本文报道了在磁性/半导体拓扑绝缘体（TI）异质结构Cr $_x$（Bi $_{1-y}$ Sb $_y$）$_{2-x}$ Te $_3$ /（Bi $_{1-y}$ Sb $_y$）$_2$ Te $_3$中的电流方向相关或单向磁电阻（UMR），它比其它已报道的系统大几个数量级。从磁场和温度的依赖关系，UMR被确定为起源于电子的磁振子的非对称散射。特别是，UMR的大幅度是自旋动量锁定和小费米波数在TI表面的结果。事实上，UMR在狄拉克点附近最大，费米波数最小。
英文摘要：
We report current-direction dependent or unidirectional magnetoresistance (UMR) in magnetic/nonmagnetic topological insulator (TI) heterostructures, Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$, that is several orders of magnitude larger than in other reported systems. From the magnetic field and temperature dependence, the UMR is identified to originate from the asymmetric scattering of electrons by magnons. In particular, the large magnitude of UMR is an outcome of spin-momentum locking and a small Fermi wavenumber at the surface of TI. In fact, the UMR is maximized around the Dirac point with the minimal Fermi wavenumber.
Many Heusler compounds are predicted to be ferromagnetic half metals in the bulk, which makes them promising compounds for spintronics. However, for devices the transport spin polarization at specific interfaces requires optimization. We show that investigations of the unidirectional magnetoresistance provide an alternative approach to access this quantity. Based on a Wheatstone-bridge design we probed the unidirectional magnetoresistance of Co2MnSi/(Ag, Cu, or Cr)(0.5 nm)/Pt (or Ta) multilayers and separate the spin-dependent unidirectional spin Hall magnetoresistance from other contributions…
Relating magnetotransport properties to specific spin textures at surfaces or interfaces is an intense field of research nowadays. Here, we investigate the variation of the electrical resistance of Ge(111) grown epitaxially on semi-insulating Si(111) under the application of an external magnetic field. We find a magnetoresistance term which is linear in current density j and magnetic field B, hence odd in j and B, corresponding to a unidirectional magnetoresistance. At 15 K, for I = 10 $μ$A (or j = 0.33 A/m) and B = 1 T, it represents 0.5 % of the zero field resistance, a much higher value com…
Twisted bilayer graphene (TBG) exhibits fascinating correlation-driven phenomena like the superconductivity and Mott insulating state, with flat bands and a chiral lattice structure. We find by quantum transport calculations that the chirality leads to a giant unidirectional magnetoresistance (UMR) in TBG, where the unidirectionality refers to the resistance change under the reversal of the direction of the current or magnetic field. We point out that flat bands significantly enhance this effect. The UMR increases quickly upon reducing the twist angle and reaches about 20\% for an angle of 1.5…
Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-heusler crystal. Besides the angular dependence on magnetization, the competition of Rashba and Dresselhaus-like spin-orbit couplings results in the dependence of these effect…
We present a detailed analysis of harmonic longitudinal and Hall voltage measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs) and unidirectional spin Hall magnetoresistance (USMR) is achieved by introducing the second heavy metal (HM) with the opposite sign of the spin Hall angle. The extracted SOT efficiencies are larger for the trilayers as compared to the bilayers, confirming the enhanced values reported for the trilayers with perpendicularly magnetized Co. The maximum effective spin Hall angl…
We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co …
The interplay between electronic transport and antiferromagnetic order has attracted a surge of interest as recent studies have shown that a moderate change in the spin orientation of a collinear antiferromagnet may have a significant effect on the electronic band structure. Among numerous electrical probes to read out such magnetic order, unidirectional magnetoresistance (UMR), where the resistance changes under the reversal of the current direction, can provide rich insights into the transport properties of spin-orbit coupled systems. However, UMR has never been observed in antiferromagnets …
Transverse acoustic waves in an inhomogeneous medium are analogues to electromagnetic waves and will exhibit topological behavior due to the Berry gauge potential in the momentum space due to inhomogeneity. The inhomogeneous (or gradient) medium can be created using an applied strain gradient in a semiconductor thin film (silicon) since the phonon frequency and dispersion will be a function of the local strain along the strain gradient direction. As a consequence, topological phonon mediated spin and heat transport can be engineered in the semiconductor thin films. Here, we present evidence of…
