France



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.&lt;br/&gt;&lt;br/&gt;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.&lt;br/&gt;&lt;br/&gt;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.&lt;br/&gt;&lt;br/&gt;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.&lt;br/&gt;&lt;br/&gt;Acknowledgements. C.R. and S.P. acknowledges the project TWEET, grant no. 2017YCTB59 by MIUR.

**References:**

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

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INTERMAG 2021

 Coexistence of topological and Rashba states in ferroelectric SnTe

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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:**

technical paper

The bilayer heterostructures composed of an ultrathin ferromagnetic metal (FM) and a nonmagnetic material hosting strong spin-orbit coupling (SOC) are a principal resource for spin-orbit torque (SOT) [1] and spin-to-charge conversion [2] effects in next generation spintronics. The key to understand these effect is current-driven nonequilibrium spin density [3]. For example, it generates SOT when it is noncollinear to the direction of local magnetization and it can arise due to variety of microscopic mechanisms, including the spin Hall effect, spin-orbit proximity effect and different interfacial scattering mechanisms. The recently discovered two-dimensional (2D) magnetic materials [4] offer new avenue for highly efficient and gate- or disorder-tunable SOT in van der Waals (vdW) heterostructures composed of few monolayers of atomically thin materials where the spin Hall effect from the bulk is absent. Using first-principles quantum transport calculations, which combine nonequilibrium Green functions with noncollinear density functional theory [1], we predicted [5] that injecting unpolarized charge current parallel to the interface of bilayer-CrI3/monolayer-TaSe2 vdW heterostructure will induce SOT-driven dynamics of magnetization on the first monolayer of CrI3 that is in direct contact with metallic transition metal dichalcogenide (TMD) TaSe2. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we find that this can reverse the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting bilayer CrI3 from antiferromagnet to ferromagnet which can be detetected by passing vertical current and is of potentially great interest to magnetic memory applications since it does not require any external magnetic field. We explain the mechanism of such current-driven nonequilibrium phase transition by showing that first monolayer of CrI3 becomes conducting due to doping by evanescent wavefunctions injected by metallic TaSe2, while concurrently acquiring strong SOC via this proximity effect. Another vdW heterostructure exhibiting SOT is doubly proximitized graphene, which is neither magnetic nor hosts SOC in its isolated form, but proximity induced magnetic moments will exhibit SOT in Cr2Ge2Te6/graphene/WS2 vdW heterostructure which can be tuned by two orders of magnitude via the gate voltage [6].

**References:**

[1] B. K. Nikolić, K. Dolui, M. Petrović, P. Plecháč, T. Markussen, and K. Stokbro, in W. Andreoni and S. Yip (eds.), Handbook of Materials Modeling (Springer, Chan, 2018); arXiv:1801.05793. [2] F. Mahfouzi, N. Nagaosa, and B. K. Nikolić, Phys. Rev. B 90, 115432 (2014). [3] P.-H. Chang, T. Markussen, S. Smidstrup, K. Stokbro, and B. K. Nikolić, Phys. Rev. B 92, 201406(R) (2015). [4] M. Gibertini, M. Koperski, A. F. Morpurgo, and K. S. Novoselov, Nat. Nanotech. 14, 408 (2019). [5] K. Dolui, M. D. Petrović, K. Zollner, P. Plecháč, J. Fabian, and B. K. Nikolić, Nano Lett. 20, 2288 (2020). [6] K. Zollner, M. D. Petrović, K. Dolui, P. Plecháč, B. K. Nikolić, and J. Fabian, Phys. Rev. Res. 2, 043057 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6d48e3a29a576e94d159077e7a25839b.jpg)
Spin-orbit-torque-operated van der Waals heterostructures from Ref. [5].

![](https://assets.underline.io/uploads/markdown_image/1/image/af911046f4ff240162efc2a61c514019.jpg)
Spin-orbit-torque-operated van der Waals heterostructures from Ref. [6].

