France



An outstanding feature of topological quantum materials is their unique spin topology in the electronic band structures with novel charge to spin conversion effects. Here, I will present the novel charge to spin conversion phenomenon in topological insulators, Weyl semimetals, and its van der Waals heterostructures with graphene.&lt;br/&gt;&lt;br/&gt;The discovery of topological Weyl semimetals has revealed opportunities to realize several extraordinary physical phenomena in condensed matter physics. Weyl semimetals with strong spin orbit coupling, broken inversion symmetry, and novel spin textures are predicted to exhibit a charge to spin conversion effect that can efficiently convert the charge current to a spin current. We report a direct experimental observation of charge-spin conversion and its inverse effect in semimetal WTe2 at room temperature as shown in Fig. 1 (Ref. 1). We also observe an evolution of the charge to spin conversion signals from WTe2 and graphene hybrid device with a geometrical design (Ref. 2). The spin precession measurements of the signal at different gate voltages and ferromagnet magnetization show the robustness of the charge to spin conversion in WTe2 at room temperature. These results can be useful for designing heterostructure devices and in the architectures of 2D spintronic circuits.&lt;br/&gt;&lt;br/&gt;We could also detect an unconventional charge to spin conversion in WTe2, which is different from the conventional spin Hall and Rashba Edelstein effects (Ref. 3). Such a large and unconventional spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin orbit interaction with a novel spin texture in the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization in WTe2 using both charge to spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing Weyl materials WTe2 as nonmagnetic spin sources in all electrical van der Waals spintronic circuits and high performance nonvolatile spintronic technologies.&lt;br/&gt;&lt;br/&gt;Unique electronic spin textures in topological insulators are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to enhancing their performance, electrical gate tunability, interference from trivial bulk states, and heterostructure interfaces. We address some challenges by integrating 2D graphene with a 3D topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity induced spin orbit phenomena (Ref. 4). In these heterostructures, we experimentally demonstrate a gate tunable spin galvanic effect (SGE) at room temperature, allowing for efficient conversion of a nonequilibrium spin polarization into a transverse charge current (Ref. 5). Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin to charge conversion in the graphene and TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin galvanic signal. These findings provide an efficient route for realizing all electrical and gate tunable spin orbit technology using TIs and graphene in heterostructures.&lt;br/&gt;&lt;br/&gt;Fig. 1. Charge to spin conversion in WTe2 at room temperature. a. Device geometry and Hanle measurement of conventional charge to spin conversion in WTe2 (Ref. 1). B. Measurement of unconventional charge to spin conversion in WTe2. The measurement of spin valve and Hanle signals both in parallel and antiparallel configurations at room temperature show the unconventional nature of charge to spin conversion (Ref 3).&lt;br/&gt;&lt;br/&gt;Fig. 2. Spin galvanic effect in graphene and topological insulator heterostructure at room temperature. a. A schematic representing the spin galvanic effect, where spin polarized carriers diffuse in the graphene and TI heterostructure, acquire a transverse momentum and produce a charge current. b. A scanning electron microscopy picture of a graphene and TI hybrid device and measurement scheme. c. Schematics of band structures of proximitized graphene develop Rashba spin split bands in the valence and conduction bands. d. The SGE signal R measured with the inplane magnetic field Bx. e. The SGE was detected via spin precession with the out of plane field Bz at different gate voltages in the valence and conduction band of proximitized graphene (Ref 5).

**References:**

1. Observation of charge to spin conversion in Weyl semimetal at room temperature. B Zhao, D Khokhriakov, Y Zhang, H Fu, B Karpiak, AM Hoque, X Xu, Y Jiang, B Yan, SP Dash. Phys. Rev. Res. 2 (1), 013286 (2020).&lt;br/&gt;2. Charge-spin conversion signal in WTe2 van der Waals hybrid devices with a geometrical design.&lt;br/&gt;B. Zhao, A. Md. Hoque, D. Khokhriakov, B. Karpiak, S.P. Dash. Appl. Phys. Lett. 117, 242401 (2020).&lt;br/&gt;3. Unconventional charge-to-spin conversion Weyl Semimetal WTe2. B Zhao, B Karpiak, D Khokhriakov, A Johansson, AM Hoque, X Xu, Y Jiang, I Mertig, SP Dash. Advanced Materials, 2000818 (2020).&lt;br/&gt;4. Tailoring emergent spin phenomena in Dirac material heterostructures. D Khokhriakov, A. W. Cummings, K Song, M Vila, B Karpiak, A Dankert, S Roche and SP. Dash. Science Advances, 4, 9, eaat9349 (2018).&lt;br/&gt;5. Gate-tunable Spin-Galvanic Effect in Graphene - Topological insulator van der Waals Heterostructures at Room Temperature. D Khokhriakov, AM Hoque, B Karpiak, SP Dash. Nature Communication 11, 3657 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/42eab8e2dc3b7cce2be8f59122fff436.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/4c4b9ee8b0fe7d9ef3c951698c9f42bd.jpg)



