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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 thickness&lt;sup&gt;1&lt;/sup&gt;. Based on these superior properties, 2D-layered vdW ferromagnetic materials have become the basis for constructing low-dimensional spintronics devices&lt;sup&gt;2&lt;/sup&gt;, in which Fe&lt;sub&gt;3&lt;/sub&gt;GeTe&lt;sub&gt;2&lt;/sub&gt; (FGT)&lt;sup&gt;3&lt;/sup&gt;, Cr&lt;sub&gt;2&lt;/sub&gt;Ge&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;6&lt;/sub&gt;&lt;sup&gt;4&lt;/sup&gt; and CrI&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;5&lt;/sup&gt; as the main representative materials exhibit strong perpendicular magnetic anisotropy and other important characteristics in single layer. However, the Curie temperature (T&lt;sub&gt;c&lt;/sub&gt;) 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 T&lt;sub&gt;c&lt;/sub&gt; in both theory and experiment. Here, we report that the interfacial engineering effect could effectively increase the T&lt;sub&gt;c&lt;/sub&gt; of the 2D vdW ferromagnetic material FGT from 230 K to 400 K, through heteroepitaxy with topological insulator of Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;. 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 Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;, 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. &lt;i&gt;Nat. Mater.&lt;/i&gt; 2007, &lt;i&gt;6&lt;/i&gt; (3), 183–191.&lt;br/&gt;2. Lin, X.; Yang, W.; Wang, K. L.; Zhao, W. Two-Dimensional Spintronics for Low-Power Electronics. &lt;i&gt;Nat. Electron.&lt;/i&gt; 2019, &lt;i&gt;2&lt;/i&gt; (7), 274–283.&lt;br/&gt;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. &lt;i&gt;npj 2D Mater. Appl.&lt;/i&gt; 2017, &lt;i&gt;1&lt;/i&gt; (1), 30.&lt;br/&gt;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. &lt;i&gt;Nature&lt;/i&gt; 2017, &lt;i&gt;546&lt;/i&gt; (7657), 265–269.&lt;br/&gt;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. &lt;i&gt;Nature&lt;/i&gt; 2017, &lt;i&gt;546&lt;/i&gt; (7657), 270–273.

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INTERMAG 2021

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

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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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technical paper

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 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)

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:**

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

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

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