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Connecting Fermi-surface topology andspin-orbit torquesin Weyl Semimetal/Ferromagnet Heterostructures

Amilcar Bedoya-Pinto

Weyl Semimetals(WSMs), materials with three-dimensional topologically protected electronic states, show highly interesting physical properties including surface Fermi-arcs, the chiral magneto-transport anomalyand extremely high electron mobilities. Still, its potential for device applications needs to be exploitedthrough the preparation of thin films, which would enable the design of functional heterostructures. Onepromising application field of WSMs is spin-orbitronics, as the Fermi-surface is expected to play an importantrole in the spin-to-charge conversion efficiency, according to theoretical investigations[1,2]. In this work, we report the growth of epitaxial, single-crystallineNbP and TaP Weyl Semimetal thin films[3]by means of molecular beam epitaxy, and their successful integration in spin-torquedevices. We have assessed the structural quality of the films(Fig.1a)featuring an atomically flat, ordered surface, essential for the observation of topological bandsby photoemission (Fig. 1b). Furthermore, werely on the preparation of high-quality in-situ TaP/Permalloy interfacesto investigatethe spin-orbit torques produced by the topological WSM by means of spin-torque ferromagnetic resonance (ST-FMR). First resultsof TaP/Py/MgO device structures at room temperature show readily signatures of largespin-orbit torques induced by the Weyl Semimetal: (i) a very strong symmetric component of the voltagelineshape across the resonancerelated to damping-like torques(Fig.1c), much different than the FMR responseof a reference ferromagnet, and(ii) a clear scalingof the resonance linewidth by applying an external DC bias through the bilayer(Fig. 1d). The connectionbetween Fermi-surface topology and spin-to-charge conversionis addressedby performing angle-resolved photoemission measurements on the TaP thin film surfaces prior to the in-situ depositionof the magnetic layers, and probing the spin-torque efficiencyalong the high-symmetry directions of theWSM, where the surface states areexpected to have asubstantialcontribution

[1] Sun, Y, et.al. Strong Intrinsic SpinHall Effect in the TaAs Family of Weyl Semimetals. Phys. Rev. Lett. 117, 146403 (2016)
[2] Johannson, A. et.al. Edelstein effect in Weyl semimetals. Phys. Rev. B 97, 085417 (2018)
[3] Bedoya-Pinto,A.et.al. Realization of NbP and TaP Weyl Semimetal Thin Films, ACS Nano, 14, 4, 4405 (2020)

Nonlocal detection of out-of-plane magnetization in a magnetic insulator by thermal spin drag

Can Onur Avci

Perpendicularly magnetized ferrimagnetic insulators (FMI) have been drawing increasing attention in spintronics research. Recent achievements of efficient current-induced switching and domain wall motion [1-5] in perpendicular FMIs, combined with theirhighly tunable propertiesopen novel avenues for practical applications. Despite theFMIs’electrically insulating nature, the spin Hall magnetoresistance (together with its anomalous Hall component) [6] provides us with the relevant tools to detect theirmagnetization vector ina local geometry using a Hall bar device. Later on, it was discovered that thein-plane magnetization vectorofan FMIcan be detectedalsoin a nonlocal geometry by long distance magnon transport[7,8]. However, the detection of the perpendicular magnetization vector of an FMI in a nonlocal geometry remained a challenge for a long time. In this work[9],wedemonstratethat,by usingan engineered temperature gradient,one can detect theout-of-plane magnetization of anFMIby simply measuring the transverse voltage drop across the Ptstrip placed on top. This is due to a conceptually new mechanism that combines the spin currents driven by an out-of-plane(∇#$)and in-plane(∇%$)temperature gradientsin a Pt/FMI bilayer generated by a single nonlocal heat source. When the magnetization has a component oriented perpendicular tothe plane, ∇#$drives a spin current into Pt with out-of-plane polarization due to the spin Seebeck effect. ∇%$then drags the resulting spin-polarized electrons in Pt parallel to the plane against the gradient direction. This finally produces an inverse spin Hall effect voltage in Pt, transverse to ∇%$and proportional to the out-of-plane component of theFMI’smagnetization(see Fig.1). This simple method enables the detection of the perpendicular magnetization component in anFMIin a nonlocal geometryand opens up new routes towardsengineeringtemperature gradients togenerateandmanipulate thermal magnons and pure spin currents.

