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Pure spin currents have been ubiquitous in contemporary spintronics research. Despite its pro- found physical and technological significance, the detection of pure spin current has largely remained indirect, which is usually achieved by probing spin-transfer torque effects or spin-to-charge conversions. By using scanning transmission X-ray microscopy, we report the direct detection and spatial mapping of spin accumulation in a nonmagnetic Cu layer without any direct charge current injection. Such a pure spin current is induced by spin pumping from a NiFe layer and is not accompanied by concomitant charge motion. The observed frequency dependence indicates that the signal is dominated by a coherent, pure spin current, but the magnitude of the spin accumulation suggests also possible additional thermal contributions. Our technique takes advantage of the X-ray magnetic circular dichroism and the synchronization of microwave with X-ray pulses, which together provide a high sensitivity for probing transient magnetic moment. From the detected X-ray signals, we observe two distinct resonance modes induced by spin pumping, which, based on micromagnetic simulations, we attribute to non-linear microwave excitations. Our result provides a new pathway for detecting pure spin currents that originate from many spintronics phenomena, such as spin Hall and spin Seebeck effects, and which can be applied to both metal and insulator spin current sources [1]. Work at Argonne, including experimental design and measurement, sample fabrication, data analysis, and manuscript preparation, was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division under Contract No. DE-AC02-06CH11357. Use of the Stanford Synchrotron Radiation Light source, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. W.Z. acknowledges the DOE visiting Faculty Program.References: 1.) “Direct Observation of spin accumulation in Cu induced by spin pumping” J. Ding, W. Zhang, B. Jungfleisch, J.E. Pearson, H. Ohldag, V. Novosad, A. Hoffmann, Phys. Rev. Res vol 2(1), p. 013262 (2020)

MMM 2020

Direct Observation of Spin Accumulation in Cu Induced by Spin Pumping

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

When the crystalline symmetries that protect a higher-order topological phase are not preserved at the boundaries of the sample, gapless hinge modes or in-gap corner states cannot be stabilized. Therefore, careful engineering of the sample termination is required. Similarly, magnetic textures, whose quantum fluctuations determine the supported magnonic excitations, tend to relax to new configurations that may also break crystalline symmetries when boundaries are introduced. Here we uncover that antiskyrmion crystals provide an experimentally accessible platform to realize a magnonic quadrupole topological insulator, whose hallmark signatures are robust magnonic corner states. Furthermore, we show that tuning an applied magnetic field can trigger the self-assembly of antiskyrmions carrying a fractional topological charge along the sample edges. Crucially, these fractional antiskyrmions restore the symmetries needed to enforce the emergence of the magnonic corner states. Using the machinery of nested Wilson loops, adapted to magnonic systems supported by noncollinear magnetic textures, we demonstrate the quantization of the bulk quadrupole moment, edge dipole moments, and corner charges.References: [1] T. Hirosawa, S. A. Díaz, J. Klinovaja, and D. Loss, arXiv:2005.05884

Magnonic Quadrupole Topological Insulator in Antiskyrmion Crystals

Magnetic skyrmions [1,2], topologically protected magnetic quasi-particles, are at the core of many recently proposed spintronic devices. Most of the materials systems shown hosting magnetic skyrmions at room temperature [3-5] are characterized by [heavy metals]\ferromagnet interfaces to generate a strong Dzyaloshinskii-Moriya interaction (DMI) and need an external magnetic field to stabilize the skyrmions. From an application point of view, both the use of heavy metals and the need for an external magnetic field are limitations. The former are usually rare, which makes them expensive; the latter is not compatible with nanoelectronics due to scalability issues. Accordingly, it is highly desirable to have a robust and tunable protocol to build systems made only with light metals which can host magnetic skyrmions in zero external magnetic field. Here we report [6] the observation of isolated magnetic skyrmions at room temperature and zero magnetic field, stabilized through interlayer exchange coupling [7,8] between a reference magnet and a free magnet (Fig. 1). In our stack, both magnets and the non-magnetic spacer in between are made out of 3d transition metals, and by carefully changing the thickness of the spacer we can tune the size of the observed skyrmions, whose 3D spin-texture is directly imaged by spin-polarized low energy electron microscopy. In addition, via density functional theory calculations we gain a deeper understanding on the origin of the DMI in such systems, allowing us to extract the strength of the DMI, which compares well with experimental results. Our findings open up the possibility to develop cheap skyrmion-based spintronic devices suitable for general-use applications which go beyond modern nanoelectronics.References: [1] A. Bogdanov and A. Hubert, J. Magn. Magn. Mater. 138, 255 (1994). [2] N. Romming et al., Science 341, 636 (2013). [3] C. Moreau-Luchaire et al., Nat. Nanotechnol. 11, 444 (2016). [4] S. Woo et al., Nat. Mater. 15, 501 (2016). [5] A. Soumyanarayanan et al., Nat. Mater. 16, 898 (2017). [6] R. Lo Conte et al., Nano Lett. (2020), https://pubs.acs.org/doi/10.1021/acs.nanolett.0c00137. [7] G. Chen et al., Appl. Phys. Lett. 106, 242404 (2015). [8] A. K. Nandy et al., Phys. Rev. Lett. 116, 177202 (2016).

