自旋Seebeck效应简介

自旋电子学作为一个新兴的学科,是未来电子学发展的重要方向之一.而近年来发现的自旋泽贝克（Seebeck）效应则为自旋电子学的研究提供了不少新现象.文章通过对自旋Seebeck效应的一些科研进展的介绍,较详尽地阐明了自旋Seebeck效应的定义和常用的利用逆自旋霍尔（Hall）效应来进行观测的机制与方法,并对不同种类材料中的自旋Seebeck效应及其可能的成因进行了分析介绍.     

Origin of large spin Hall magnetoresistance in Fe/CuOx bilayers

We have compared the spin Hall magnetoresistance (SMR) in Fe/Pt and Fe/CuO_x (with natural oxidation) bilayers with varying the thickness of Fe layer. A larger SMR in Fe/CuO_x bilayers has been found when the thickness of Fe layer is 3 nm. Moreover, the SMR of the two bilayers decrease with increasing the thickness of Fe from 3 nm to 10 nm, but that of Fe/CuO_x drops more sharply due to shunting current effect. Through harmonic measurements, the emergent spin current is proved to be generated in the Fe/CuO_x bilayers. The mixed phase of CuO_x has been confirmed including CuO, Cu₂O and Cu, which performs strong spin-orbit coupling and produce large spin current. On the other hand, the interface-generated spin current should be ruled out. All the results have been compared with those in Fe/Al₂O₃ bilayers with negligible spin current.     

Large enhancement of domain wall-induced anomalous magnetoresistance in ferrimagnetic Tb/Co wires: The effect of injecting spin Hall current

We demonstrated domain wall (DW)-induced anomalous magnetoresistance (MR) generated in asymmetric and symmetric ferrimagnetic Tb/Co multilayered, and Tb–Co alloyed wires. The extraordinary Hall effect (EHE)-induced circulating currents in the vicinity of DWs between longitudinal voltage probes are assigned to the anomalous MR. A large anomalous MR ~1.5% was obtained in the asymmetric Tb/Co multilayered wire. The large MR can be attributed to an addition of spin Hall current with a long coherence length from an adjacent Pt layer. These results open new possibilities for the use of ferrimagnetic multilayered wires beyond multi-function devices.     

Large planar Hall effect in perpendicularly magnetized W/CoFeB/MgO structures

The planar Hall effect (PHE) in W/CoFeB/MgO structure with perpendicular magnetic anisotropy was investigated as a function of CoFeB thickness (t_CoFeB). The PHE is measured by sweeping the in-plane magnetic field at various azimuthal angles as well as by rotating strong magnetic field which is enough to saturate the magnetization. We observed a huge PHE in the W/CoFeB/MgO sample, which is even larger than anomalous Hall effect (AHE). This is distinct from the results in Ta/CoFeB/MgO samples showing a much smaller PHE than AHE. Since the PHE is insensitive to the t_CoFeB while the AHE is proportional to the t_CoFeB, the unprecedented PHE can be attributed to the W layer with a large spin-orbit coupling.     

Perspectives of electrically generated spin currents in ferromagnetic materials

Spin-orbit coupling enables charge currents to give rise to spin currents and vice versa, which has applications in non-volatile magnetic memories, miniature microwave oscillators, thermoelectric converters and Terahertz devices. In the past two decades, a considerable amount of research has focused on electrical spin current generation in different types of nonmagnetic materials. However, electrical spin current generation in ferromagnetic materials has only recently been actively investigated. Due to the additional symmetry breaking by the magnetization, ferromagnetic materials generate spin currents with different orientations of spin direction from those observed in nonmagnetic materials. Studies centered on ferromagnets where spin-orbit coupling plays an important role in transport open new possibilities to generate and detect spin currents. We summarize recent developments on this subject and discuss unanswered questions in this emerging field.     

High field electron-spin transport and observation of the Dyakonov-Perel spin relaxation of drifting electrons in low temperature-grown gallium arsenide

High field electron-spin transport in low temperature-grown gallium arsenide is studied. We generate electron spins in the samples by optical pumping. During transport, we observe the Dyakonov-Perel (DP) [M.I. Dyakonov, V.I. Perel, Zh. Eksp. Teor. Fiz. 60 (1971) 1954] spin relaxation of the drifting electrons. The results are discussed and are compared with those obtained in calculations of the DP spin relaxation frequency of the hot electrons. A good agreement is obtained.     