[1]Avci et al.Nat. Mater.16, 309 (2017)
[2]Shao et al.Nat. Commun. 9, 3612 (2018)
[3]Avci etal.Nat. Nanotech. 14, 561 (2019)
[4]Velez et al.Nat. Commun. 10, 4750 (2019)
[5]Ding et al.PRB 100, 100406(R)(2019)
[6]Nakayama et al.PRL 110, 206601 (2013)
[7]Cornelissen et al.Nat. Phys. 11, 1022 (2015)
[8]Goennenwein etal.107, 172405 (2015)
[9]Avci et al.PRL124, 027701 (2020)

Electrical Spin Current Generation in Ferromagnets

Vivek Amin

Ferromagnets generate spin currents under an applied electric field. For example, charge currents in ferromagnets are spin-polarized because majority and minority carriers have different conductivities. However, in the presence of spin-orbit coupling, electrons can carry a substantial spin current flowing perpendicularly to the electric field with spin directions both longitudinal and transverse to the magnetization.
In this talk, we discuss several mechanisms to electrically generate spin currents in ferromagnets. These mechanisms are closely related to the anomalous and planar Hall effects but yield spin currents with spin directions transverse to the magnetization. Such spin currents can be detected through the torques they exert at layer boundaries [1]. We present first principles transport calculations giving the strength and magnetization dependence of the electrically generated spin currents allowed by symmetry via both intrinsic [2] and extrinsic [3] mechanisms. We find that in transition metal ferromagnets, the spin currents with spin direction transverse to the magnetization can have an associated conductivity comparable to the spin Hall conductivity in Pt.

[1] W. Wang, T. Wang, V. P. Amin, Y. Wang, A. Radhakrishnan, A. Davidson, S. R. Allen, T. J. Silva, H. Ohldag, D. Balzar, B. L. Zink, P. M. Haney, J. Q. Xiao, D. G. Cahill, V. O. Lorenz, and X. Fan, Anomalous Spin-Orbit Torques in Magnetic Single-Layer Films, Nature Nanotechnology, 14, 819-824, 2019
[2] V. P. Amin, J. Li, M. D. Stiles, and P. M. Haney, Intrinsic Spin Currents in Ferromagnets, Phys. Rev. B 99, 220405(R), 2019
[3] V. P. Amin, J. Zemen, and M. D. Stiles, Interface generated spin currents, Phys. Rev. Lett. 121, 136805 (2018)

Magneto-Raman Spectroscopy to Identify Spin Structure in Low-Dimensional Quantum Materials