Tuning zero-field magnetic skyrmions in all-light-metals multilayers

Magnetic skyrmions and antiskyrmions are topologically protected non-collinear spin textures. They have opposite topological charges of +1 and – 1 owing the differently winding of the in-plane spin component about a fixed direction of the core1. The existence of them in a material system mostly depends on the symmetry of the underlying crystal structure. For example, cubic B20 compounds host Bloch skyrmion of isotropic spin texture2, whereas D2d symmetry compounds have anisotropic spin texture named 'antiskyrmion'3,4. Antiskyrmions comprise of alternating chiral boundaries of helicoids and cycloids. They found in two varieties of shape, namely round and square-shaped antiskyrmion5. Here for the first time, using the Lorentz microscopy technique, we have discovered elliptical shape skyrmions in the same material which comprises of Bloch walls only 5. We named them as "elliptical Bloch skyrmions". The elliptical skyrmion's major axis is oriented along the helical propagation directions, [100] and [010], and have opposite chiralities. They are found as spare and lattice states. Interestingly, both topological structure phases can also be stabilized in zero biased fields and below, over wide temperature ranges. It is found that in D2d long-range dipole-dipole interactions play a significant rule for the stabilization of those different opposite topological spin textures. Micromagnetic simulations also confirms these nano-objects. Antiskyrmions are mostly found in room temperature and above, and in lower temperatures, elliptical Bloch skyrmion is observed. At a critical temperature of 268 K, both these nano-objects are stabilized. We also performed experiment at room temperature on the nano-stripes. The transformation and density variation of antiskyrmions and elliptical Bloch skyrmions occur here that depend on the degree of tilting6. Notably, those nano-objects in the nano-tracks produce as a single chain form in the middle of the track even without giving an in-plane field that was indeed needed in the broad width lamellae. The novel and unanticipated finding of opposite topological charges of 1in one material system make the family of D2d material exceptional.References: 1. A. Bogdanov, U. Roeßler and K.-H. Mueller, Phys. Rev. B, 66, 214410 (2002) 2. S. Muehlbauer, B. Binz and C. Pfleiderer, Science, 323, 915-919 (2009) 3. A. K. Nayak, V. Kumar and S. S. P. Parkin, Nature, 548, 561 (2017) 4. J. Jena, R. Stinshoff and S. S. P. Parkin, Nano Lett., 20, 59−65 (2019) 5. J. Jena, B. Goebel and S. S. P. Parkin, Nat. Commun., 11, 1115 (2020) 6. J Jena, B. Goebel and S. S. P. Parkin, Submitted., XX,XXX (2020)

Observation of opposite topological charges in single D2d Heusler compound

Recently, two-dimensional van der Waals (vdW) systems with a ferromagnetic behavior were found [1], [2], [3], [4], representing exciting opportunities for magneto-optics and magneto-electric applications. In those materials, the long-range magnetic order exists despite the thermal fluctuations of the Mermin-Wagner theorem because of a strong magnetic anisotropy. The compound Fe3GeTe2 (FGT) stands as a promising candidate among the vdW structures due to the tunability of its ferromagnetism under an electric field, allowing to rise the Curie temperature above 300 K [5]. In recent experiments by Park et al. on non-oxidized multilayer of FGT [6], Bloch skyrmions were observed using Differential Phase Contrast Microscopy and Lorentz Transmission Electron Microscopy and their stability cannot be explained by the existence of a chiral interaction such as Dzyaloshinskii–Moriya interaction (DMI), forbidden by the centrosymmetric structure of FGT. In this work, we numerically study the magnetic textures hosted by FGT using the micromagnetic solver Mumax3 [7] and axisymmetric simulations [8] to apprehend the phenomena at play. We demonstrate the existence of small hybrid magnetic textures, composed of Bloch skyrmionic bubbles in the middle of the structure with Néel “caps” close to the interface. These bubbles are stable in FGT for a range of external magnetic field and take sizes in the range of 20 nm for perpendicular fields in the range of a few 10 of mT comparable with the experimental observations. We would like to acknowledge S.Woo, X. Yu, A. Hassan and M. Chsiev for fruitful discussions. The ANR grant TOPSky TOPSKY (ANR-17-CE24-0025), the FLAGERA 2019 SOgraphMEM and the EU grant SKYTOP (H2020 FET Proactive 824123) are acknowledged for their financial support.References: [1] Cheng Gong et al., Nature 546, no. 7657 (June 2017): 265–69. [2] Shengwei Jiang, Jie Shan, and Kin Fai Mak, Nature Materials 17, no. 5 (May 2018): 406–10. [3] Jie Liu et al., AIP Advances 8, no. 5 (May 2018): 055316. [4] Bevin Huang et al., Nature 546, no. 7657 (June 2017): 270–73. [5] Yujun Deng et al., Nature 563, no. 7729 (November 2018): 94–99. [6] Park, Tae-Eon, et al., arXiv preprint arXiv:1907.01425 (2019). [7] Arne Vansteenkiste et al., AIP Advances 4, no. 10 (October 2014): 107133. [8] William Legrand et al., Physical Review Applied 10, no. 6 (December 2018): 064042.

