transport in the PMN-PT/LiFe5O8/Pt heterostructures. The key spin transport parameters, including the spin Hall angle, spin diffusion length, and spin mixing conductance, were extracted through the thickness dependence of spin Hall magnetoresistance. It is found that the spin Hall angle decreases with the decrease in temperature, while the spin diffusion length keeps invariant with temperature, revealing the dominance of D'yakonov–Perel' type spin transport in LiFe5O8/Pt. Remarkably, by applying the electric field onto the piezoelectric PMN-PT substrate, the spin Hall angle of Pt can be modulated 28% and exhibits a nonvolatile hysteresis relationship with the applied electric field, which primarily originates from the modulation of Pt resistivity induced by the strain coupling through the ferroelastic domain switching of the ferroelectric PMN-PT substrate. Our results elucidate the spin transport characteristics in PMN-PT/LiFe5O8/Pt heterostructures and pave the way toward novel spintronic devices with electrically tunable spin current generation and transport processes.

Experimental extraction of donor-driven spin relaxation in n -type nondegenerate germanium

Using pure spin current transport measurements in lateral spin-valve devices, we study the spin relaxation in an $n$-type nondegenerate Ge layer, which is moderately doped Ge (P: $\ensuremath{\sim}{10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$). The obtained spin diffusion length $({\ensuremath{\lambda}}_{\mathrm{Ge}})$ of the nondegenerate Ge is two to three times greater than that of heavily doped degenerate Ge (P: $\ensuremath{\sim}{10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) in the temperature range from 8 to 100 K. We find that the electron spin lifetime ($\ensuremath{\tau}$) for the nondegenerate Ge is monotonically increased with decreasing temperature $(T)$. The increase in $\ensuremath{\tau}$ at temperatures less than 50 K can be interpreted in terms of the donor-driven spin relaxation mechanism including the 1/$\sqrt{T}$ behavior in multivalley semiconductors, proposed by Song et al. [Y. Song, O. Chalaev, and H. Dery, Phys. Rev. Lett. 113, 167201 (2014)]. We note that it is important for the $\ensuremath{\tau}$ of the moderately doped nondegenerate Ge to partly consider the $T$-independent component of spin relaxation in addition to the 1/$\sqrt{T}$ component.

Room temperature magnetization dynamics of Y3Fe5O12 films capped with a Cr2O3 layer

Spin-transport across antiferromagnetic insulators (AFMIs) is a novel way to reduce power consumption in pure spin current based devices. In this paper, room temperature magnetization dynamics of Yttrium-Iron-Garnet (YIG) interfaced with an AFMI Cr2O3 has been investigated in order to study the spin transport across the AFMI. After capping with 20 nm thick Cr2O3, the Gilbert damping parameter of YIG layer is not changed enormously as compared to the non-capped sample. Cr2O3 of 20 nm thickness is in a spin disordered state at room temperature due to finite size scaling effect. So, it was expected that the spin current from YIG layer can be transmitted through the Cr2O3 layer. However, due to large spin diffusion length and large domain size of Cr2O3 (>20 nm), spin current cannot lose its angular momentum during propagation in Cr2O3 as a result of which, no change in Gilbert damping parameter is noticed. This observation will pave a way for further investigation of spin current transport across AFMIs.

Revealing the importance of interfaces for pure spin current transport

The authors measure spin transport and spin absorption across interfaces in lateral spin-valve devices and correlate this electrical characterization to direct imaging of the defect structure of the buried interfaces.

Hopping-Dominated Spin Transport in Unintentionally Doped Organic Semiconductors.

We report a spin diffusion theory to predict unusual pure spin current transport in unintentionally doped organic semiconductors. We demonstrate that the feasibility of pure spin current transport via polaron hopping at a low carrier density. Our theoretical prediction, 40 nm, for spin diffusion length (SDL) in dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) is in very good agreement with experimental data. Interestingly, SDL can be prolonged by restraining molecular geometry structure disorder and reducing the reorganization energy. In comparison with anisotropic organic materials, the SDL in isotropic ones increases up to 60%. Our results open up a new avenue to design organic spintronics devices with long SDL and low carrier density.