France



Recent discoveries of gate-tunable magnetism in ferromagnetic two-dimensional (2D) materials such as CrI&lt;sub&gt;3&lt;/sub&gt;and Fe&lt;sub&gt;3&lt;/sub&gt;GeTe&lt;sub&gt;2&lt;/sub&gt; 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 Cr&lt;sub&gt;2&lt;/sub&gt;Ge&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;6&lt;/sub&gt; (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 NbFeTe&lt;sub&gt;2&lt;/sub&gt; and magnetically doped transition metal dichalcogenides.

**References:**



**Images:**





INTERMAG 2021

 Electrical and chemical control of magnetism in layered ferromagnetic semiconductors


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The energy cost of storing and moving data is enormous, and is growing at such a rapid pace that within a few years the power demand may overwhelm some developed nations. Non-volatile memory (NVM) and neuromorphic computing are enabling critical advances in this area, but there are many scientific and technological challenges. This conference will cover the materials challenges of different memory technologies, from the fundamental processes and mechanisms, to industry perspectives on materials specifications and manufacturing.

Materials Challenges for Memory

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:**

technical paper

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In this paper, to accurately analysis the magnetic properties of high temperature superconductor, the elemental operator model is proposed. The calculated and measured results of silver-sheathed Bi2223 superconducting tape samples under external magnetic field are compared to show the effectiveness and accuracy of the proposed method. I. Introduction The hysteresis phenomenon of high temperature superconductor (HTS) is caused by the pinning of magnetic flux due to various defects, as illustrated in Fig.1, such as voids, normal inclusions, grain boundaries, and compositional variations. The consequence of flux-pinning is that the flux remains constant in the interior of the HTS until a new flux is applied. This phenomenon constitutes a memory in the HTS, which can be well described by Bean’s critical-state model. Based on the mechanisms of superconductor, the HTS magnetic characteristic can be described via an infinite set of antimagnetic operators and magnetic operators. The property of antimagnetic operator and magnetic operator can be obtained by the antimagnetic curves and elementary hysteresis loops of different switching values of magnetic field, respectively[1,2]. In this paper, in the framework of the elemental operator and wind-rose method, as mentioned in [3], the underlying principle, experimental verification, and the error analysis of the elemental operator have been introduced. II. Elemental Operator Method In the proposed model, based on the mechanisms of HTS, it is assumed that lots of interacting magnetic elemental operators and antimagnetic operators. The magnetic elemental operators, which possesses its own biaxial anisotropy and anisotropy coefficient are enclosed in the HTS material. The interacting elemental operator which possesses its own biaxial anisotropy and anisotropy coefficient. Then, the magnetization direction of each biaxial elemental operator is determined from the various energy contributions of the material anisotropy and applied field. Similar to the determination method of Stoner-Wohlfarth (S-W) particle, the stable orientation of the magnetization in the HTS structure can be obtained by minimizing the energy of the elemental operator. With this wind–rose curve, graphical interpretation can be given for the magnetization process of this biaxial elemental operator. In this Hx-Hy plane, four regions can be separated by the wind–rose curve, and different region corresponding to different number of the energy minima exists. The energy barrier of elemental operator under different magnetic field can decide the position of energy minima, which means the stable orientation of the magnetization, Besides, the determination of the actual energy minimum is related to the magnetization history. Based on the energy barrier diagrams of the elemental operator, the number of energy minimum and the stable orientation of the magnetization can be obtained. When the magnetic field applies inside the wind–rose curve, with the increase of magnetic field, four, three and two energy minima may exist corresponding to different magnetic field magnitudes. When the magnetic field applies outside the wind–rose curve, the magnetization only has one stable orientation as there is only one energy minimum. For the bulk magnetic hysteresis, a collection of biaxial elemental operators can be considered with different easy axis orientations and weights associated to each operator. To simulate the magnetic properties of different HTS materials by this model, the different distribution functions and parameters are needed. Each HTS has an optimal distribution function and parameters, which deliver the best results. With different parameters of the distribution function, this model can be employed to predict the magnetic characteristic of different HTS materials. III. Experimental verification The experiment was carried out on a Bi-2223 multi-filamentary tape with an external magnetic field parallel to a wide surface of the specimen and perpendicular to the long axis of filaments, and the magnetization versus the applied magnetic field, M−H, over the HTS sample at the temperature of 77.3 K was recorded, as shown in Fig.2. The magnetization curves measured and simulated by the presented method, respectively. The comparison illustrated that the elemental operator model that we have proposed agrees accurately with the experimental results for both the limiting hysteresis loop and major loops.