Micromagnetic Simulations Of Skyrmionic Bubbles In The Van Der Waals Structure Fe3GeTe2

Non-equilibrium phenomena of quasiparticles (QPs) in a spin-split superconductor (SC) rely on the mutual coupling between different non-equilibrium imbalances of spin, charge, heat, and spin-heat [1]. Although recent experiments and theories have shown a variety of exotic transport properties in SC-based devices with either Zeeman or exchange spin splitting [1], how QP interplays with magnon spin currents [2] remains elusive. Here, using non-local magnon spin-transport devices [3] where a singlet SC (Nb) on top of a ferrimagnetic insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect (iSHE) in such an exchange-spin-split SC [4] can be greatly enhanced by up to 3 orders of magnitude compared with that in the normal state, particularly when its interface superconducting gap matches the magnon spin accumulation. Through systematic measurements with varying the current density and SC thickness, we identify that superconducting coherence peaks and exchange spin-splitting of the QP density-of-states, yielding a larger spin excitation while retaining a modest QP charge-imbalance relaxation, are responsible for the giant QP iSHE. Our results provide a new device concept in which the non-local voltage created from long-rage magnon-carried spin is orders of magnitude controllable by turning the superconductivity on and off [5].References: [1] F. S. Bergeret, M. Silaev, P. Virtanen, and T. T. Heikkilä, Rev. Mod. Phys. 90, 041001 (2018). [2] L. J. Cornelissen, J. Liu, R. A. Duine, J. Ben Youssef, and B. J. van Wees, Nat. Phys. 11, 1022 (2015). [3] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, and B. Hillebrands, Nat. Phys. 11, 453 (2015). [4] M. Silaev, P. Virtanen, F. S. Bergeret, and T. T. Heikkilä, Phys. Rev. Lett. 114, 167002 (2015) [5] K.-R. Jeon, J.-C. Jeon, X. Zhou, A. Migliorini, J. Yoon and S. S. P. Parkin, arXiv:2004.09467 (2020).

Giant transition-state enhancement of quasiparticle spin-Hall effect in an exchange-spin-split superconductor detected by non-local magnon spin-transport

Spin-dependent scattering transport at heavy metal (HM)/magnetic insulator (MI) interface can be described in terms of three parameters: the spin-sink conductance Gs and the real and imaginary part of the spin-mixing conductance, G↑↓=Gr+iGi [1,2]. Each parameter is relevant for different spin-dependent phenomena. For instance, Gs originates from spin-flip processes and therefore is the leading parameter in electrical biasing of magnons [3], whereas Gr accounts for the spin-transfer (Slonczewski) torque to the magnetization and plays a fundamental role in spin-pumping experiments [4]. On the other hand, Gi quantifies the interfacial exchange field, which induces a field-like torque in the conduction electrons of the HM and is important for example in spin-splitting field experiments in superconductivity [5]. These conductances are broadly studied in ferrimagnetic insulators, where usually the contribution of Gr is much larger than that of Gi [6], leading to only a few reports on the exchange field at HM/MI interfaces [7]. In this work [8], we study the three spin conductance terms by spin Hall magnetoresistance (SMR) in a new system: a ferromagnetic insulator (FMI) such as EuS. By SMR measurements as a function of the temperature, and taking advantage of the newly developed microscopic theory for SMR [2], we can extract relevant microscopic parameters such as the exchange interaction between the 1s electrons in Pt and the localized magnetic moments in EuS (Jint~18 meV) [8]. An interfacial exchange field of the order of 1 meV (~15 T) acting upon the conduction electrons of Pt can be estimated from Gi, which is at least three times larger than Gr below the Curie temperature [8]. Our work provides an easy method to quantify this interfacial spin-splitting field, which is of interest in different areas of Condensed Matter Physics, such as proximity effects in superconducting hybrid systems.References: [1] T.-Y. Chen et al., Phys Rev. B 87, 144411 (2013) [2] X.-P. Zhang et al., Nano Lett. 19, 6330 (2019) [3] L. J. Cornelissen et al., Nat. Phys. 11, 1022 (2015). [4] M. Weiler et. al., Phys. Rev. Lett., 111, 176601 (2013) [5] B. Li et al., Phys. Rev. Lett. 110, 097001 (2013). [6] X. Jia et al., Europhysics Lett. 96, 17005 (2011). [7] N. Vlietstra et al., Appl. Phys. Lett. 103, 032401 (2013); T. Kosub et al., Appl. Phys. Lett. 113, 222409 (2018). [8] J. M. Gomez-Perez et al., arXiv:2004.12009 (2020)