Angela R. Hight Walker

Raman spectroscopy, imaging, and mapping are powerful non-contact, non-destructive optical probes of fundamental physics in graphene and other related two-dimensional (2D) materials, including layered, quantum materials that are candidates for use in the next quantum revolution. An amazing amount of information can be quantified from the Raman spectra, including layer thickness, disorder, edge and grain boundaries, doping, strain, thermal conductivity, magnetic ordering, and unique excitations such as charge density waves. Most interestingly for quantum materials is that Raman efficiently probes the evolution of the electronic structure and the electron-phonon, spin-phonon, and magnon-phonon interactions as a function of temperature, laser energy, and polarization. Our unique magneto-Raman spectroscopic capabilities will be detailed, enabling diffraction-limited, spatially-resolved Raman measurements while simultaneously varying the temperature (1.6 K to 400 K), laser wavelength (tunability from visible to near infrared), and magnetic field (up to 9 T) to study the photo-physics of nanomaterials. Additionally, coupling to a triple grating spectrometer provides access to low-frequency (down to 6 cm-1, or 0.75 meV) phonon and magnon modes, which are sensitive to coupling. By utilizing electrical feedthroughs, studying the strain-dependent effects on magnetic materials utilizing MEMs devices is also a novel opportunity. Current results on intriguing quantum materials will be presented to highlight our capabilities and research directions. One example leverages the Raman spectra from α-RuCl3 to probe this Kitaev magnet and possible quantum spin liquid1. Within a single layer, the honeycomb lattice exhibits a small distortion, reducing the symmetry from hexagonal to orthorhombic. We utilize polarization-dependent Raman spectroscopy to study this distortion, including polarizations both parallel and perpendicular to the c-axis. Coupling of the phonons to a continuum is also investigated. Using Raman spectroscopy to probe magnetic phenomena in the antiferromagnetic metal phosphorus trichalcogenide family2, we highlight FePS3 and MnPSe3. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that in FePS3 one of the Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of ≈3.7 THz (122 cm−1). In addition, the surprising symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3. Using resonant Raman scattering, we studied the Neel-type antiferromagnet MnPSe3 through its ordering temperature and also as a function of applied external magnetic field. Surprisingly, the previously assigned one-magnon scattering peak showed no change in frequency with an increasing in-plane magnetic field. Instead, its temperature dependence revealed a more surprising story. Combined with first-principle calculations, the potential origin of this Raman scattering will be discussed.

Finally, the magnetic field- and temperature-dependence of an exciting ferromagnetic 2D material, CrI3, will be presented3. We report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two new features in the spectra which appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3.

[1] PHYSICAL REVIEW B 100, 134419 (2019)
[2] PHYSICAL REVIEW B 101, 064416 (2020)
[3] https://arxiv.org/abs/1910.01237 (in press @Nature Comm)

Observation of magnetic skyrmions and their current-driven dynamics in van der Waals heterostructures

Seonghoon Woo

 

Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing properties and integration with other 2D family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy (PMA). Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). In this talk, we present the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe3GeTe2 (FGT). We demonstrate the ability to drive individual skyrmion by short current pulses along a vdW heterostructure, FGT/h-BN, as highly required for any skyrmion-based spintronic device. Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states. Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application.

Spin galvanic effects and magnetization dynamics in layered vdW systems

Simran Singh

The van der Waals (vdW) based layered materials and their heterostructures are a modular platform to probe spin related phenomena such as spin-charge interconversion, spin-orbit torques and magnetization dynamics. I will discuss the spin-charge interconversion in vdW bonded few-layer graphene/Platinum (Gra/Pt) heterostructures where we observe a large spin-charge interconversion signal. The spin Hall effect (SHE) in Pt and spin diffusion in graphene layers cannot explain the large spin-charge interconversion signals observed in our heterostructures. This indicates that a mechanism of spin-to-charge conversion other than the SHE (ISHE) is dominant in our devices. Based on previous photoemission studies on the Pt/Gr interface, it is plausible to ascribe the spin-charge interconversion observed in these Pt/Gr interfaces to the Rashba effect.

The layered Weyl semimetal candidate, WTe2, is predicted to host large spin-galvanic effects which can be used to efficiently manipulate the magnetization state of a magnetic system. I will discuss our recent results showing an efficient magnetization switching of a layered ferromagnet system driven by the charge current induced spin currents in WTe2. Time permitting, I will also briefly discuss low temperature broadband magnetization dynamics studies of a layered anti-ferromagnetic system which are aimed at probing the magnetic energy landscape of the layered magnetic systems.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 12.08.2020 - 15:00 (CET)