**References:**

[1] N. N. Duan, W. J. Xu, S. H. Wang, et al., Journal of Applied Physics, vol. 117, no. 17, p. 17A718( 2015). [2] N. N. Duan, W. J. Xu, S. H. Wang, et al., IEEE Transactions on Magnetics, vol. 51, no. 3, p. 7300904(2015). [3] W. J. Xu, N. N. Duan, S. H. Wang, et al., IEEE Transactions on Industrial Electronics, vol. 64, no.3, pp. 2459-2467(2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/41dabfe68862d3b320947bd99a3b67c8.jpg)
Different state of HTS under external magnetic field

![](https://assets.underline.io/uploads/markdown_image/1/image/4ebaada7745baf31ece68e99909b43c4.jpg)
Hysteresis loops of Bi2223 HT3-PA sample at 77K

 Magnetic Characteristic Analysis of High Temperature Superconductors by the Elemental Operator Model

1. Introduction Iron loss calculation is important for efficiency prediction of electric machines. To establish a general iron loss model which is applicable to a wide range of magnetization and frequency, the physical phenomena such as micro-eddy currents, pinning effect, etc. during the dynamic domain wall motion should be one option [1]. The authors have proposed a dynamic iron loss modeling method considering the pinning effect macroscopically and the calculation accuracy is confirmed to be improved [2]. However, in the electromagnetic field analysis, the static hysteresis phenomenon was not considered. In [3], a method considering the static hysteresis phenomenon in the electromagnetic field analysis by introducing the reactive magnetic field Hhys is proposed. In this paper, we applied this method and proposed a method to calculate Hhys simply by using the Fourier series expansion. The iron losses and the dynamic hysteresis loops under different frequencies are obtained and compared with the measured ones. 2. Analysis model and method The analysis model is one non-oriented silicon steel (JIS C 2550-1986: 50A470, thickness: 0.5mm). Only half region is analyzed due to symmetry. Sinusoidal flux are imposed by using the Dirichlet boundary conditions. The nonlinear eddy current analysis with A-Φ (A: magnetic vector potential and Φ: electrical scalar potential) method is carried out to obtain the flux distribution and eddy current distribution. The static hysteresis phenomenon is considered by using the reactive magnetic field Hhys expressed by a Fourier series. The B-H hysteresis loops due to the static hysteresis phenomenon and eddy currents are calculated using the obtained flux distribution. B of each time step is calculated by averaging all the flux density in the steel region and H is calculated by averaging all the flux density in the coating region then divided by the permeability in the vacuum. 3. Results and Discussions The obtained iron losses are shown in Fig. 1 and compared with the measured ones. It shows that the iron loss calculation accuracy is much improved by considering the static hysteresis phenomenon. The obtained dynamic hysteresis loops under different frequencies are shown in Fig. 2 and compared with the measured ones. It shows that by considering the static hysteresis phenomenon in the electromagnetic field analysis, the accuracy of dynamic hysteresis loops are much improved. More investigation will be given in the full paper.

**References:**

[1] Y. Gao, Y. Matsuo, and K. Muramatsu, “Investigation on simple numeric modeling of anomalous eddy current loss in steel plate using modified conductivity,” IEEE Trans. on Magn., vol. 48, no. 2, pp. 635-638, 2012. [2] W. Guan, H. Qian, F. Fang, Y. Luo, J. Yang, B. Chen, Y. Gao, and K. Muramatsu, “Numerical modeling of excess loss in SiFe sheet considering pinning effect,” IEEE Trans. on Applied Superconductivity, vol. 26, no. 7, Art. no. 0608804, 2016. [3] K. Yamazaki and Y. Sakamoto, “Electromagnetic field analysis considering reaction field caused by eddy currents and hysteresis phenomenon in laminated cores,” IEEE Trans. on Magn., vol. 54, no. 3, Art. no. 1300204, 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8fc2099b05a6fb55c4adc9d809686c93.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/689cdb0f0ae1f69e2fd9431c96c48d80.jpg)

 Iron loss calculation of non-oriented silicon steel considering hysteresis using Fourier series expansion