INTERMAG 2021

 Charge Spin Conversion in Topological Materials and Heterostructures INVITED

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An outstanding feature of topological quantum materials is their unique spin topology in the electronic band structures with novel charge to spin conversion effects. Here, I will present the novel charge to spin conversion phenomenon in topological insulators, Weyl semimetals, and its van der Waals heterostructures with graphene. The discovery of topological Weyl semimetals has revealed opportunities to realize several extraordinary physical phenomena in condensed matter physics. Weyl semimetals with strong spin orbit coupling, broken inversion symmetry, and novel spin textures are predicted to exhibit a charge to spin conversion effect that can efficiently convert the charge current to a spin current. We report a direct experimental observation of charge-spin conversion and its inverse effect in semimetal WTe2 at room temperature as shown in Fig. 1 (Ref. 1). We also observe an evolution of the charge to spin conversion signals from WTe2 and graphene hybrid device with a geometrical design (Ref. 2). The spin precession measurements of the signal at different gate voltages and ferromagnet magnetization show the robustness of the charge to spin conversion in WTe2 at room temperature. These results can be useful for designing heterostructure devices and in the architectures of 2D spintronic circuits. We could also detect an unconventional charge to spin conversion in WTe2, which is different from the conventional spin Hall and Rashba Edelstein effects (Ref. 3). Such a large and unconventional spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin orbit interaction with a novel spin texture in the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization in WTe2 using both charge to spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing Weyl materials WTe2 as nonmagnetic spin sources in all electrical van der Waals spintronic circuits and high performance nonvolatile spintronic technologies. Unique electronic spin textures in topological insulators are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to enhancing their performance, electrical gate tunability, interference from trivial bulk states, and heterostructure interfaces. We address some challenges by integrating 2D graphene with a 3D topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity induced spin orbit phenomena (Ref. 4). In these heterostructures, we experimentally demonstrate a gate tunable spin galvanic effect (SGE) at room temperature, allowing for efficient conversion of a nonequilibrium spin polarization into a transverse charge current (Ref. 5). Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin to charge conversion in the graphene and TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin galvanic signal. These findings provide an efficient route for realizing all electrical and gate tunable spin orbit technology using TIs and graphene in heterostructures. Fig. 1. Charge to spin conversion in WTe2 at room temperature. a. Device geometry and Hanle measurement of conventional charge to spin conversion in WTe2 (Ref. 1). B. Measurement of unconventional charge to spin conversion in WTe2. The measurement of spin valve and Hanle signals both in parallel and antiparallel configurations at room temperature show the unconventional nature of charge to spin conversion (Ref 3). Fig. 2. Spin galvanic effect in graphene and topological insulator heterostructure at room temperature. a. A schematic representing the spin galvanic effect, where spin polarized carriers diffuse in the graphene and TI heterostructure, acquire a transverse momentum and produce a charge current. b. A scanning electron microscopy picture of a graphene and TI hybrid device and measurement scheme. c. Schematics of band structures of proximitized graphene develop Rashba spin split bands in the valence and conduction bands. d. The SGE signal R measured with the inplane magnetic field Bx. e. The SGE was detected via spin precession with the out of plane field Bz at different gate voltages in the valence and conduction band of proximitized graphene (Ref 5).

**References:**

1. Observation of charge to spin conversion in Weyl semimetal at room temperature. B Zhao, D Khokhriakov, Y Zhang, H Fu, B Karpiak, AM Hoque, X Xu, Y Jiang, B Yan, SP Dash. Phys. Rev. Res. 2 (1), 013286 (2020). 2. Charge-spin conversion signal in WTe2 van der Waals hybrid devices with a geometrical design. B. Zhao, A. Md. Hoque, D. Khokhriakov, B. Karpiak, S.P. Dash. Appl. Phys. Lett. 117, 242401 (2020). 3. Unconventional charge-to-spin conversion Weyl Semimetal WTe2. B Zhao, B Karpiak, D Khokhriakov, A Johansson, AM Hoque, X Xu, Y Jiang, I Mertig, SP Dash. Advanced Materials, 2000818 (2020). 4. Tailoring emergent spin phenomena in Dirac material heterostructures. D Khokhriakov, A. W. Cummings, K Song, M Vila, B Karpiak, A Dankert, S Roche and SP. Dash. Science Advances, 4, 9, eaat9349 (2018). 5. Gate-tunable Spin-Galvanic Effect in Graphene - Topological insulator van der Waals Heterostructures at Room Temperature. D Khokhriakov, AM Hoque, B Karpiak, SP Dash. Nature Communication 11, 3657 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/42eab8e2dc3b7cce2be8f59122fff436.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/4c4b9ee8b0fe7d9ef3c951698c9f42bd.jpg)

technical paper

While very recent, the introduction of 2D materials in Magnetic Tunnel Junctions (MTJs) has already shown some promising properties (atomic thickness control, diffusion barrier, spin filtering…)[1]. Graphene and the 2D insulator h-BN have been the first 2D materials to show strong impact on spin transport in MTJs. It was shown that strong spin filtering occurred with the creation of an insulating spin channel in metallic graphene and metallic channel in insulating h-BN[2][3]. The recent advent of the wide Transition Metal Dichalcogenides (TMD) family of 2D semiconductors (MoS2, WS2…) opened new opportunities for further tailoring of spintronics properties. Preliminary results highlighted that maintaining interfaces and materials quality (such as preventing ferromagnets oxidation) remains a crucial issue. Here, we will present results on band filtering in WS2 based spin valves. We will detail a protocol to fabricate spin valves based on CVD grown WS2, with step-by-step characterizations in support (Raman spectroscopy, photoluminescence, AFM measurements…) which aims at preserving interfaces spin properties by avoiding oxidations and degradations. The WS2 layers are integrated in Co/Al2O3/WS2/Co magnetic tunnel junctions acting as hybrid spin valve structures (Figure 1). We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface. The fabrication process is further validated by the measurements of magnetoresistance spin signals above state of the art for 2D semiconductors based MTJs. For monolayer based junctions, the spin polarization extracted from the WS2/Co interface is measured to be positive. Interestingly, for bi- and tri-layers WS2, the observed spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction (Figure 2). This unusual thickness dependent property will be explained in light of the peculiar thickness band-structure evolution of WS2, with Density Functional Theory calculations in support. This allows us to unravel a model of band filtering enabling spin selection and sign inversion through a selective K-Q transition in the band structure. Our work thus demonstrates the potential of WS2 for K-Q band filtering and its tailoring in MTJs. As this band structure evolution is common to many other TMDs, our work also opens the way to the integration of members of this very large 2D materials family, in order to reveal their spin transport properties in Magnetic Tunnel Junctions [4].

