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The proximity effect in two-dimensional materials opens ways to achieve important functions for future spintronic devices. In van der Waals heterostructures (vdWHs), transition metal dichalcogenides (TMD) can be used to enhance the spin-orbit coupling of graphene leading to highly efficient spin-to-charge conversion (SCC) by spin Hall effect (SHE) that is predicted to be controllable by a gate voltage. Here, we report for the first time the observation of the SHE in graphene proximitized with WSe2 in a vdWHs. These kind of vdWHs are a promising platform to study a variety of spin-dependent phenomena [1,2]. By Hanle precession measurements, we quantify the spin transport and SCC parameters from 10 K up to room temperature. Exceptional for graphene/TMD devices, the sole mechanism is the SHE for all measurements and no Rashba-Edelstein effect is observable. Importantly, we are able to amplify and turn off the SCC by applying a back-gate voltage, demonstrating the long-awaited milestone of an electrically-tunable SHE. The amplified SCC shows a high efficiency, measured with an unprecedented SCC length of larger than 20 nm, defined as the product of the spin Hall angle (ΘSH) and the spin diffusion length (λs). These results show the capability of two-dimensional materials to advance towards the implementation of novel spin-based devices and future applications.References: [1] W. Yan et al., Nat. Commun. 7, 13372 (2016). [2] C. K. Safeer et al., Nano Lett. 19, 1074–1082 (2019).

MMM 2020

Proximity-Induced Spin Hall Effect in Graphene/WSe2 van der Waals Heterostructures with Tunable, Highly Efficient Spin-to-Charge Conversion

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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technical paper

Even if magnetic van der Waals crystals where known since decades, two-dimensional (2D) magnetism has been obtained experimentally only a few years ago[1]. Since its discovery in Cr2Ge2Te6 and CrI3, an intense research effort has started, triggered by their high potential in studying 2D magnetic states[2-4]. A wide range of new physical properties emerging from the reduced dimensionality can be explored and exploited like the control of magnetic properties through gating. New devices based on van der Waals magnetic heterostructures have been proposed, such as spin-filter magnetic tunnel junctions based on CrI3 heterostructures, showing a tunneling magnetoresistance higher than their counterparts based on conventionally grown magnetic thin films. The potential of 2D magnets in topological spintronics is also under investigation, with for example recent works on the stabilization of skyrmions in Fe3GeTe2. However, in order to use van der Waals magnets as building blocks for relevant spintronics devices, a magnetic material with a Curie temperature (TC) above room temperature is required. Unfortunately, most of the available van der Waals magnets have a low intrinsic TC. Even though a 2D magnetic state at room temperature can be achieved through gating for example, it remains a challenging task and only few materials are reported to have a TC above 300K down to the monolayer limit, like MnSex, VSe2 and more recently for few-layers CrTe2[5]. Here, we report our work on CrTe2 from bulk to 40nm thin mechanically exfoliated flakes using Nitrogen-Vacancy (NV) magnetometry, a well-suited technique for the study of 2D magnets[6]. We find an in-plane ferromagnetic order at room temperature and we extract a value for the saturation magnetization Ms of 24.5 ± 3.0 kAm-1 at room temperature.References: [1] N. Samarth, Nature 546, 216 (2017) [2] K. S. Burchet al., Nature 563, 47 (2018) [3] X. Z. Cheng Gong, Science 363, 6428 (2019) [4] M. Gibertini et al., Nature Nanotechnology 14, 408 (2019) [5] A. Purbawati et al., ACS Appl. Mater. Interfaces (2020) [6] L. Thiel et al., Science 364, 973 (2019)

Room-temperature ferromagnetism in CrTe2 thin flakes imaged by single spin magnetometry

The discovery of intrinsic magnetism in 2D materials has opened an exciting new platform for spintronics, allowing fundamental studies of efficient mechanisms for electrically controlling magnetic materials. In this work we show that the magnetization orientation of the semiconducting vdW magnet Cr2Ge2Te6 (CGT) can be detected electrically and can be manipulated efficiently by current-induced spin-orbit torques from heavy-metals like Pt and Ta. We provide a direct quantitative measurement of these torques in CGT/Pt. Our results indicate that the interfacial spin-current transparency in these devices is similar to metallic-ferromagnet/Pt samples but is higher than in oxide-ferromagnet/Pt structures and that the interlayer exchange coupling in multilayer CGT is strong enough so that all layers reorient in response to the spin-orbit torque. We also characterize strong magnetothermal signals arising from the interplay between thermal gradients and magnon spin currents in these devices, as detected by the spin-Seebeck effect. These results demonstrate the promise of semiconducting vdW magnets in 2D material-based spin-orbitronic and magneto-thermal devices