 Spin-Orbit Torque in van der Waals Heterostructures of Magnetic Two-Dimensional Materials

Spintronic beyond CMOS devices need a charge/voltage readout of their magnetic state for efficient and direct integration with other electronic components [1]. The magnetoresistance readout, though efficient for a memory system, is limiting in logic technology as it does not generate an electromotive force to drive other circuit elements [2]. We propose a readout mechanism, using local spin injection devices, that uses large spin-orbit coupling effects to generate charge current/voltage as an output [3,4]. We show that the inverse spin Hall effect in Ta can generate an output voltage of 0.32 mV in a local spin injection device and, we demonstrate the favorable dimensional scaling of this novel readout mechanism [5]. Layered 2D materials that have large inverse Edelstein effect lengths, long scattering time constants and high in-plane resistivity can increase the output voltage of spin injection devices beyond heavy metals. In this talk, we will discuss our experimental work in Bi2Te3, Sb2Te3, and BiSb2Te3 to optimize spin to charge transduction efficiency.

**References:**

[1] Nikonov, D. E. & Young, I. A., Benchmarking of beyond-CMOS exploratory devices for logic integrated circuits. IEEE J. Explor. Solid-State Computat. Devices Circuits 1, 3–11 (2015) [2] Manipatruni, S., Nikonov, D. E. & Young, I. A. Beyond CMOS computing with spin and polarization. Nat. Phys. 14, 338 (2018). [3] Manipatruni, S., Nikonov, D.E., Lin, CC. et al. Scalable energy-efficient magnetoelectric spin–orbit logic. Nature 565, 35–42 (2019). [4] Pham, V.T., Groen, I., Manipatruni, S. et al. Spin–orbit magnetic state readout in scaled ferromagnetic/heavy metal nanostructures. Nat Electron 3, 309–315 (2020). [5] C. Lin et al., "Experimental demonstration of integrated magneto-electric and spin-orbit building blocks implementing energy-efficient logic," 2019 IEEE International Electron Devices Meeting (IEDM).

**Images:**

 Optimization of spin to charge transduction efficiency for magnetic state readout.

The Dzyaloshinskii-Moriya interaction (DMI) plays a tremendous role for fast domain wall motion and allows creation of magnetic skyrmions which are very promising for spintronic devices. A standard ingredient in such devices is an interface between ferromagnetic metal and heavy nonmagnetic metal (HM), where the HM provides large spin orbit coupling as the energy source to obtain strong DMI[1]; or using Rashba effect at interface between a light element and ferromagnetic metal [2]. However, using a single two-dimensional (2D) magnet to obtain DMI and create skyrmions is rare even though the discoveries of truly 2D magnets such as Cr2Ge2Te6 CrI3, VSe2, MnSe2 and Fe3GeTe2 etc. have opened up new opportunities for spintronic technology. We demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides MnXY (X/Y = S, Se, Te, X ≠ Y) in which the difference between X and Y on the opposites sides of Mn breaks the inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are comparable to those of state-of-the-art ferromagnet/heavy metal (FM/HM) heterostructures. In addition, by performing Monte Carlo simulations, we unveil that at low temperatures the ground states of the MnSeTe and MnSTe monolayers can transform from ferromagnetic states with worm-like magnetic domains into the skyrmion states by applying an external magnetic field. At increasing temperature, the skyrmion states start fluctuating above 50 K before evolution to a completely disordered structure at higher temperature. The present results pave the way for new device concepts utilizing chiral magnetic structures in specially designed 2D ferromagnetic materials[3]. Moreover, we unveil that topological magnetism and its electric control can both be achieved in the two-dimensional (2D) magnetoelectric multiferroics. We verify that significant DMI can promote the stabilization of sub-10 nm skyrmion in 2D multiferroics with perpendicular magnetic anisotropy such as CrN monolayer. [4]

**References:**

[1] Hongxin Yang, André Thiaville, Stanislas Rohart, Albert Fert, Mairbek Chshiev, Physical Review Letters 115, 267210(2015) [2] Hongxin Yang, Gong Chen, Alexandre AC Cotta, Sergey A Nikolaev, Edmar A Soares, Waldemar AA Macedo, Kai Liu, Andreas K Schmid, Albert Fert, Mairbek Chshiev Nature Materials 17, 605 (2018) [3] Jinghua Liang, Weiwei Wang, Haifeng Du, Ali Hallal, Karin Garcia, Mairbek Chshiev, Albert Fert, Hongxin Yang Phys. Rev. B 101, 184401 (2020) Editors’ suggestion [4] Jinghua Liang, Qirui Cui, Hongxin Yang, Physical Review B 102, 220409 (2020) (R)

 Significant Dzyaloshinskii-Moriya Interaction in Two-Ddimensional Janus Structures and its Electrically Control in 2D Magnetoelectric Multiferroics