**References:**

[1] Piquemal-Banci et al. J. Phys. D: Appl. Phys. 50, 203002 (2017). [2] Piquemal-Banci et al. ACS Nano 12, 4712 (2018). [3] Piquemal-Banci et al. Nature Communications 11, 5670 (2020) [4] Zatko et al. ACS Nano 12, 14468 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6a415d1ecdb931f2b917bf91efc06300.jpg)
Fig.1: Schematic of the Co/Al2O3/WS2/Co Magnetic Tunnel Junction.

![](https://assets.underline.io/uploads/markdown_image/1/image/449f969d4cc41755969c71cac4422490.jpg)
Fig. 2: Spin Polarization of Co/WS2 versus the number of WS2 layers.

 Spin Filtering Manipulation in WS2-based Magnetic Tunnel Junctions

Long before the recent fascination with two-dimensional materials, the critical behaviour and universality scaling of phase transitions in low-dimensional systems has been a topic of great interest. Particularly intriguing is the case of long-range magnetic order in two dimensions, once considered to be excluded in systems with continuous symmetry by the Mermin-Wagner theorem,but later revisited from the viewpoints of topological vortex order, symmetry breaking fields and finite size effects. Recent experiments on van der Waals monolayer systems [1] show that a sizable out-of-plane magnetic anisotropy is able to stabilize 2D long-range ferromagnetic order, as demonstrated in CrI3, CrBr3 and Fe3GeTe2, while a spontaneous magnetic ordering has remained elusive for an in-plane 2D magnetic system in the monolayer limit. In this work, we construct a large-area, nearly ideal easy-plane system, a CrCl3 monolayer on Graphene/6H-SiC (0001) grown by molecular-beam epitaxy (Figs.1a and 1b), which exhibits ferromagnetic ordering as unambiguously determined by element-specific X-ray magnetic dichroism [2]. Hysteretic behaviour of the field-dependent magnetization is sustained up to a temperature of 10 K (Fig.1c), and angular dependent measurements evidence a clear in-plane easy axis, unlike all other van der Waals monolayer magnets reported to date. Moreover, the analysis of the critical exponents of the temperature-dependent magnetization show a scaling behaviour that is characteristic of a 2D-XY system (Fig. 1d). These observations suggest the first realization of a finite-size Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a quasi-freestanding monolayer magnet with a XY universality class; accessible through the bottom-up growth of a van der Waals layer with an in-plane hexagonal crystal symmetry and negligible substrate interaction. The realization of XY magnetism in two dimensions opens the door to explore exotic physics such as spin superfluidity [3], and further constitutes an ideal platform to stabilize truly 2D topological spin textures, such as merons [4]. Apart from the novel physics discovered herein, the ability to grow homogeneous monolayer magnets by molecular beam epitaxy adds versatility and scalability to integrate 2D magnets in devices and large-area functional heterostructures. The strategies to achieve a high Curie temperature in the truly 2D limit will be also discussed in view of applications.

**References:**

[1] K. S. Burch, D. Mandrus, J. G. Park, Magnetism in two-dimensional van der Waals materials. Nature 563, 47–52 (2018). [2] A. Bedoya-Pinto et al., Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer. https://arxiv.org/abs/2006.07605 (2020) [3] S. Kim, et al., Transport signature of the BKT transition in 2D easy-plane magnets. https://arxiv.org/abs/2003.08956 (2020) [4] X. Lu, R. Fei, L. Zhu, L. Yang, Meron-like topological spin defects in monolayer CrCl3. Nat. Commun. 11, 4724 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/0a1c26c8222c2c586f29dc1ea27170fb.jpg)
Figure 1. (a) Schematic crystal structure of CrCl3/Graphene/6H-SiC layers in top view and cross-section configurations. (b) Scanning tunneling microscopy images of the CrCl3 layer grown on Graphene/6H-SiC (0001). The overview image reveals that both Graphene and CrCl3 follow the SiC topography with atomic terraces. The atom resolved image features a moiré pattern, which results from the superposition of the hexagonal unit cell of CrCl3 and graphene.(c) Hysteresis loops of the XMCD signal at the Cr L3 edge, as a function of temperature, indicating clear ferromagnetic behaviour. Remanence and coercive field vanish at 12.5K. (d) Modified Arrott-Plots for the temperature-and field dependent XMCD data. A consistent set of critical exponents is inferred (β=0.235, γ= 2.2), matching with the predictions of the 2DXY model. The XMCD data in Figures 3c and 3d were acquired at the VekMaG beamline in BESSY, Berlin.

 Intrinsic 2DXY ferromagnetism in a van der Waals monolayer grown by molecular beam epitaxy

Recently, two-dimensional transition metal dichalcogenides (TMDs) emerge as intriguing platforms for spintronics study because of their controllability in structure, stacking order, conductivity, non-trivial energy band topology and interplay between spin and pseudospin [1,2]. With strong spin-orbit coupling [3], TMDs also show great potentials in manipulation of magnetic moment by using of spin-orbit torque (SOT), however, most of the present study is restricted to the mechanically exfoliated samples with small sizes and relatively low conductivities [4,5]. Therefore, the efficiency of magnetization switching in TMD/FM bilayer might not be comparable with conventional HM/FM bilayer and not suitable for large-scale application. Here, we develop a manufacturable recipe to fabricate large-area thin films of PtTe2, a type-II Dirac semimetal, to study their capability of generating SOT [6]. PtTe2/Py bilayer was fabricated and characterized by spin-torque Ferromagnetic resonance (ST-FMR) technology. Large SOT efficiency together with high conductivity results in a giant spin Hall conductivity (0.2-2×105 h/2e (Ωm)−1) of PtTe2 thin films, which is one of the largest values among the presently reported TMDs. We further demonstrate that the SOT from PtTe2 layer can switch a perpendicularly magnetized CoTb layer efficiently. This work paves the way for employing PtTe2-like TMDs for wafer-scale spintronic device applications.