Magnetic films with perpendicular magnetic anisotropy (PMA) are an important subject of research, as they are used in nonvolatile memory devices and sensors. Moreover, they are also interesting as ferromagnetic systems with nontrivial configurations of magnetization. This is due to the fact that in PMA materials the influence of magnetocrystalline anisotropy and magnetostatic self-interaction are opposite to each other. Being projected onto the film plane, the emerging magnetization distribution can look like a stripe or bubble domain state, or as a labyrinth domain structure [1]. Along with this, the magnetization near the film boundaries differs from the magnetization inside the film, and the domain walls (DWs) have a vortex structure [2]. As shown in [3, 4], this system may contain a large number of vertical and horizontal Bloch lines (VBLs and HBLs), as well as Bloch points (BPs). In this work, we numerically investigate the process of magnetization reversal in a Co(001) PMA film containing an antidot array [5] (a system of columnar defects). In our case, these defects are square holes. We show that these holes can lead to appearance of stable HBLs located on the DWs and pinned to adjacent holes. The formation of HBLs and VBLs is accompanied by the transfer of the topological charge. We numerically calculated the distribution of magnetization in a 180° DW located between two holes. For this, we have minimized the total magnetic energy in a cuboid with dimensions X = 150 nm, Y = 200 nm, Z = 800 nm, using the mumax3 [6] solver (Y is the film thickness). Periodic boundary conditions were imposed along the z axis. To visualize HBLs and VBLs, we numerically calculated the density of the gyrotropic vector [3]. The topological charges of BPs were calculated by integrating the gyrotropic vector over closed surfaces. The following magnetization distributions were found: (a) two VBLs without BPs, (b) the HBL connecting holes and containing a BP, (c) the HBL connecting holes and not containing a BP. The domain structure in a cuboid with dimensions X = 1024 nm, Y = 1024 nm and Z = 200 nm, containing an antidot array was also simulated (Z is the film thickness). The period of the rectangular array was chosen to be 100 nm or 200 nm. Hysteresis loops were calculated by minimizing the total energy including the Zeeman energy. The magnetic field step was chosen to be 250 Oe. The initial condition for each step was chosen as the result of the previous energy minimization. The simulation results are shown in Figs. 1, 2. The static domain structure in Fig. 1 was obtained for the period of the array 100 nm. We can see that the complex configuration of magnetization within the antidot array entails a nontrivial distribution of HBLs and BPs. In Fig. 2 the hysteresis loops are shown for the case of continuous film and for antidot arrays with periods 100 nm and 200 nm (the magnetic field is perpendicular to the film surface).

**References:**

[1] L. Fallarino, O. Hovorka and A. Berger. Phys. Rev. B, Vol. 94, 064408 (2016). [2] M. Kisielewski, A. Maziewski, V. Zablotskii. JMMM Vol. 316, p. 277 (2007). [3] E. Zh. Baykenov, I. M. Izmozherov, V. V. Zverev. Bull. Russ. Acad. Sci.: Phys. Vol. 83, p. 809 (2019). [4] I. M. Izmozherov, E. Zh. Baykenov, V. V. Zverev. Phys. Met. Metallogr. Vol. 122, No. 3 (2021) (in print). [5] T. Schneider, M. Langer, J. Alekhina, et al. Sci. Rep. Vol. 7, 41157 (2017). [6] A. Vansteenkiste, J. Leliaert, M. Dvornik, et al. AIP Advances. Vol. 4, 107133 (2014).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/7ebecceb44c66a9aacc2926578898de6.jpg)
Fig.1. Domain structure in an antidot Co film. The HBLs are marked.

![](https://assets.underline.io/uploads/markdown_image/1/image/58c058a74cb35089c9aba5435b426f7d.jpg)
Fig.2. Hysteresis loops for the case of continuous film and for antidot arrays with periods 100 nm and 200 nm.

 3D Simulations of Domain Wall Structure and Hysteresis in Cobalt Films with Columnar Defects