Manipulation of the van der Waals Magnet Cr2Ge2Te6 by Spin-Orbit Torques

Recent observations of long-ranged magnetic ordering in van der Waals bonded, layered magnetic materials down to the single layer limit has led to a plethora of research dedicated to the study of two-dimensional (2D) magnets, with plenty of opportunities to investigate fundamental physics and potential quantum applications. With these materials, the properties are typically strongly correlated to the number of layers, with new physics occurring in the few-layer (~few nm) regime. In this sense, common techniques used to measure magnetic behaviors such as neutron scattering and SQUID are at a disadvantage due to sample size requirements. On the other hand, Raman spectroscopy, which has diffraction-limited spatial resolution, is a powerful, non-destructive optical method to probe magnetism in 2D layered materials through inelastic scattering as a function of temperature, laser energy, polarization, and magnetic field. One material of particular interest is chromium tri-iodide (CrI3), a ferromagnet at bulk thicknesses below the Curie temperature but with the remarkable property of layered antiferromagnetism in thin multilayers. Here, we will report on a magneto-Raman spectroscopy study on multi-layered CrI3 [1], focusing on two new features in the spectra which appear at temperatures below the magnetic ordering temperature and were previously assigned to high frequency magnons. 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. In addition, through DFT-calculations and symmetry arguments, we conclude that the new modes are not magnons as previously believed, but instead are zone-folded phonons.References: [1] A. McCreary et al. arXiv:1910.01237.

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

Spin pumping refers to the transfer of spins from precessional moments in a ferromagnet to a non-magnetic material. In a ferromagnetic/non-magnetic bi-layered system, spin pumping manifests itself as a damping enhancement in the ferromagnetic layer and a spin current in the non-magnetic layer. This effect can be strong if the non-magnetic layer is a heavy metal thin film, because such a film can work as a spin sink and produce a very weak spin backflow. The effect can be even stronger if the non-magnetic component is a topological insulator, because it can efficiently convert a spin current to a charge current and thereby serve as a very efficient spin absorber, due to the spin-momentum locking of the topological surface states (TSS). In principle, TSS should also be present in other topological materials, such as Dirac or Weyl semimetals. These TSS are also expected to exhibit spin-momentum locking, but their interplays with magnetism are largely unexplored. This presentation reports strong damping enhancement in a ferromagnetic NiFe thin film due to a neighboring α-Sn thin film. Multiple evidences suggest that the damping enhancement is associated with TSS in the topological Dirac semimetal (TDS) phase of α-Sn. The TDS α-Sn films are realized on InSb substrates. The measurements used α-Sn(6nm)/Ag(2nm)/NiFe(20nm) layered structures where Ag works as a spacer to physically separate α-Sn and NiFe. Ferromagnetic resonance studies show that the damping in these structures is ~4.8 bigger than in the bare NiFe film. Control measurements indicate that this damping enhancement is mostly due to the TSS in the α-Sn, rather than the bulk of the α-Sn film or the Ag spacer; it is absent in β-Sn(6nm)/Ag(2nm)/NiFe(20nm) where the Sn film is topologically trivial. This work demonstrates that TDS may be as promising as topological insulators in terms of spintronics applications.

Topological Surface States-Caused Large Damping Enhancement in Dirac Semimetal-Ferromagnetic Metal Layered Structures

Electric-field control of magnetization is essential for the next-generation spintronic technologies such as nonvolatile magnetic memory. In particular, the voltage-control of the magnetic anisotropy (VCMA) in ferromagnets promises an energy-efficient reversal of magnetization based on the clocking scheme, which earlier has been demonstrated by using a pulsed voltage in magnetic tunnel junction (MTJ). However, the VCMA method in MTJ still requires a high gate voltage and a constant-bias magnetic field, which is a disadvantage for practical applications. In contrast, as another important method for magnetization reversal, current-induced spin-orbit torque (SOT) in topological insulator (TI) based magnetic heterostructures has been studied in recent experiments [1], which succeeded in reducing the switching power consumption by current pulse injected parallel to a bias magnetic field [2]. In this research, inspired by the VCMA and SOT approaches for magnetization control, we combine them and present two distinct clocking methods for the magnetization switching in TI based magnetic heterostructures. First, we analytically formulate the uniaxial magnetic anisotropy in magnetic TIs as a function of the Fermi energy. Next, as shown in Fig. 1, we propose a transistor-like device with the functionality of a nonvolatile magnetic memory adopting the VCMA writhing method that require no external magnetic fields and the anomalous-Hall-effect based readout scheme. For the magnetization reversal, the estimated source-drain current density and gate voltage are the orders of 104 - 105 A/cm2 and 0.1V, respectively, which are much smaller than those of traditional heavy-metal/ferromagnet heterostructures as well as the MTJs. We also discuss the possibility of using the proposed method for magnetization reversal in TI/ferromagnetic-insulator bilayers with magnetic proximity at the interface.References: [1] Y. Fan et al., Nat. Nanotechnol. 11, 352 (2016). [2] K. Yasuda et al., Phys. Rev. Lett. 119, 137204 (2017).