**References:**

[1] K. Deng, G. L. Wan, and S. Y. Zhou et al., Nat. Phys. 12, 1105 (2016) [2] X. Xu, W. Yao, D. Xiao and T. F. Heinz, et al., Nat. Phys. 10, 343 (2014) [3] H. T. Yuan, M. S. Bahramy, and N. Nagaosa et al., Nat. Phys. 9, 563 (2013) [4] Q. M. Shao, G. Q. Yu, et al., Nano Lett. 16, 7514 (2016) [5] D. MacNeill, G. M. Stiehl, and D. C. Ralph et al., Nat. Phys. 13, 300 (2017) [6] H.J. Xu, G.Q. Yu, and X.F. Han et al., Adv. Mater. 32, 2000513 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2e07e845455a23b928ba584487bf0019.jpg)
Figure 1 (a) Wafer-scale PtTe2 thin films with thicknesses of ~ 10 nm. (b) left, Cross-sectional HAADF-STEM image of a PtTe2/Py stack, where the interface is indicated by the white dashed lines; right, the layout of ST-FMR device with GSG connection. (c)Thickness dependence of efficiency of SOT(ξSOT) and spin Hall conductivity σs.

 High spin Hall conductivity in large-area type-II Dirac semimetal PtTe2

The chalcogenides GeTe [1] and SnTe [2] are the prototypes of ferroelectric Rashba semiconductors (FERSC), a class of materials in which ferroelectricity is coupled to the Rashba spin texture, enabling the non-volatile ferroelectric control of spin-to-charge conversion [3]. While the properties of GeTe as FERSC have been widely investigated [4, 5], SnTe is mostly known as topological material, probably due to the low Curie temperature (100 K) as bulk ferroelectric. However, attempts to bring the critical temperature of ferroelectric SnTe to room temperature [6] are motivated by the prediction of giant intrinsic spin Hall conductivity in its Rashba phase [7], with the possibility of controlling the spin-to-charge conversion ferroelectrically. Here we propose to identify some conditions leading to stable ferroelectric SnTe at room temperature by exploiting the dipolar interaction with ferroelectric GeTe. First, we study the growth of epitaxial thin SnTe(111) films on GeTe(111)/Si(111) by molecular beam epitaxy. Then, by spin and angular photoemission spectroscopy experiments, we access the band dispersion of the SnTe in such heterostructure as a function of temperature. The work shows the coexistence of topological and Rashba states in SnTe(111), suggesting that the ferroelectric distortion of SnTe on GeTe survives up at room temperature, possibly thanks to the dipolar interaction with the ferroelectric GeTe underneath. We find a nice agreement with density functional theory calculations [2] which also predict the tunability of topological and Rashba bands with the orientation of the ferroelectric polarization. These preliminary results open the way to the investigation of SnTe/GeTe for reconfigurable spin-based transistors based on the ferroelectric control of the spin transport. Acknowledgements. C.R. and S.P. acknowledges the project TWEET, grant no. 2017YCTB59 by MIUR.

**References:**

[1] D. Di Sante et al., Adv. Mater., Vol. 25, p. 509 (2013). [2] E. Plekhanov, S. Picozzi et al., Phys. Rev. B, Vol. 90, p. 161108(R) (2014) [3] Paul Noël et al., Nature, Vol. 580, p. 483-486 (2020) [4] C. Rinaldi et al., Nano Letters, Vol. 18, p. 2751 (2018). [5] C. Rinaldi et al., APL Mater., Vol. 4, p. 032501 (2016). [6] K. Chang et al., Science, Vol. 353, p. 274 (2016) [7] H. Wang et al., npj Comput. Mater., Vol. 6, p. 7 (2020)

**Images:**

 Coexistence of topological and Rashba states in ferroelectric SnTe

The family of two dimensional (2D) magnets has been expanded with tremendous speed since the two pioneering 2D magnetic insulators, Cr2Ge2Te6 [1] and CrI3 [2], were isolated in 2017, and has exhibited great potential in developing spintronic devices for next generation data storage [3]. Over the past four years, the magnetism of 2D magnets has been extensively studied by the full arsenal of probing techniques. Due to their sensitivity to external stimuli, various modulations, such as electron gating [4]–[6], have been taken to manipulate the magnetic properties of van der Waals (vdWs) systems. Such 2D magnets can be incorporated to form heterostructures with clean and sharp interfaces, which gives rise to exotic phenomena as a result of the interfacial proximity effect. Very recently, we have demonstrated the first inch-scale epitaxial two-dimensional ferromagnetic superlattices (Fe3GeTe2/CrSb)n, in which Fe shows a significantly reduced moment (1.05 µB/atom) [7] comparing to bulk Fe3GeTe2 [8]. Here we report a detailed x-ray magnetic circular dichroism (XMCD) study of the spin and orbital moments of this system. Fig. 1a shows the schematic diagram of the (4-layer Fe3GeTe2/CrSb)6 superlattices. These were grown in a Perkin Elmer 430 MBE on sapphire(0001) with a base vacuum of 10-9 Torr. The Fe3GeTe2 layers were grown at the substrate temperature of ∼310 °C, with the source temperatures of Fe (99.99%), Ge (99.999%) and Te (99.999%) at 1165 °C, 1020 °C and 285 °C, respectively, co-evaporated from Knudsen cells. The substrate temperature was changed to 280 °C when growing CrSb films with Cr (99.99%) and Sb (99.999%) cell temperatures of 1180 °C and 400 °C, respectively. The synchrotron-based x-ray magnetic circular dichroism (XMCD) technique was performed to unambiguously determine the spin and orbital moments of the superlattices. Oppositely circular polarized x-rays with 100% degree of polarization were used successively to resolve XMCD signals from each of the magnetic elements. The light-helicity was switched in a fixed magnetic field (5 T), which was applied in normal incidence with respect to the film plane and in parallel with the beam, as shown in Fig. 1b. The XMCD was obtained by subtracting the two x-ray absorption spectra (XAS), (σ- - σ+), with different helicity. Fig. 1c and 1d show typical pairs of XAS and XMCD spectra of the (Fe3GeTe2/CrSb)6 superlattices obtained at 3 K in total electron yield (TEY). The XAS of Fe L2,3 edges well resembles that of bulk Fe3GeTe2 [8], whilst that of Cr show multiple structures for both spin-orbit split core levels. The strongly dichroic spectra of Fe and Cr clearly indicate the ferromagnetic coupling between the Fe3GeTe2 and the CrSb layers in the superlattices. By applying sum rules, we obtained ms = 1.58 ± 0.1 µB/atom and ml = 0.22 ± 0.02 µB/atom for the Fe and ms = 0.94 ± 0.09 µB/atom and ml = 0.29 ± 0.03 µB/atom for the Cr, respectively. Unlike the reduced moment of the Fe3GeTe2 in (Fe3GeTe2/CrSb)3 [7], that in (Fe3GeTe2/CrSb)6 shows a bulk-like moment [8]. Future work to explore the tuning of the spin polarized band structure of both the 2D ferromagnetic superlattices via the interface engineering will be of great interest and have strong implications for both fundamental physics and the emerging spintronics technology.