Spin-orbit torques induced by spin Hall and spin swapping currents of a separate ferromagnet in a magnetic trilayer

Spin-orbit torques (SOTs) have been investigated most widely in normal metal/ferromagnet bilayers where the spin Hall effect of normal metal is a main source of spin currents. Recently, ferromagnets are found to also serve as spin-current sources through spin-orbit coupling. In this work, we theoretically investigate SOT acting on ferromagnet₂ in ferromagnet₁/normal metal/ferromagnet₂ trilayers, which is caused by the spin Hall and spin swapping effects of ferromagnet₁. Our result provides an analytical expression of SOT in the trilayers, which may be useful for quantifying the spin Hall and spin swapping effects of ferromagnets and also for designing and interpreting SOT experiments where a ferromagnet is used as a spin-current source instead of a normal metal.     

Magnetocapacitance study of the fractional-quantum-Hall effect: Quasiparticle charge and spin polarization

By the method of capacitance spectroscopy and of magnetotransport we have investigated the and fractional-quantum-Hall-effect (FQHE) states in gated GaAs AlGaAs heterojunctions with tuned electron areal density. Our experimental results confirm the theoretical prediction of the fractional quasiparticle charge in the FQHE state and of the existence of spin-aligned quasiholes and spin-reversed quasielectrons in the fully spin-polarized FQHE state.     

Gauge potential formulations of the spin Hall effect in graphene

Two different gauge potential methods are engaged to calculate explicitly the spin Hall conductivity in graphene. The graphene Hamiltonian with spin-orbit interaction is expressed in terms of kinematic momenta by introducing a gauge potential. A formulation of the spin Hall conductivity is established by requiring that the time evolution of this kinematic momentum vector vanishes. We then calculated the conductivity employing the Berry gauge fields. We show that both of the gauge fields can be deduced from the pure gauge field arising from the Foldy-Wouthuysen transformations.     

The origin of spin current in YIG/nonmagnetic metal multilayers at ferromagnetic resonance

Spin pumping in yttrium-iron-garnet(YIG)/nonmagnetic-metal(NM) layer systems under ferromagnetic resonance(FMR) conditions is a popular method of generating spin current in the NM layer.A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications.It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer.Here,by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM_1/NM_2(NM_1 =Cu or Al,NM_2 =Pt or Ta),we unambiguously showed that spin current in NM,instead of from the precessing YIG magnetization,came from the magnetized NM surface(in contact with thin YIG),either due to the magnetic proximity effect(MPE) or from the inevitable diffused Fe ions from YIG to NM.This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect(ISHE).The voltage signal is attributed to the magnetized NM surface,hardly observed in the conventional FMR experiments,and was greatly amplified when the electrical detection circuit was switched on.     

Inertial spin Hall effect in non-commutative space

In the present Letter the study of inertial spin current (that appears in an accelerated frame of reference) is extended to Non-Commutative (NC) space. In the Hamiltonian framework, the Dirac Hamiltonian in an accelerating frame is computed in the low energy regime by exploiting the Foldy–Wouthuysen scheme. The NC θ-effect appears from the replacement of normal products and commutators by Moyal ?-products and ?-commutators. In particular, the commutator between the external magnetic vector potential and the potential induced by acceleration becomes non-trivial. Expressions for θ-corrected inertial spin current and conductivity are derived explicitly. We have provided yet another way of experimentally measuring θ. The θ bound is obtained from the out of plane spin polarization, which is experimentally observable.     

Spin and spin-valley Hall effects in a honeycomb lattice with antiferromagnetism and spin-orbit couplings

We study linear response to a longitudinal electric field on an antiferromagnetic honeycomb lattice with intrinsic and Rashba spin-orbit couplings (SOCs). It is found that the spin-valley Hall effect could emerge alone or coexist with the spin Hall effect. The spin and spin-valley Hall conductivities exhibit some peculiarities that depend on the distinct topological states of the graphene lattice. Furthermore, the spin and spin-valley Hall conductivities could be remarkably modulated by changing the Fermi level. Our findings suggest that the antiferromagnetic honeycomb lattice with SOCs is an excellent platform for potential applications of spintronics and valleytronics.     