Electric-Field Control of Spin-Orbit Torque and Magnetic Anisotropy in Topological Insulator Heterostructures

The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer[1,2] stimulates a thorough Raman spectroscopic study of one such material, FePS3, a large spin (S = 2) Mott insulator where the Fe atoms form a honeycomb lattice. Bulk FePS3 was shown to be a quasi-2D Ising antiferromagnet, with additional features in the Raman spectra emerging below the Neel temperature (TN ~ 120 K). Using temperature- and magnetic field-dependent Raman spectroscopy as an optical probe of magnetic structure, we demonstrate[3] that one of these Raman-active modes (ψ4) below TN is a magnon with a frequency of ~ 3.7 THz (~122 cm-1). Contrary to previous work[4], which interpreted this feature as a phonon, our Raman data[3] shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, were we find the g-factor ~2. Other Raman-active modes emerging below TN are attributed to zone-folded phonons in the antiferromagnetic state. The anomalous temperature dependence of these modes along with an analysis of temperature-dependent magnetization data, suggests a persistence of short-range magnetic order above TN.References: [1] C. Gong, L. Li, Z. Li, et al., Nature 546, 265 (2017). [2] B. Huang, G. Clark, E. Navarro-Moratalla, et al., Nature 546, 270 (2017). [3] A. McCreary, J. R. Simpson, T. T. Mai, et al., Phys. Rev. B 101, 064416 (2020). [4] J.-U. Lee, S. Lee, S. H. Ryoo, et al., Nano Lett. 16, 7433 (2016).

Temperature- and Magnetic Field-Dependent Raman Spectroscopy of Layered, Antiferromagnetic FePS3

Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. In Fe-based superconductors where the FeAs bonds are confined in 2D monolayers (MLs), the magnetic ground state of the Fe spins is antiferromagnetic1. On the other hand, in Fe-based ferromagnetic semiconductors such as (In,Fe)As where the FeAs bonds are distributed randomly in a three-dimensional (3D) InAs matrix, the Fe spins are coupled ferromagnetically via interactions with electron carriers2. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral FeAs bonds embedded with a regular interval in a semiconductor InAs matrix, which resemble the crystal structure of Fe-based superconductors. The samples were grown by low-temperature molecular beam epitaxy on InAs (001) or semi-insulating GaAs (001) substrates. Lattice images by scanning transmission electron microscopy (STEM) reveal that the zinc-blende structure of the host InAs is well maintained, and the Fe atoms are mainly confined within a thickness of 2-3 MLs (Fig. 1a). We study magnetic and transport properties of a series of FeAs/InAs superlattices (thickness is fixed at 12 nm) containing (1ML of FeAs / t ML of InAs) x N periods (t = 20, 10, 7, 5; N = 1, 3, 5, 7) (the inset of Fig. 1(b)). All the samples exhibit intrinsic ferromagnetism, and the TC value changes rapidly as t-3 (Fig 1b). Furthermore, extremely large magnetoresistance (MR) (~500%, Fig. 1c) are observed, whose origin can be attributed to the giant MR occurring in these multilayer structures. These interesting features indicate that the FeAs/InAs superlattice structures are promising for functional spintronic devices3. This work was partly supported by Grants-in-Aid for Scientific Research (17H04922, 18H05345, 19H05602, 19K21961), CREST program (JPMJCR1777) and PRESTO program (JPMJPR19LB) of JST, and Spin-RNJ.References: [1] H. Hosono et al. Materials Today 21, 278-302 (2018). [2] P. N. Hai, L. D. Anh et al., Appl. Phys. Lett. 101, 182403 (2012). [3] L. D. Anh et al, ArXiv:2005.10181 (2020).

Ferromagnetism and giant magnetoresistance in zinc-blende FeAs/InAs superlattice structures

In recent years, spin-based technologies, in which particle spin is utilized for information processing, have become promising for “beyond-CMOS” solid-state devices. Graphene and other layered materials have rapidly established themselves as intriguing building blocks for spintronics applications [1]. Because of the graphene intrinsic low spin-orbit interaction, spins can flow snugly through its crystal lattice over long distances, resulting in an ideal spin communication channel. At the same time, the graphene’s low spin-orbit interaction inhibits spin manipulation, which is the cornerstone for successfully implementing spin-based devices. Nevertheless, this bottleneck can be overcome by combing graphene with other layered materials in artificial van der Waals heterostructures. In this talk, I will present a set of experiments where we study the spin-relaxation in graphene-transition metal dichalcogenides heterostructures [2]. In such engineered van der Waals systems, spin-orbit coupling in graphene is enhanced by proximity-induced effects. As a consequence, the spin dynamics becomes anisotropic [2, 3], with a spin relaxation that depends on the spin orientation. Interestingly, these systems can be also exploited for efficient spin-charge interconversion driven by the spin Hall effect and the inverse spin galvanic effect at room temperature, with conversion efficiencies tailored by electrostatic gating [4].References: 1. W. Han et al., Nature Nanotechnology 9, 794 (2014). 2. L. A. Benítez, J. F. Sierra et al., Nature Physics 14, 303 (2018). 3. L. A. Benítez, J. F. Sierra et al., APL Materials 7, 120701 (2019). 4. L. A. Benítez, W. Savero Torres, J. F. Sierra, et al., Nature Materials 19, 170 (2020).