Recent discoveries of gate-tunable magnetism in ferromagnetic two-dimensional (2D) materials such as CrI3and Fe3GeTe2 highlight the unique potential of this class of materials for novel spintronic devices. Understanding the interplay between magnetic order, free electron density, and electric field in the 2D limit is essential for uncovering the full potential of these materials. In this talk, we will first discuss electrical control of magnetism in Cr2Ge2Te6 (CGT), a van der Waals ferromagnetic semiconductor, in an electric double-layer transistor (EDLT) geometry. We show that degenerately electron-doped CGT exhibits enhanced Curie temperature of up to 200 K, and rotation of its easy axis from out-of-plane to in-plane orientation. We demonstrate that similar changes in Curie temperature and easy axis can be induced by Na intercalation, which is accompanied by heavy electron doping of the host. We will further discuss our exploration of magnetism in other van der Waals materials such as NbFeTe2 and magnetically doped transition metal dichalcogenides.

**References:**



**Images:**

 Electrical and chemical control of magnetism in layered ferromagnetic semiconductors

Low-dimensional 2D semiconductor magnetic materials have been attracting much attention, due to their potential applicability in a wide range of technologies, such as data storage, sensing, spintronic devices or in biomedical applications. In this respect, materials composed by 2D layers bonded to one another through weak van der Waals interactions present enormous potential for several of these applications. They often exhibit strongly anisotropic behaviour and can be cleaved into very thin specimens, frequently monolayer crystals. In this regard, chromium triiodide (CrI3) is a Van der Walls material with an intrinsic ferromagnetic nature, with a relatively high Curie temperature of about 61 K in its bulk form. Previous research on CrI3 shows that although this material has been of great interest to researchers in recent years, its synthesis by alternative methods has not been considered. In other words, in most of the reported research, a bulk sample has been prepared by the chemical vapour transport (CVT) method and CrI3 monolayer, bilayer or multi-layer samples are then, typically, made by mechanical exfoliation methods. This is a cumbersome sample preparation procedure and the resultant samples are prone to light, air and moisture induced degradation. To prevent the latter, new and efficient synthetic strategies and storage methods are required. Additionally, in order to design efficiently aligned 2D magnetic material devices, it is very important to control their morphology and magnetization orientation, to take advantage of the CrI3 anisotropic magnetic properties. In this regard, the electrospinning technique is a relatively simple and low-cost technique, which has been significantly developed in recent years. Electrospinning is based on the extrusion of a thin fibre from an electrically charged polymer precursor solution, mixed with the active functionalizing inclusions, provoked by a strong applied DC electric field. The solution is loaded into a syringe with its needle connected to a high voltage source. At the threshold voltage, when the electrostatic forces overcome the surface tension of the precursor solution, a jet is continuously emitted that dries in-flight, forming the fibres. They are then collected as a mesh on a grounded collector plate. Electrospinning is a very powerful tool for controlling the morphology and molecular orientation of the polymer chains along the fibres. The fibre diameters can vary from several nanometers up to several micrometres, depending on the parameters of the process, while their lengths can reach several meters. This method is, then, very effective for creating oriented inclusions encapsulated in the polymer matrix, allowing the production of functionalized fibre arrays with anisotropic properties. As such, in this wok, we demonstrate the implementation of this approach for synthesising highly oriented chromium triiodide (CrI3) magnetic inclusions inside nano/microfibres with a polyethylene oxide matrix (PEO) by the electrospinning technique. The samples are identified as PCx in which x represents the mass amount of CrI3 as the precursor. The structural, microstructural, and spectroscopic analysis shows uniformly dispersed CrI3 nanosized inclusions inside the fibres, as seen in HAADF-STEM images (Figure 1). The inclusions present a C2/m monoclinic structure [1] at room temperature, where their c-axis is perpendicular to the fibre mat plane and the ab layers are in-plane. Analysis of the magnetic properties show that the samples have a ferromagnetic - paramagnetic phase transition at ~ 55-56 K, lower than that of bulk CrI3 [2,3]. Noticeably, a field-driven metamagnetic transition is observed below ~ 45 K, from M vs. H curves, when the applied magnetic field is perpendicular to the fibre mat plane (H^ab), while it is strongly reduced when the field is in-plane (H||ab) (Figure 2). This metamagnetic transition occurs at around a transition field (H*) of 19 kOe at 5 K and shifts to lower fields by increasing the temperature (inset of Figure 2). This anisotropic behaviour is attributed to the field-induced changes from antiferromagnetic to ferromagnetic interlayer magnetic moment alignment along the CrI3 c-axis stacked layers [4], induced by the perpendicular applied magnetic field. These CrI3 electrospun fibres then show an efficient cost-effective route to synthesize magnetic composite fibres with highly oriented van der Walls inclusions, for spintronic applications, taking advantage of their anisotropic 2D layered materials properties.