**References:**

[1] C. Gong et al., “Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals,” Nature, vol. 546, no. 7657, pp. 265–269, 2017. [2] B. Huang et al., “Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit.,” Nature, vol. 546, no. 7657, pp. 270–273, 2017. [3] X. Lin, W. Yang, K. L. Wang, and W. Zhao, “Two-dimensional spintronics for low-power electronics,” Nat. Electron., vol. 2, no. July, pp. 22–24, 2019. [4] Y. Deng et al., “Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2,” Nature, vol. 563, no. 7729, pp. 94–99, Nov. 2018. [5] B. Huang et al., “Electrical control of 2D magnetism in bilayer CrI 3,” Nat. Nanotechnol., vol. 13, no. 7, pp. 544–548, 2018. [6] I. A. Verzhbitskiy et al., “Controlling the magnetic anisotropy in Cr2Ge2Te6 by electrostatic gating,” Nat. Electron., pp. 1–6, Jun. 2020. [7] S. Liu et al., “Two-dimensional ferromagnetic superlattices,” Natl. Sci. Rev., vol. 7, no. 4, pp. 745–754, Apr. 2020. [8] J. X. Zhu et al., “Electronic correlation and magnetism in the ferromagnetic metal Fe3GeTe2,” Phys. Rev. B, vol. 93, no. 14, p. 144404, 2016.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/964f0f91868b201d33c6236147e2d64c.jpg)
Figure 1. Element-specific magnetic states measured by XAS and XMCD of (Fe3GeTe2/CrSb)6 superlattice. (a) Schematic of (Fe3GeTe2/CrSb)6 superlattices, including the 4-layer Fe3GeTe2 (~ 3.2 nm) and CrSb (~ 1.6 nm). (b) Schematic of experimental setup of XMCD measurement. (c) Typical pair of XAS and XMCD spectra at Fe L2,3 edge obtained using TEY mode at 3 K and 5 T. (d) Typical pair of XAS and XMCD spectra at Cr L2,3 edge obtained using TEY mode at 3 K and 5 T. (XAS spectra are offset vertically for clarity).

 Bulk-like Magnetic Moments in Epitaxial Two-dimensional Superlattices

Recent discoveries of magnetic van der Waals (vdW) materials provide the ideal platform for exploring intrinsic two-dimensional (2D) magnetism down to the 2D limit and potential opportunities for new spin-related applications. However, according to Ginzburg criterion, 2D magnetism is associated with strong intrinsic magnetization fluctuations, which will break down the approximation of mean field theory in the neighborhoods of critical points [1]. In this talk, we will show the evidence for the magnetization-fluctuation-induced effective g factor anisotropy in the 2D ferromagnetic insulators CrXTe3 (X = Si, Ge) by means of ferromagnetic resonance (FMR). After deriving the classical and quantum models of magnetic resonance [2, 3], we confirm the dramatic anisotropic shift of the measured g factor to originate from fluctuations with anisotropic interactions. The deduction of the g factor behind the fluctuations is consistent with the spin-only state (g≈2.050(10) for CrSiTe3 and 2.039(10) for CrGeTe3). In addition, we will show that the abnormal enhancement of g shift, supplemented by specific heat and magnetometry measurements. We believe the results presented in this talk strongly suggest that CrSiTe3 exhibits a more typical two-dimensional nature than CrGeTe3 and may be closer to the quantum critical point.

**References:**

[1] B. Zappoli, D. Beysens, and Y. Garrabos, The ginzburg criterion, in Heat Transfers and Related Effects in Supercritical Fluids (Springer Netherlands, Dordrecht, 2015) pp. 371-372. [2] H. Beljers and J. Smit, Phil. Res. Rep 10 (1955). [3] K. Nagata, I. Yamamoto, H. Takano, and Y. Yokozawa, J. Phys. Soc. Jpn. 43, 857 (1977).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e990b75278b41650b030200dc271187b.png)
FIG. 1. (a) Coplanar waveguide with a rectangular single crystal placed parallel to the ab plane. The domain wall is represented by a blue solid line. (b) Illustration of three different resonant modes from the side view. (c) Frequency- and field-dependent FMR spectra (in-plane, β = π/2) for CrSiTe3 at 5 K. The solid lines represent fitting for the single domain mode and multi-domain mode. (d) Typical microwave transmission at 28 GHz sliced from (c). The resonant peak of the multidomain mode is formed by the superposition of a series of peaks with different angles α.