The unidirectional magnetoresistance (UMR) is one of the most complex spin-dependent transport phenomena in ferromagnet/non-magnet bilayers, which involves spin injection and accumulation due to the spin Hall effect (SHE) or Rashba-Edelstein effect (REE), spin-dependent scattering, and magnon scattering at the interface or in the bulk of the ferromagnet. While UMR in metallic bilayers has been studied extensively in very recent years, its magnitude is as small as 10$^-$$^5$, which is too small for practical applications. Here, we demonstrate a giant UMR effect in a heterostructure of BiSb topo…
Unidirectional magnetoresistance driven by nonequilibrium antiferromagnetic magnons
South China University of Technology（华南理工大学）；Norwegian University of Science and Technology（挪威科技大学）；Peking University（北京大学）；Nanyang Technological University（南洋理工大学）
The structure inversion asymmetry at surfaces and interfaces give rise to the Rashba spin-orbit interaction (SOI), that breaks the spin degeneracy of surface or interface states. Hence, when an electric current runs through a surface or interface, this Rashba effect generates an effective magnetic field acting on the electron spin. This provides an additional tool to manipulate the spin state in materials such as Si and Ge that, in their bulk form, possess inversion symmetry (or lack structural inersion asymmetry). The existence of Rashba states could be demonstrated by photoemission spectrosc…
Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi2Te3 topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches -22% at 8T. The interplay between negative and positive magnetoresistance can be understood in terms of the compet…
We report a metal-insulator like transition in single crystalline 3D topological insulator Bi2Te3 at a temperature of 230K in presence of an external magnetic field applied normal to the surface. This transition becomes more prominent at larger magnetic field strength with the residual resistance value increasing linearly with the magnetic field. At low temperature, the magnetic field dependence of the magnetoresistance shows a transition from logarithmic to linear behavior and the onset magnetic field value for this transition decreases with increasing temperature. The logarithmic magnetoresi…
Large unsaturated magnetoresistance (XMR) with magnitude about 1000% is observed in topological insulator candidate TaSe3 from our high field (up to 38 T) measurements. Two oscillation modes, associated with one hole pocket and two electron pockets in the bulk, respectively, are detected from our Shubnikov-de Hass (SdH) measurements, consistent with our first-principles calculations. With the detailed Hall measurements performed, our two-band model analysis exhibits an imperfect density ratio n_h/n_e closing 0.9 at T< 20 K , which suggests that the carrier compensations account for the XMR in …
We study the unidirectional magnetoresistance (UMR) and the nonlinear Hall effect (NLHE) in the ferromagnetic Rashba model. For this purpose we derive expressions to describe the response of the electric current quadratic in the applied electric field. We compare two different formalisms, namely the standard Keldysh nonequilibrium formalism and the Moyal-Keldysh formalism, to derive the nonlinear conductivities of UMR and NLHE. We find that both formalisms lead to identical numerical results when applied to the ferromagnetic Rashba model. The UMR and the NLHE nonlinear conductivities tend to b…
Spin-orbit effects appearing in topological insulators (TI) and at Rashba interfaces are currently revolutionizing how we can manipulate spins and have led to several newly discovered effects, from spin-charge interconversion and spin-orbit torques to novel magnetoresistance phenomena. In particular, a puzzling magnetoresistance has been evidenced, bilinear in electric and magnetic fields. Here, we report the observation of bilinear magnetoresistance (BMR) in strained HgTe, a prototypical TI. We show that both the amplitude and sign of this BMR can be tuned by controlling, with an electric gat…
We demonstrate the existence of Giant proximity magnetoresistance (PMR) effect in a graphene spin valve where spin polarization is induced by a nearby magnetic insulator. PMR calculations were performed for yttrium iron garnet (YIG), cobalt ferrite (CFO), and two europium chalcogenides EuO and EuS. We find a significant PMR (up to 100%) values defined as a relative change of graphene conductance with respect to parallel and antiparallel alignment of two proximity induced magnetic regions within graphene. Namely, for high Curie temperature (Tc) CFO and YIG insulators which are particularly impo…
Observation of spin-orbit magnetoresistance in metallic thin films on magnetic insulators
由Rashba自旋轨道相互作用引起的磁阻效应被预测，但尚未观察到，在由正常金属和铁磁绝缘体组成的双层膜。在这里，我们提出了一个实验观察这种新型的自旋-轨道磁电阻（SOMR）效应的双层结构Cu[Pt]/Y3 Fe 5 O 12（YIG），其中Cu/YIG界面装饰有纳米尺寸的Pt岛。这种新的MR显然不是由体自旋-轨道相互作用引起的，因为Cu中的自旋-轨道相互作用可以忽略，Pt岛的不连续性。当Pt岛不存在或远离Cu/YIG界面时，SOMR消失，因此我们可以明确地将其归因于Pt装饰增强的界面处的Rashba自旋轨道相互作用。数值模拟结果与SOMR实验结果在磁场的角度依赖性和Cu厚度依赖性方面一致。我们的研究结果表明，实现自旋操纵的接口工程。