**References:**

[1] M-A. McGuire et al., Chemistry of Materials, Vol. 27, p. 612-620 (2015) [2] Y. Liu et al., Scientific reports, Vol. 9, p.1-8 (2019) [3] S. Mondal et al., Phyical. Review B, Vol. 99 p. 180407 (2019) [4] V. Bayzi Isfahani et al., Nanotechnology (2021), https://doi.org/10.1088/1361-6528/abd4a3

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/62650995b8c36b3b252b07b545b68332.jpg)
Figure 1. HAADF-STEM images showing the nanosized CrI3 inclusions inside the fibres.

![](https://assets.underline.io/uploads/markdown_image/1/image/ea4d662a3d47ba0d94cb2fd4796bbe64.jpg)
Figure 2. M vs. H hysteresis curves obtained with the applied magnetic field perpendicular to the fiber mat plane. Inset: Variation of metamagnetic transition field (H*) with temperature.

 Magnetic nano/microfibres with highly oriented van der Waals CrI3 inclusions by electrospinning

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

In the last years 2D materials have attracted a huge attention thanks to the amazing properties that arise when thickness approaches the single layer level and thanks to the large number of functionalities that they offer. Recently, they started to appear as high potential candidates for spintronics devices since, thanks to their intrinsic two-dimensional nature, they create thin, tuneable and free of defects barriers with sharp interfaces. Despite this potential, investigation of 2D materials for spintronics is still at the beginning. Only very recently few works started to appear using other 2D materials beyond graphene [1,2,3]. In particular, concerning the family of transition metal dichalcogenides (TMDCs), promising theoretical calculations were first reported predicting an extremely large magnetoresistance (MR) signal up to 300% in Fe/MoS2/Fe MTJs [4]. However, experimental results are still far from these expectations. Indeed, most of experimental works have to face the main technological issue of avoiding bottom FM electrode oxidation during fabrication process, limiting in turn MR signals. Here we will show an in-situ fabrication approach to avoid oxidation of the FM electrodes during the device fabrication process that tackles this fundamental problem. This allowed the successful fabrication of NiFe/MoS2/Co MTJs with multilayer MoS2 mechanically exfoliated flakes (Fig. 1) showing the highest results reported so far for MoS2-based spin valve devices (Fig. 2) [5] and confirming that our in situ fabrication protocol allows maintaining high quality non-oxidized interfaces between the ferromagnetic electrodes and the 2D layer. Beyond interfaces and material quality, we suggest that an overlooked more fundamental physics issue related to spin-current depolarization could explain the limited MR observed so far in MoS2-based magnetic tunnel junctions [5,6]. This points to a path towards the observation of larger spin signals in line with theoretical predictions above 100%. We envision the impact of our work to be beyond MoS2 and its broader transition-metal dichalcogenides family by opening the way to an accelerated screening of other 2D materials that are yet to be explored for spintronics.

**References:**

[1] W. Wang, A. Narayan, L. Tang et al, Nano Lett., Vol. 15, p. 5261–5267, (2015). [2] A. Dankert, P. Pashaei, M. Venkata Kamalakar et al., ACS Nano, Vol. 11, p. 6389–6395, (2017). [3] W.C. Wong, S. M. Ng, H. F. Wong et al., IEEE Trans. Magn., Vol. 53, p.1600205 (2017). [4] K. Dolui et al., Phys. Rev. B, Vol. 90, p. 041401(R), (2014). [5] M. Galbiati et al., Phys. Rev. Appl. Vol. 12, p. 044022, (2019). [6] M. Galbiati et al., ACS Appl. Mater. Interfaces, 10, 30017 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fe90119ad6598a010681d8782003c9a8.jpg)
Fig 1: Schematic of the NiFe/MoS2/Co spin valve.

![](https://assets.underline.io/uploads/markdown_image/1/image/d863f48060c6f728a5ec5e58746534c6.jpg)
Fig. 2: Magnetoresistance signal measured in this NiFe/MoS2/Co spin valve.

 Spin Valves with Exfoliated 2D Materials: MoS2

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 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).

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