![](https://assets.underline.io/uploads/markdown_image/1/image/55280833cc11cb44be02ba76d089135f.jpg)
FIG. 3. (a,b) Temperature dependence of the effective g factor with the field applied both in-plane and out-of-plane for CST and CGT. The real g factor is calculated with g =1/3gab+2/3gc.

 Critical Fluctuations Induced g-factor Anisotropy in the Two-Dimensional Ferromagnetic Insulators CrXTe3 (X=Si, Ge)

Since the discovery of single-layer graphene in 2004, two-dimensional (2D) van der Waals (vdW) materials represented by it have demonstrated excellent electrical, magnetic, mechanical and optical physical properties under the structure of one or several layers of atomic thickness1. Based on these superior properties, 2D-layered vdW ferromagnetic materials have become the basis for constructing low-dimensional spintronics devices2, in which Fe3GeTe2 (FGT)3, Cr2Ge2Te64 and CrI35 as the main representative materials exhibit strong perpendicular magnetic anisotropy and other important characteristics in single layer. However, the Curie temperature (Tc) of above materials has not reached room temperature yet, which has greatly hinderer the subsequent development for device application. Therefore, recently researchers have been committed to explore 2D vdW ferromagnetic materials for room-temperature Tc in both theory and experiment. Here, we report that the interfacial engineering effect could effectively increase the Tc of the 2D vdW ferromagnetic material FGT from 230 K to 400 K, through heteroepitaxy with topological insulator of Bi2Te3. A theoretical calculation was further carried out to describe the magnetic properties by using first-principles calculations and the self-consistent Hubbard U approach (DFT+Uscf) together with the Monte Carlo (MC) simulations. After combination with Bi2Te3, the intralayer interactions in FGT was calculated to dramatically increase compared to that in pure FGT, well explaining the Tc enhancement. Our results may open up a new door to benefit the magnetic order in the 2D limit and realize spintronic devices based on 2D-layered vdW ferromagnetic materials with room temperature performances towards industrialization.

**References:**

1. Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6 (3), 183–191. 2. Lin, X.; Yang, W.; Wang, K. L.; Zhao, W. Two-Dimensional Spintronics for Low-Power Electronics. Nat. Electron. 2019, 2 (7), 274–283. 3. Liu, S.; Yuan, X.; Zou, Y.; Sheng, Y.; Huang, C.; Zhang, E.; Ling, J.; Liu, Y.; Wang, W.; Zhang, C.; Zou, J.; Wang, K.; Xiu, F. Wafer-Scale Two-Dimensional Ferromagnetic Fe3GeTe2 Thin Films Grown by Molecular Beam Epitaxy. npj 2D Mater. Appl. 2017, 1 (1), 30. 4. Gong, C.; Li, L.; Li, Z.; Ji, H.; Stern, A.; Xia, Y.; Cao, T.; Bao, W.; Wang, C.; Wang, Y.; Qiu, Z. Q.; Cava, R. J.; Louie, S. G.; Xia, J.; Zhang, X. Discovery of Intrinsic Ferromagnetism in Two-Dimensional van Der Waals Crystals. Nature 2017, 546 (7657), 265–269. 5. Huang, B.; Clark, G.; Navarro-Moratalla, E.; Klein, D. R.; Cheng, R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden, D. H.; Yao, W.; Xiao, D.; Jarillo-Herrero, P.; Xu, X. Layer-Dependent Ferromagnetism in a van Der Waals Crystal down to the Monolayer Limit. Nature 2017, 546 (7657), 270–273.

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 Room-temperature ferromagnetism in 2D vdW Fe3GeTe2 and its potential application

Transition metal dichalcohedides (TMDs) are a group of compounds with the chemical formula MX2, where M is a transition metal (e.g. Mo, W), X is a chalcogen (Te, Se, S, etc.). Such materials have a layered structure, where metal atom sheets are sandwiched between two chalcogen sheets. The atomic bonding within the X-M-X layer is covalent, while the interlayer coupling is realized due to the van der Waals interaction. Therefore, such materials are considered structurally and electronically two-dimensional. In addition, TMDs attract attention for their promising electronic properties [1, 2]. TMD WTe2, MoTe2 and MoxW1-xTe2 are known to exhibit the topological Weyl semimetal (TWS) features when crystalized in an orthorhombic structure [3, 4]. Such materials have promising prospects for use in spintronics and micro- and nanoelectronics due to unusual electronic and magnetic properties (e.g. extremely large magnetoresistance, high charge carrier mobility, spin-polarized transport) owing to their nontrivial topology of the electronic band structure. Since TWSs were discovered relatively recently, at present such materials have not yet been extensively studied and are a hot research direction. Therefore, the purpose of this work is a comprehensive study of the electrical, galvanomagnetic and optical properties of the WTe2 single crystal. WTe2 single crystals were grown by the chemical vapour transport method as described in [5]. XRD analysis revealed that WTe2 have an orthorhombic structure with the lattice parameters a = 3.435(8) Å, b = 6.312(7) Å and c = 14.070(4) Å. The chemical composition of the samples was confirmed by X-ray microanalysis using a FEI Inspect F scanning electron microscope equipped with an EDAX attachment. The temperature dependences of the electrical resistivity ρ(T) and galvanomagnetic properties (magnetoresistivity and Hall Effect) were measured by the standard four-probe method in the temperature range from 1.8 to 300 K in magnetic fields of up to 9 T. An electric current flowed in the (00l) plane of the sample, and the magnetic field vector was directed perpendicular to the (00l) plane. The optical properties were measured using the Beattie method with one reflection from the (00l) plane of the samples in the spectral range of 0.17–5.0 eV at room temperature. The studies carried out showed that the temperature dependence of the electrical resistivity ρ(T) has a “metallic” behaviour and ρ monotonically increases with temperature from 20 to 860 μOhm×cm according to a quadratic law at T &lt; 60 K, reaches a linear dependence with a tendency to saturation at higher temperatures. T2 – dependence of ρ at low temperatures is known to be explained by several mechanisms, in particular, strong electron-electron scattering. In addition, in [6, 7], it is shown that the electron–phonon–surface interference scattering mechanism can lead to ρ ~ T2 in pure metals. Since the WTe2 single crystal has a layered structure, interface scattering of current carriers can cause ρ ~ T2. The magnetoresistivity Δρxx/ρ of the WTe2 single crystal is found to reach 1700% at T = 1.8 K in a magnetic field of 9 T. The field dependence Δρxx/ρ(B) changes according to a law close to quadratic in the investigated range of fields. This behaviour of the magnetoresistivity is inherent in compensated conductors with a closed Fermi surface in the region of high effective magnetic fields [8]. The state of electron-hole compensation leads to a high mobility of current carriers, which is also characteristic of TWSs. The Hall Effect data obtained allow us to conclude that the majority charge carriers are electrons with the concentration n ~ 1019 cm-3 and the mobility μ ≈ 7500 cm2/V×s at T = 1.8 K. Optical studies show that, in the investigated spectral region, the optical conductivity spectrum σ(E) is one broad band centered at 3.4 eV, formed by interband transitions. In the infrared region of the spectrum, low values of the imaginary part ε2 and positive values of the real part of ε1 of the complex permittivity are observed, which gives grounds to conclude that there is no contribution from free carriers of up to the boundary of the investigated spectral region. The revealed features, such as the large magnetoresistivity, high mobility of charge carriers, along with their relatively low concentration, insignificant contribution from free carriers to optical conductivity, are apparently the manifestation of the topological nature of the material, however, to clarify it, further studies are required. The research was carried out within the state assignment of the Ministry of Education and Science of the Russian Federation (theme “Spin”, No. AAAA-A18-118020290104-2), supported in part by RFBR (project No. 20-32-90069) and the Government of Russian Federation (Decree No. 211, Contract No. 02.A03.21.0006).