The recently discovered antiferromagnetic topological insulator MnBi$_2$Te$_4$ hosts a variety of exotic topological quantum phases such as the axion insulator and Chern insulator states. Here we report systematic gate voltage dependent magneto transport studies in six septuple-layer MnBi$_2$Te$_4$. In p-type carrier regime, we observe positive linear magnetoresistance when MnBi$_2$Te$_4$ is polarized in the ferromagnetic state by an out-of-plane magnetic field. Whereas in n-type regime, distinct negative magnetoresistance behaviors are observed. The magnetoresistance in both regimes is highly…
We observe an unusual behavior of the spin Hall magnetoresistance (SMR) measured in a Pt ultra-thin film deposited on a ferromagnetic insulator, which is a tensile-strained LaCoO3 (LCO) thin film with the Curie temperature Tc=85K. The SMR displays a strong magnetic-field dependence below Tc, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across Tc and remains measurable even above Tc. Moreover, no hysteresis is observed in the field dependen…
某些材料和多层具有依赖于磁场的电阻（称为磁阻）的倾向，已经发现了商业应用，例如巨磁阻硬盘读取头。但磁阻也可以是物质中电子和磁性相互作用的有力探测器。例如，磁阻可以用来分析多带电导率、传导不均匀性、局部磁矩和（分数）朗道能级结构。对于具有强自旋轨道相互作用的材料，磁电阻可以用作弱反定域或非平凡Berry相的探针，例如在拓扑绝缘体表面态中。对于三维拓扑绝缘体，大的线性磁阻通常被用作潜在的非平凡拓扑结构的指示，尽管这种效应的起源尚未确定。在这里，我们观察到一个大的磁电阻在导电体状态的Bi $_2$ Te $_3$。我们发现，这种类型的大的磁阻是由于螺旋自旋动量锁定（即自旋旋转与动量方向）和单向自旋排列之间的竞争，通过施加磁场。翘曲效应被发现提供（准）线性依赖于磁场。我们提供了一个定量模型的螺旋度破缺诱导的磁阻，可以应用到广泛的材料，表面或界面弱到强的自旋轨道相互作用，如当代氧化物界面，散装Rashba系统，和拓扑绝缘体表面状态。
This article aims at giving a general presentation of spintronics, an important field of research developing today along many new directions in physics of condensed matter. We tried to present simply the physical phenomena involved in spintronics -- no equations but many schematics. We also described the applications of spintronics, those of today and those expected to have an important impact on the next developments of the information and communication technologies.
Additional Evidence for the Surface Origin of the Peculiar Angular-Dependent Magnetoresistance Oscillations Discovered in a Topological Insulator Bi_{1-x}Sb_{x}
Current induced anisotropic magnetoresistance in topological insulator films
拓扑绝缘体在本体中是绝缘的，但具有由时间反转对称性保护的自旋动量锁定的金属表面状态。这些表面态的存在已被角分辨光电子能谱（ARPES）和扫描隧道显微镜（STM）证实。通过输运测量检测这些表面状态，这可能在第一次似乎是最直接的途径，被证明比预期的更具挑战性。在这里，我们报告了一个详细的电子输运研究高品质的Bi 2 Se 3拓扑绝缘体薄膜。在面内磁场、沿偏置电流方向和垂直于偏置电流方向上的测量结果显示了相反的磁阻效应。我们认为，这种对比的行为是有关的自旋和电流方向的锁定提供证据的螺旋自旋结构的拓扑表面状态。
The abstract notion of topology has led to profound insights into real materials. Notably, the surface and edges of topological materials can host physics, such as unidirectional charge or spin transport, that is unavailable in isolated one- and two-dimensional systems. However, to fully control the mixing and interference of edge-state wave functions, one needs robust and tunable junctions. We propose to achieve this control using an antiferromagnetic topological insulator that supports two distinct types of gapless unidirectional channels on its surface, one from antiferromagnetic domain wal…
Through a thorough magneto-transport study of antiferromagnetic topological insulator MnBi2Te4 (MBT) thick films, a positive linear magnetoresistance (LMR) with a two-dimensional (2D) character is found in high perpendicular magnetic fields and temperatures up to at least 260 K. The nonlinear Hall effect further reveals the existence of high-mobility surface states in addition to the bulk states in MBT. We ascribe the 2D LMR to the high-mobility surface states of MBT, thus unveiling a transport signature of surface states in thick MBT films. A suppression of LMR near the Neel temperature of MB…
Possible origin of extremely large magnetoresistance in the topological insulator CaBi2 single crystal
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；University of Chinese Academy of Sciences（中国科学院大学）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Songshan Lake Materials Laboratory（松山湖材料实验室）
Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure
Antiferromagnetic (AFM) spintronics, where magneto-transport is governed by an antiferromagnet instead of a ferromagnet, opens fascinating new perspectives for both fundamental research and device technology, owing to their intrinsic appealing properties like rigidness to magnetic field, absence of stray field, and ultrafast spin dynamics. One of the urgent challenges, hindering the realization of the full potential of AFM spintronics, has been the performance gap between AFM metals and insulators. Here, we demonstrate the insulator-metal transition and persistently large anisotropic magnetore…
Asymmetric magnetoresistance in 3$d$-5$d$ based La$_{0.67}$Ca$_{0.33}$MnO$_3$/SrIrO$_3$ bilayer: The role of unidirectional anisotropy
Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学）；Indian Institute of Technology, Hauz Khaz（印度理工学院豪兹卡兹校区）；Special Centre for Nanoscience, Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学纳米科学特别中心）；UGC-DAE Consortium for Scientific Research, Indore（印度古物与考古学联合会-原子能部科学研究中心）