Strong Interfacial Exchange Field in a Heavy Metal/Ferromagnetic Insulator System Determined by Spin Hall Magnetoresistance

Relatively little work has been done to date to understand the transport of spin waves generated via ferromagnetic resonance (FMR) in anisotropic high-temperature superconductors. To this end, we have fabricated bilayers of yttrium barium copper oxide (YBCO) with the half-metallic ferromagnet lanthanum strontium manganite (LSMO) via pulsed laser deposition on (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) (001) substrates. We fix the LSMO thickness at 30 nm and vary the YBCO thickness from 2-17 nm. We have verified that the YBCO superconducts with Tc varying from 55-88 K depending on YBCO thickness (with a minimum thickness of 5 nm required for superconductivity). Since the variation in Tc with thickness is not as strong in single YBCO layers grown on LSAT substrates we believe that the magnetic exchange field from the LSMO, rather than a epitaxial strain effect, is the governing factor for Tc. To characterize spin wave transport, we performed FMR measurements on the bilayers as a function of temperature and YBCO thickness. We find that the resonance linewidth of bare LSMO increases with decreasing temperature, which is attributed to an increased contribution from two-magnon scattering at lower temperatures. Additionally, with the addition of 17 nm of YBCO, we see a near doubling of the FMR linewidth at all temperatures. Although more work needs to be done to fully characterize the system, these results possibly point to spin wave dissipation in the YBCO layer, which has not previously been observed.

Spin transport in La0.67Sr0.33MnO3/YBa2Cu3O7-δ bilayers

Spin orbit torques (SOTs) are being actively considered for use in the next generation memory devices for magnetization switching, spin oscillators and racetrack memories, including those using magnetic skyrmions. SOTs are fundamentally based on charge-to-spin conversion by the spin-Hall or Rashba effect. Taking advantage of the spin current, the torques can be used to switch magnetic insulators or the free layer of magnetic tunnel junctions with high energy efficiency. Therefore, it is important to accurately measure the charge-to-spin conversion efficiency. SOTs are principally of interest in nanostructured samples, samples with minimum dimension less than a micron. However, the magnitude and form of the torques are most often characterized in micron scale samples, using spin torque ferromagnetic resonance (ST-FMR), a technique that involves analyzing the resonance linewidth [1] or lineshape [2]. On microstructures, these two analysis methods are quite consistent. Here we present ST-FMR results on permalloy Ni80Fe20 nanowires --- with widths varying from 150 to 800 nm --- that show that the standard model used to analyze the resonance linewidth and lineshape give different results [3]. Fig. 1 a-c show examples of ST-FMR spectra. Fig. 1d shows the efficiency of nanowires with different width, which shows greatly enhanced efficiency when the lineshape method is used. We present a ST-FMR model that properly accounts for the sample shape. It shows a much better consistency between the two methods, as illustrated in Fig. 2. Micromagnetic simulations are used to verify the model. These results and the more accurate nanowire model presented are of importance for characterizing and optimizing charge-to-spin conversion efficiencies in nanostructures.References: [1] S. Petit et al., Physical Review Letters 98, 077203 (2007) [2] L. Liu et al., Physics Review Letters 106, 036601 (2011). [3] J.-W. Xu et al., arXiv:2004.03784(2020). [to appear in Physical Review Applied]

Charge-to-spin conversion efficiency in ferromagnetic nanowires by spin torque ferromagnetic resonance: Reconciling lineshape and linewidth analysis methods