**References:**

1. J. A. Wilson and A. D. Yoffe, Adv. Phys., Vol. 18, p. 193 (1969). 2. P. K. Das, D. D. Sante, F. Cilento et al., Electron. Struct., Vol. 1, p. 014003 (2019). 3. Y. Wu, D. Mou, N. H. Jo et al., Phys. Rev. B, Vol. 95, p. 121113(R) (2016). 4. I. Belopolski, D. S. Sanchez, Y. Ishida et al., Nat. Commun., Vol. 7, p. 13643 (2016). 5. A. N. Domozhirova, S. V. Naumov, S. M. Podgornykh et al., Accepted for publication in Journal of Physics: Conference Series (2021). 6. V. E. Startsev, V. P. D'yakina, V. I. Cherepanov et al., Zh. Eksp. Teor. Fiz., Vol. 79, p. 1335 (1980). 7. V. V. Marchenkov, Low Temp. Phys., Vol. 37, p. 852 (2011). 8. I. M. Lifshitz, M. Ya. Azbel and M. I. Kaganov, Electronic Theory of Metals (Moscow: Nauka) (1971).

**Images:**

 Electrical Resistivity, Galvanomagnetic and Optical Properties of WTe2 Single Crystal

Chalcogenide thin films have become of interest in energy conversion, as thermoelectric materials, and for spintronic applications. Amongst them, antimony telluride (Sb2Te3) and bismuth telluride (Bi2Te3) have gained attention as Topological Insulators (TI) [1,2]. In order to make a step toward technology transfer, it is of major importance to achieve epitaxial quality-TI on large area, Si-based substrates. We have recently developed Metal Organic Chemical Vapor Deposition (MOCVD) processes to grow Bi2Te3 and Sb2Te3 thin films on top of 4” Si(111) substrates [1,3]. In this contribution we report clear evidence of the existence of topologically-protected surface states (TSS) in both ~90 nm-thick Bi2Te3 and ~30 nm-thick Sb2Te3 films by making use of magnetotransport (MR) and angle resolved photoemission spectroscopy (ARPES) studies. MR measurements were performed in the Van der Pauw configuration on ~1 x 1 cm2 samples without any processing or capping layers, in the 5-295 K temperature range. Following MR, samples were analysed by ex situ ARPES. In order to make this possible, prior to ARPES, the samples were cleaned under vacuum by 1.5 keV Ar ion sputtering for about 15 sec at 10-5 mbar. The sputtering cycles were repeated as many times as necessary to obtain a clean surface free of C and O contaminants, as verified by in situ X-ray photoelectron spectroscopy. As a final step, annealing under vacuum was performed at 290 °C for ~10 min, in order to recover the damage induced by Ar+ sputtering. Finally, flat and well-ordered surfaces were obtained, as checked by streaky reflection high-energy electron diffraction patterns. ARPES spectra were acquired at room temperature with a 100 mm hemispherical electron analyzer equipped with a 2D CCD detector (SPECS). The He I (21.22 eV) resonant line was used to excite photoelectrons and the energy resolution of the system was greater than 40 meV. Both Sb2Te3 and Bi2Te3 films exhibited a metallic behaviour, reaching a resistivity of 0.83 mΩ cm and 1.4 mΩ cm at 5 K, respectively. From Hall measurements, we identified the carrier type, being holes in Sb2Te3 and electrons in Bi2Te3, as expected. The evolution of the carrier density with the lowering of the temperature turned out to be different for the two samples: an increasing of the hole density for the Sb2Te3 and a decreasing of the electron density for the Bi2Te3 were observed. The corresponding mobilities displayed a maximum at 5 K, suggesting a suppression of bulk conduction at low temperature with a potentially higher contribution from the TSS. Quite interestingly, at 5 K, we detected a 430% increase of electron mobility in Bi2Te3, to be compared with a marginal 6% increase of hole mobility in the case of Sb2Te3. In both Sb2Te3 and Bi2Te3, MR measurements highlighted the presence of clear weak antilocalization (WAL) at low temperature, as shown in figures 1 and 2. WAL was interpreted in the framework of the Hikami-Larkin-Nagaoka (HLN) model as a first proof of the existence of 2D-conduction channels connected to TSS [4]. The two HLN parameters α (being connected to the number of conducting channels) and the coherence length (lφ) were extracted by fitting the magnetoconductance values (MC). The α values were 0.3 and 0.8 for Sb2Te3 and Bi2Te3 respectively, meaning that in Bi2Te3 the 2D-conduction is highly dominating when compared to Sb2Te3, in agreement with the corresponding temperature behaviour of electron and hole mobilities. At 5.5 K, the lφ reached the value of 74 nm in Bi2Te3 and 55 nm in Sb2Te3, again indicating a more favourable TSS-connected transport in Bi2Te3 than in Sb2Te3. Comparing the obtained α and lφ values with those reported in the literature for Bi2Te3 grown by MBE [5], we observe a very similar value for α and a slightly lower coherence length for our material. For what concerns Sb2Te3, the obtained α and lφ values are still lower than those previously reported for Sb2Te3 grown by MBE [6]. As clearly shown in the insets of figures 1 and 2, ARPES measurements evidenced the typical Dirac-like band structure represented by a linear dispersion relation in both Sb2Te3 and Bi2Te3 (Fermi level EF is placed at 0 eV). ARPES showed that for both Sb2Te3 and Bi2Te3, the Dirac point is not exactly at EF, cutting the valence band in Sb2Te3 and the conduction band in Bi2Te3, in accordance with Hall measurements. ARPES data were in nice agreement with the partial overlap between TSS and bulk conduction observed at low T in transport measurements. This is most likely the explanation why, for both materials, we did not reach the ideal α=1 value expected for a pure TSS conduction. Our results showed that the TI properties of Bi2Te3 and Sb2Te3 grown by MOCVD on large areas Si substrates, are approaching those obtained by state-of-the-art methods, such as MBE, thus making a fundamental step toward potential technology transfer of TI. On the other hand, to enhance the TSS contribution in the MOCVD-grown TIs and, therefore, their functionalities, the Fermi level must be moved in the bulk band gap, closer to the Dirac point. [7]