Using magnetic tunnel junctions to compute like the brain

Mark Stiles, NIST

Computers, originally designed to do precise numerical processing, are now widely used to do more cognitive tasks. These include categorical challenges like image and voice recognition, as well as robotic tasks like driving a car and making real-time decisions based on sensory input. While the human brain does not do precise numerical processing well, it excels at these other tasks, leading researchers to look to the brain for inspiration on efficient ways to engineer cognitive computers. Of particular interest are energy and space optimization. Computers can now perform many of these cognitive tasks as well as humans, and often faster, but at the cost of much higher total energy consumption and much greater space. Some improvements are being found at the top of the computational stack from algorithms that are more brainlike, and some at the bottom from novel electronic devices that emulate features of the brain. However, the greatest progress can be found by working simultaneously across the computational stack.

Magnetic tunnel junctions have several features that make them attractive potential devices for these applications. One feature is that they are already integrated into fabrication plants for complementary-metal-oxide-semiconductor (CMOS) integrated circuits. They can be readily integrated with existing CMOS technology to take advantage of its many capabilities. Another feature is that they are multifunctional. With only slight changes in fabrication details, they can be modified to provide non-volatile memory, truly random thermal fluctuations, or gigahertz oscillations. Magnetic tunnel junctions can be used as a memory to store synaptic weights, but when the weights change too frequently the energy cost of repeatedly writing them becomes inefficient. Reducing the retention time of the memory reduces the cost of writing them, leading to a trade-off between energy efficiency and reliability. The seemingly random patterns of neural spike trains have inspired a number of computational approaches based on the random thermal fluctuations of superparamagnetic tunnel junctions. I discuss some of these approaches and the design choices we have made in implementing a neural network based on superparamagnetic tunnel junctions.

PDF file of the talk available here

Crystal time-reversal symmetry breaking and spin splitting in collinear antiferromagnets

Libor Šmejkal

Relativistic bandstructure of solids generates functionalities of modern quantum, topological and spintronics materials1. Common collinear antiferromagnets exhibit Kramers spin degenerate bands2 and for many decades were believed to be excluded from spin splitting physics. Our recent prediction of crystal time-reversal symmetry breaking by anisotropic magnetization densities due to the collinear antiferromagnetism combined with nonmagnetic atoms3 changes this perspective. Unlike the conventional spin-orbit interaction induced spin splitting, our antiferromagnetic spin splitting is of exchange origin, can reach giant eV values, and can preserve spin quantum number.
In this talk, we will discuss the basic properties of this new type of antiferromagnetic spin splitting, its local magnetic symmetry origin and symmetry criteria for its emergence and we will catalogue broad class of material candidates. Furthermore, we will show that this antiferromagnetic spin splitting generates a crystal Hall effect controllable via rearrangement of nonmagnetic atoms3. Finally, we will present an experimental discovery of crystal Hall effect in ruthenium dioxide antiferromagnet4.

[1] Šmejkal, L., Mokrousov, Y., Yan, B. & MacDonald, A. H. Topological antiferromagnetic spintronics. Nat. Phys. 14, 242 (2018).
[2] Šmejkal, L., Železný, J., Sinova, J. & Jungwirth, T. Electric Control of Dirac Quasiparticles by Spin-Orbit Torque in an Antiferromagnet. Phys. Rev. Lett. 118, 106402 (2017), arXiv (2016)
[3] Šmejkal, L., González-Hernández, R., Jungwirth, T. & Sinova, J. Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets. Sci. Adv. 6, eaaz8809 (2020), arXiv (2019)
[4] Feng, Z., Zhou, X., Šmejkal, L. et al. Observation of the Crystal Hall Effect in a Collinear Antiferromagnet. arXiv (2020)