Spin Transport and Spin-Orbit Proximity Phenomena in van der Waals Heterostructures INVITED

Graphene has been known as an excellent material for long-distance spin transport due to its weak spin-orbit coupling (SOC). However, the same reason makes graphene an adverse candidate for different spintronics applications in which strong SOC is required, such as the Datta and Das spin transistor proposal or spin-charge interconversions. It has recently been predicted theoretically that SOC can be induced in graphene so that spin-orbit phenomena such as spin Hall effect (SHE) or Rashba-Edelstein effect can be obtained. In our work, by using van der Waals heterostructure-based lateral spin valves, we experimentally demonstrate spin-to-charge conversion (SCC) due to SHE in graphene via spin-orbit proximity with MoS2, a transition metal dichalcogenide (TMD)1. The combination of long-distance spin transport and large spin-to-charge conversion in a van der Waals heterostructure gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. We performed similar experiments in graphene in proximity with WSe22. Here we observed gate-tunable SHE with SCC efficiency larger than in some of the best SCC materials such as topological insulators. Similarly, we observed efficient SHE in graphene combined with an insulator, Bi2O33. Here, the SHE can be due to extrinsic mechanisms such as skew scattering in graphene decorated with adatoms. Using a similar approach, we observed large multidirectional SCC in Weyl semimetal MoTe22. Here, due to the low symmetry of MoTe2 crystal, we detect, along with the conventional SCC, an unconventional SCC where the spin polarization and the charge current are parallel. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration. All in all, these exceptional effects obtained by the unique properties of 2D materials open exciting opportunities in a variety of future spintronic applications.References: 1. C. K. Safeer, et al. Nano Letters, 19, 2, 1074-1082 (2019). 2. F. Herling, et al. APL Materials 8, 071103 (2020). 3. C. K. Safeer, et al. Nano Letters, 20, 6, 4573-4579 (2020). 4. C. K. Safeer, et al. Nano Letters, 19, 12, 8758-8766 (2019).

Spin-to-charge conversion in van der Waals heterostructures INVITED

The classical Einstein-de Haas (EdH) effect [1] is a AC mechanical torque arising from a time rate of change of net magnetization, and represents the intrinsic relationship between magnetism and mechanical angular momentum. Nanofabrication of torque sensing devices is opening new avenues for exploration and applications of the EdH effect. The scale-up of mechanical resonance frequencies with continued miniaturization enhances EdH torques, which increase linearly with drive frequency, relative to frequency-independent magnetic cross-product torques. Previously, miniaturized EdH experiments have been performed at the microscale using mechanical modes in the audio frequency range (13 kHz). The present work brings the measurements to radio frequencies. Single-crystal Yttrium Iron Garnet (YIG) disks with magnetic vortex ground states are mounted on nanomechanical resonators having fundamental torsional modes close to 3 MHz. For DC bias fields below about 200 A/m the maximum cross-product torques, historically an important source of systematic error in EdH experiments, remain smaller than the EdH torques [3]; no control of the field geometry to null cross-product torques is required. Quadrature lock-in measurements allow cross-product and EdH torque signals to be recorded simultaneously, owing to a 90○ relative phase shift arising between the driving torques, and referenced to the phase of the driving field. The simultaneous measurement scheme enables determination of the magnetomechanical ratio, g′, without requiring separate experimental inputs. Results from measurements around the full vortex hysteresis loop, and comparisons with micromagnetic simulations of cross-product and EdH torques, also will be presented and discussed.References: [1] A. Einstein and W.J. de Haas, Proceedings - KNAW 18, 696 (1915). [2] T.M. Wallis, J. Moreland, and P. Kabos, Appl. Phys. Lett. 89, 122502 (2006) [3] K. Mori, M.G. Dunsmore, and M.R. Freeman, arXiv:2005.08406 [cond-mat.mes-hall] (2020) 

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Proximity-Induced Spin Hall Effect in Graphene/WSe2 van der Waals Heterostructures with Tunable, Highly Efficient Spin-to-Charge Conversion

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