**References:**

[1] R. Cecchini, R. Mantovan,.. and M. Longo, Phys. Status Solidi PRL, Vol 12(8), 1800155 (2018) [2] Y. L. Chen, J. G. Analytis,.. and Z. X. Shen, Science 325, 178-181 (2009) [3] M. Rimoldi, R. Cecchini,.. and R. Mantovan, RSC Advances, Vol 10(34), 19936-19942 (2020) [4] S. Hikami, A. I. Larkin, Y. Nagaoka, Prog. Theor. Phys., Vol 63(2), 707-710 (1980) [5] A. Roy, S. Guchhait, .. and S. K. Banerjee, Apl, Phys. Lett., 102(16) 163118, (2013) [6] Y. Takagaki, A. Giussani, & R. Calarco, J. Phys.: Condens. Matter 25(34), 345801 (2013) [7] C. Chang, P. Tang,.. & Q. Xue, Phys. Rev. Lett.115(13) 136801 (2015)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/944d6ea6f697f1478d5927dfced2b881.jpg)
1) HLN fit of the magnetoconductance and ARPES analysis of a Sb2Te3 film.

![](https://assets.underline.io/uploads/markdown_image/1/image/dc881fa1b2c83587acbe82b0640f1377.jpg)
2) HLN fit of the magnetoconductance and ARPES analysis of aBi2Te3 film.

 Magnetotransport and ARPES studies of large-area Sb2Te3 and Bi2Te3 topological insulators grown by MOCVD on Si

Superconductors (SCs) with Ising spin-orbit coupling (SOC), which internally pin electron spins to out-of-plane (OOP) directions such that OOP Cooper pairing can be stabilized against both orbital and paramagnetic effects of in-plane (IP) applied magnetic fields [1-3], have been of particular focus as a central ingredient for the topological protection of spin-triplet Cooper pairs and Majorana fermions [4,5]. Here, we experimentally examine how this so-called Ising superconductivity [1-3] affects the conversion efficiency of magnon spin to quasiparticle charge in a superconducting flake of 2-H NbSe2, transferred on top of Y3Fe5O12-based non-local magnon devices [6]. Comparing with a reference device composed of a spin-singlet superconducting Nb thin film [7], we show that the Y3Fe5O12-induced IP exchange spin-splitting in the NbSe flake is hindered by its inherent OOP SO-field, which in turn limits the transition-state enhancement of spin-to-charge conversion efficiency. Our results may have implications for proximity-controlled equal-spin tripet pairing [8] via tuning of IP exchange and OOP SO fields in Ising SC/insulating ferromagnet bilayers.

**References:**

[1] J. M. Lu, O. Zeliuk, I. Leermakers, N. F. Q. Yuan, U. Zeitler, K. T. Law, and J. T. Ye, Science 350, 1353 (2015). [2] Y. Saito et al., Nature Phys. 12, 144 (2016). [3] X. Xi, Z. Wang, W. Zhao, J. Park, K. T. Law, H. Berger, L. Forro, J. Shan, and K. F. Mak, Nat. Phys. 12, 139 (2016). [4] S. M. Frolov, M. J. Manfra, and J. D. Sau, Nat. Phys. 16, 718 (2020). [5] Benjamin T. Zhou, Noah F. Q. Yuan, Hong-Liang Jiang, and K. T. Law, Phys. Rev. B 93, 180501(R) (2016). [6] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, and B. Hillebrands, Nat. Phys. 11, 453 (2015). [7] K.-R. Jeon, J.-C. Jeon, X. Zhou, A. Migliorini, J. Yoon, and S. S. P. Parkin, ACS Nano 14, 15874 (2020). [8] J. Linder and J. W. A. Robinson, Nat. Phys. 11, 307 (2015).

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