There is electromagnetic interference in the DC power supply system of metro, which will produce electrochemical corrosion to the adjacent metal structure of rail. Buried metal pipeline is one of the main objects of electrochemical corrosion. Due to the characteristics of buried pipeline laying and the randomness of electromagnetic leakage, it is difficult to directly and effectively analyze the corrosion law of buried pipeline under electromagnetic interference. In addition, due to the lack of corrosion detection, it is easy to cause metro operation accidents. 1.Introduction In recent years, many scholars have indirectly evaluated the electromagnetic interference degree and electrochemical corrosion of buried metal by the pipe-to-soil voltage [1-2]. Due to the characteristics of buried pipeline laying and the randomness of electromagnetic distribution, it is difficult to directly and effectively analyze the corrosion law. Some scholars have tried to investigate the electromagnetic interference of buried pipelines from the perspective of electrochemical corrosion mechanism [3-4]. However, the existing research is mainly focused on the static corrosion problem, which is lack of considering the operation of metro vehicles, so there is no direct basis for quantitative analysis of the corrosion area of buried pipelines. In this paper, the model and verification platform for dynamic electromagnetic interference are established, and the impact of metro vehicle operation state on corrosion area is analyzed quantitatively, as well as the cumulative effect of electrochemical corrosion under weak electromagnetic interference. 2.Simulation experiment This research set up an experimental platform for simulating stray current corrosion under the motion of subway vehicles as shown in Fig.1. Based on the corrosion experiment and simulation of buried pipeline, the dynamic corrosion process and law of buried pipeline metal are studied under the action of Variable electromagnetic interference. The graphite electrode (positive) indicates a moving metro vehicle, and its movement is realized by a slider block controlled by a stepper motor. The movement process of the slider block simulates the actual movement of the subway vehicle. The programmable DC power supply provides dynamic current that leaks during the movement of the graphite electrode (positive). The leakage current of the positive graphite electrode forms an electrochemical corrosion environment in the simulated soil solution, and finally flows back through two graphite electrodes (negative), and the buried pipeline specimen was placed in a simulated soil solution. The above process truly simulates the double side power supply system of metro. The finite element simulation model was used to simulate the entire process of buried pipeline corrosion under the dynamic electromagnetic interference. Finally, the experimental results and simulation results were compared and analyzed. 3.Results When the graphite electrode (positive electrode) moves to the 40th second, the corrosion current density distribution of buried pipeline is shown in Figure 2 (a). When the corrosion current value is negative, it indicates the current density of the outgoing pipeline, and when the corrosion current value is positive, it indicates the current density of the incoming pipeline. Therefore, the corresponding anode area and cathode area are divided on the buried pipeline. The metal of buried pipeline in anode area is corroded by electronic loss caused by corrosion current. The pipeline metal in anode area is corroded by the electronic loss caused by corrosion current. The closer the anode area of the buried pipeline is, the greater the corrosion current density of the outgoing pipeline is. Obviously, the metal surface of both ends of the buried pipeline is seriously corroded. With the frequent start and stop of metro vehicles, the anode area of buried pipeline is constantly changing. 4.Conclusions The results show that under the dynamic electromagnetic interference, the metal of buried pipeline is bipolar, and the anode area (corrosion area) changes with the running state of metro vehicles. Therefore, in the case of no excavation detection of buried pipelines, the corrosion area caused by electromagnetic interference can be quantitatively identified by analyzing the operation of metro vehicles. And the cumulative effect of stray current cannot be neglect under the condition of uniform speed of metro vehicles.

**References:**

1.Zakowski, W. Sokólski, Corrosion Science., Vol.41, p.2213(1999). 2.S. R.Allahkaram, M. Isakhani-Zakaria, Journal of Natural Gas Science and Engineering.,Vol.26, p.453(2015). 3.K. Tang, Materials Today Communications., Vol.20, p.100564(2019). 4.S. Qian, Y. F. Cheng, Construction and Building Materials.,Vol.148, p.675(2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/90129ab39a06aa2fb7896aeea1e13316.jpg)
Fig.1 Corrosion experiment device for buried pipelines under stray current

![](https://assets.underline.io/uploads/markdown_image/1/image/a8c5a08f26844c87bed15fc0555feda5.jpg)
Fig.2 Comparative analysis of stray current corrosion of buried pipelines under electromagnetic interference

 Quantitative Analysis of Buried Pipeline Corrosion under Dynamic Electromagnetic Interference in DC-Subway