Optical spectroscopy of 2-dimensional antiferromagnetic materials

Hyeonsik Cheong

Magnetism in low dimensional systems is a fascinating topic for the fundamental physics as well as for possible applications in future spintronic devices. Although ferromagnetic 2-dimensional (2D) materials are attracting the most interest, antiferromagnetic 2D materials are equally interesting for the rich physics they reveal. However, antiferromagnetic ordering is much more difficult to investigate because the lack of net magnetization hinders easy detection of antiferromagnetic ordering. Neutron scattering, which is a powerful tool to detect antiferromagnetic order in bulk materials, cannot be used for atomically thin samples due to the small sample volume. Raman spectroscopy has proven to be a powerful tool to detect antiferromagnetic ordering by monitoring magnetically induced changes in the Raman spectrum. In this talk, I will review recent achievements in the study of antiferromagnetism in 2 dimensions using Raman spectroscopy. FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for 2-dimensional order-disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering. On the other hand, NiPS3, which shows an XXZ-type antiferromagnetic ordering in bulk, exhibits antiferromagnetic ordering down to 2 layers with a slight decrease in the transition temperature, but the magnetic ordering is suppressed in the monolayer limit. A Heisenberg-type antiferromagnet MnPS3 also exhibits ordering down to 2 layers with a small decrease in the transition temperature. Furthermore, a recent discovery of a peculiar excitonic transition that exhibit a dramatic decrease of the linewidth below the transition temperature will be reported.

Spin-orbit torques based on topological spin texture and magnon

Hyunsoo Yang

Layered topological materials such as topological insulators (TIs) and Weyl semimetals are a new class of quantum matters with large spin-orbit coupling, and probing the spin texture of these materials is of importance for functional devices. We reveal spin textures of such materials using the bilinear magneto-electric resistance (BMR), which depends on the relative orientation of the current with respect to crystallographic axes [1,2]. We also visualize current-induced spin accumulation in topological insulators using photocurrent mapping [3]. Topological surface states (TSS) dominated spin orbit torques are identified in Bi2Se3 [4], and magnetization switching at room temperature using Bi2Se3 as a spin current source is demonstrated [5]. Nevertheless, the resistive nature of TIs can cause serious current shunting issues, leading to a large power consumption. In order to tackle this issue, we propose two approaches.
Weyl semimetals have a larger conductivity compared to TIs and they can generate a strong spin current from their bulk states. The Td-phase Weyl semimetal WTe2 can be produced with high quality, simplifying interfacial studies and facilitating device applications. Utilizing the magneto-optical Kerr microscopy, we show the current-driven magnetization switching in WTe2/NiFe with a low current density and a low power [6].
The current shunting issue can be also overcome by the magnon-mediated spin torque, in which the angular momentum is carried by precessing spins rather than moving electrons. Magnon-torque-driven magnetization switching is demonstrated in the Bi2Se3/NiO/Py devices at room temperature [7]. By injecting the electric current to an adjacent Bi2Se3 layer, spin currents were converted to magnon torques through an antiferromagnetic insulator NiO. The presence of magnon torque is evident for larger values of the NiO-thickness where magnons are the only spin-angular-momentum carriers. The demonstration reveals that the magnon torque is sufficient to control the magnetization, which is comparable with previously observed electrical spin torque ratios of TIs [5].
Looking towards the future, we hope that these studies will spark more works on harnessing spin currents from topological materials and revealing interesting spin textures at topological material/magnet interfaces. All magnon-driven magnetization switching without involving electrical parts could be achieved in the near future. The results will invigorate magnon-based memory and logic devices, which is relevant to the energy-efficient control of spin devices.

[1] P. He et al., Nat. Phys. 14, 495 (2018)
[2] P. He et al., Nat. Comm. 10, 1290 (2019)
[3] Y. Liu et al., Nat. Comm. 9, 2492 (2018)
[4] Y. Wang et al., Phys. Rev. Lett. 114, 257202 (2015)
[5] Y. Wang et al., Nat. Comm. 8, 1364 (2017)
[6] S. Shi et al., Nat. Nano. 14, 945 (2019)
[7] Y. Wang et al., Science 366, 1125 (2019)