光自旋霍尔效应及其研究进展

光束在经过非均匀介质后,自旋角动量相反(左、右旋圆偏振)的光子在垂直于入射面的横向相互分离,造成光束的自旋分裂,这种现象叫做光自旋霍尔效应.它类似于电子系统中的自旋霍尔效应:自旋光子扮演自旋电子的角色,而折射率梯度则起外场作用.光自旋霍尔效应为操控光子提供了新的途径,在纳米光学、量子信息和半导体物理方面具有重要的应用前景;同时由于它与凝聚态和高能物理中的带电粒子自旋霍尔效应有高度的相似性和共同的拓扑根源,所以又为测量自旋霍尔效应这类弱拓扑现象提供了独特而又方便的机会.文章简单介绍了光自旋霍尔效应,并总结了近几年国内外的研究进展.     

Electrically controllable spin transport in bilayer graphene with Rashba spin-orbit interaction

We study the spin transport in bilayer graphene nanoribbons (BGNs) in the presence of Rashba spin-orbit interaction (SOI) and external gate voltages. It is found that the spin polarization can be significantly enhanced by the interlayer asymmetry or longitudinal mirror asymmetry produced by external gate voltages. Rashba SOI alone in BGNs can only generate current with spin polarization along the in-plane y direction, but the polarization components can be found along the x, y and z directions when a gate voltage is applied. High spin polarization with flexible orientation is obtained in the proposed device. Our findings shed new light on the generation of highly spin-polarized current in BGNs without external magnetic fields, which could have useful applications in spintronics device design.     

Circularly polarized light stimulation of spin transport in zinc-blende semiconductors

An experimental and theoretical study on the optically stimulated spin transport in zinc-blende semiconductors is presented. The first part of the paper describes an experiment which investigates the effect of a longitudinal electric field on the spin-polarized carriers induced by a circularly polarized light. Since the photo-generated hole spins relaxation is extremely fast, the experiment observes only the effect resulting from spin-polarized electrons accumulating at the transverse edges of the sample, as a result of left-right asymmetries in scattering for spin-up and spin-down electrons in the presence of spin–orbit (SO) interaction. It is found that the effect depends on the longitudinal electric field and doping density as well as on temperature. The results are discussed. The second part of the paper deals with a theoretical investigation using norm-conserving pseudopotential and Green function formalism to analyse the SO mechanism responsible for the light-induced Hall voltage. The findings resulting from the investigation are discussed and are compared with experimental data.     

Direct comparison of anisotropic magnetoresistance and planar Hall effect in epitaxial Fe3O4 thin films

The angular dependence of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) were studied as a function of temperatures from the same epitaxial Fe₃O₄ film on MgO(001) substrates. The PHE contains only a twofold angular dependence, but the AMR below 200 K is constituted with both twofold and fourfold symmetric terms. Our results also prove that the origin of the fourfold symmetry of AMR is related to the lattice symmetry rather than the spin scattering near the antiphase boundaries.     

Temperature dependence of spin injection efficiency in an epitaxially grown Fe/GaAs hybrid structure

The electrical spin injection from Fe into an n-doped GaAs channel through Schottky-tunnel-barrier is observed from 1.8 K to room temperature. The magnitude of local spin valve signal (ΔR/R₀) decreases as the temperature increases. In each temperature, we calculated the injected polarization (η) considering the spin drift effect induced by the electric field. The interfacial polarizations of 19.3% and 12.6% are acquired for Fe/GaAs junction at T=1.8 and 300 K, respectively. The temperature dependence of spin injection efficiency is matched with interface resistance variation. As the temperature increases, Schottky-tunnel-barrier property is diminished so that the spin injection efficiency would be reduced.     

Spin polarized carrier injection driven magneto-optical Kerr effect in Cr-doped ZnO nanorods

Hydrothermal growth of Cr doped ZnO nanorods (NRs) thin films was grown on glass substrates. The strong ferromagnetic (FM) and antiferromagnetic (AFM) ordering was observed in 3% and 7% of Cr doped samples. The optical excitation of the Cr doped ZnO NRs is responsible for injection of the spin-polarized carriers in the 3% Cr doped ZnO NRs through longitudinal magneto-optical Kerr effect (MOKE). We determined a negative anisotropic magnetoresistance ($～23\text{\%}$) originated from spin orbit coupling due to sp-d exchange interaction. we calculated the process of photon induced inverse spin Hall angle (${\theta }_{\mathrm{ISHE}}～3.94\times {10}^{?2}$) close to the MOKE saturated rotation angle (${\theta }_k～0.046$) for 3% Cr: ZnO. These results can open a new path of optical spin detectors for next-generation spintronic device technology.