The recent discovery of anti-skyrmions[1] and elliptical block skyrmions[2] in lamellae formed from single crystals of inverse tetragonal ternary Heusler compounds makes these compounds extremely interesting. Related binary compounds host chiral domain walls whose current induced motion in films that are only 1 unit cell thick has recently been observed. These films were prepared using a novel method of chemical templating[3]. We show that strain can dramatically affect the current induced domain wall motion in such films, changing both its magnitude and, remarkably, even its direction. Anti-skyrmions are extremely interesting objects that have high stability[4] and whose size can be tuned by more than an order of magnitude [5] by varying the thickness of the host lamella. These properties derive from the symmetry of the bulk Dzyaloshinskii-Moriya vector exchange interaction that stabilizes them that itself is derived from their underlying crystal structure. We have found similar properties in other materials with related symmetries[6]. The current induced motion of anti-skyrmions has not yet been realized but theoretical models suggest that the skyrmion Hall angle will vary strongly with current direction and, for certain crystal directions, will be zero. Whilst we have found no evidence, to date, for current induced motion of such objects from volume spin-transfer torques, models show that spin-orbit torques should give rise to their motion. For this purpose, ultra-thin 2D layers are needed that can be coupled to heavy metal layers in which spin currents can be generated. In this talk we discuss progress in preparing high quality, epitaxial, ultra-thin films of ternary inverse tetragonal Heusler compounds that are only 1 to 2 unit cells thick and the exploration of non collinear spin textures in these films as well as their current induced motion. Finally, we discuss the first observation of fractional anti-skyrmions and Block skyrmions in nano-stripes formed from inverse tetragonal ternary Heusler compounds.

**References:**

[1] Nayak, A.K., et al., Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature, 2017. 548(7669): p. 561-566. [2] Jena, J., et al., Elliptical Bloch skyrmion chiral twins in an antiskyrmion system. Nat. Commun., 2020. 11: p. 1115. [3] Filippou, P.C., et al., Chiral domain wall motion in unit-cell thick perpendicularly magnetized Heusler films prepared by chemical templating. Nat. Commun., 2018. 9: p. 4653. [4] Saha, R., et al., Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn1.4Pt0.9Pd0.1Sn Nat. Commun., 2019. 10: p. 5305. [5] Ma, T., et al., Tunable Magnetic Antiskyrmion Size and Helical Period from Nanometers to Micrometers in a D2dHeusler Compound. Adv. Mater., 2020: p. 2002043. [6] Srivastava, A.K., et al., Observation of Robust Néel Skyrmions in Metallic PtMnGa. Adv. Mater., 2020. 32: p. 1904327.

**Images:**

 Ultra-thin Heusler films and lamellae that host non-collinear spin textures

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

The isolation of a large variety of two-dimensional materials (2DM) and their co-integration in van der Waals heterostructures has opened new avenues for innovative material design and engineering. In partcular, the structural interface quality in the atomically smooth 2DMs has led to alternative and versatile strategies beyond simply combining materials functionalities. Indeed, properties of interest can be imprinted in a chosen 2DM by proximity-induced effects. Such an approach is compelling for spintronic devices, which harness their functionality from thin layers of magnetic and non-magnetic materials and their interfaces. In this talk, I will present recent experiments in which we investigate the spin dynamics in graphene-transition metal dichalcogenides heterostructures. I will show that the spin-orbit coupling in graphene can be strongly enhanced by proximity effects. As a consequence, the spin relaxation becomes highly anisotropic, with spin lifetimes that are markedly different depending on the spin orientation [1,2]. I will further demonstrate that the proximity-induced spin-orbit coupling leads to strongly enhanced spin-to-charge interconversion [3]. By performing spin precession experiments in appropriately designed Hall bars (Fig. 1) , we are able to separate the contributions of the spin Hall and the spin galvanic effects. Remarkably, their corresponding conversion efficiencies can be tailored by electrostatic gating in magnitude and sign, peaking near the graphene charge neutrality point and having a large magnitude even at room temperature (Fig. 2).

**References:**

[1] L. A. Benítez, J. F. Sierra, W. Savero Torres, et al., Nature Phys. 14 (2018) [2] L. A. Benítez, J. F. Sierra, W. Savero Torres, et al., APL Materials 7, 120701 (2019) [3] L. A. Benítez, W. Savero Torres, J. F. Sierra, et al., Nature Mater. 19, 170 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/0c37217fbfd7b923b7029b34ffa3ace3.jpg)
Fig. 1. Spin-to-charge interconversion (SCI) in graphene by proximity of a TMDC. A current I generates a transverse spin current owing to the SHE (red arrows) and an in-plane spin density due to the inverse SGE (blue arrow).

![](https://assets.underline.io/uploads/markdown_image/1/image/cbd947cb028a646afecb3febb8cb18b5.jpg)
Fig. 2. Tunable SCI as a function of carrier density, modulated by the gate voltage Vg.

 Spin-Orbit Proximity Phenomena in Van der Waals Heterostructures

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

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.

Electrical and chemical control of magnetism in layered ferromagnetic semiconductors

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