Generation and manipulation of pure spin current play an important role in fabrication of power efficient spintronics devices1. Spin pumping is very efficient way of creation of pure spin current in ferromagnetic (FM)/nonmagnetic (NM) system by microwave excitation which can be converted into a voltage by inverse spin Hall effect (ISHE) Various materials possessing high spin orbit coupling (SOC) like Pt, Ta, W, Bi2Se3 etc., have been utilized for efficient spin to charge conversion2,3. However, these are limited by very small spin diffusion length (λsd ~10 nm). In this context organic semiconductors (OSC) e.g. C60 are emerging as a potential candidate due to large value of λsd. The low value of SOC in OSC limits them for efficient spin to charge conversion. However, when the OSC is deposited on a substrate the structural distortion may exhibit enhancement of SOC. Here we report simultaneous observation of spin pumping and ISHE measurements in the CoFeB/C60 bilayers by ferromagnetic resonance (FMR) in the frequency range 6 – 17 GHz. During the measurement the sample was kept on the wide coplanar waveguide (CPW) in the flip-chip manner. Angle dependent ISHE measurements have been performed at fixed frequency of 7 GHz and 11 mW power by using a nanovoltmeter at the two edges of the sample. Figure1 shows dc voltage (Vmeas) and FMR signal versus applied magnetic field (H) plot. At 180-degree ISHE signal is reversed as the magnetic field direction is reversed, which indicates that signal is not coming from any artefacts and it is due to spin pumping. Also, we have calculated the spin Hall angle for CoFeB/C60 bilayer system is 0.04 which is higher than the reported value.References: 1. Y. Tserkovnyak, A. Brataas, et al., Rev. Mod. Phys. 77, 13751421 (2005) 2. B. B. Singh et al., physica status solidi (RRL) Rapid Research Letters 13, 1800492 (2019) 3. B. B. Singh et. al., Phys. Rev. Appl. 13, 044020 (2020)

Observation of inverse spin hall effect at CoFeB/C60 interface

2D-transition metal dichalcogenides (TMD) are known to have high spin-orbit coupling, an essential property for spintronic applications [1,2]. TMD materials with this property can replace the heavy metals for next generation spintronic devices. In this context MoS2 has been of great interest in recent years due to its unique electrical and optoelectronics properties. It can be fabricated in very thin layers due to weak Van der waals forces, which makes it suitable to investigate interface dominated Rashba and Edelstein effects. However, fabrication of MoS2 by chemical or exfoliation methods makes it difficult for spintronics applications. In this work, we demonstrate inverse spin Hall effect in MoS2(t)/CoFeB(10nm)/MgO(2nm) heterostructures where ‘t’ varies from 7 to 30nm.The heterostructure was fabricated by RF and DC sputtering of MoS2 and CoFeB, respectively, on thermally oxidized Si substrate. MgO capping layer was deposited by thermal evaporation. Here the spin current was generated from CoFeB and injected into MoS2 layer via spin pumping using ferromagnetic resonance (FMR). The resulting spin to charge conversion at interface was measured using nanovoltmeter. Various spin rectification effects were separated by performing angle dependent measurements [3]. For the sample with MoS2(30nm), a spin pumping voltage of -1.6 mV was measured which was dominant over the rectification effects. The spin mixing conductance value was found to be ~ 3.72×1018 m-2 which is comparable to that of MoS2 and Pt obtained from literature. Figure 1 shows the measured voltage signal for the sample Si/SiO2/MoS2(30nm)/CoFeB(10nm)/MgO(2nm). In order to understand the origin of this voltage, power and frequency dependent measurements were also performed. The linear increase in the symmetric component of dc voltage with the microwave power further indicated spin pumping [4].References: [1] Qing Hua , Kourosh Kalantar-Zadeh, Andras Kis, Jonathan N. Coleman and Michael S. Strano Nature Nanotechnology, 7, 699–712 (2012) [2] Zehua Hu, Zhangting Wu, Cheng Han, Jun He, Zhenhua Ni and Wei Chen, Chemical Society Reviews, 47, 3100-3128, (2018) [3] Braj B. Singh, Sukanta K. Jena, Manisha Samanta, Kanishka Biswas, Biswarup Satpati and Subhankar Bedanta, Phys. Status Solidi RRL,Volume 13, Issue 3,1800492 (2019) [4] K. Ando, S. Takahashi, J. Ieda, Y. Kajiwara, H. Nakayama, T. Yoshino, K. Harii, Y. Fujikawa, S. Maekawa and E. Saitoh, J. Appl. Phys.109, 103913 (2011).

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