Session A40: Skyrmions in Two Dimensional Systems

Live

Magnetic skyrmions [1] are at the core of many recently proposed spintronic devices. Most of the thin film multilayers hosting magnetic skyrmions [2-3] are characterized by [heavy metals]\ferromagnet interfaces to generate a strong Dzyaloshinskii-Moriya interaction (DMI) and usually an external magnetic field is needed to stabilize the skyrmions. From an application point of view, both the use of heavy metals (rare and expensive) and the need for an external magnetic field (incompatible with nanoelectronics due to scalability issues) are limitations. Accordingly, it is highly desirable to have [light metal]-based systems capable of hosting skyrmions at zero field.
Here, I will first discuss the observation of magnetic domain walls with uni-directional sense of rotation in [light-metal]\ferromagnet multilayers. Density functional theory calculations allows us to understand the origin and predict the strength of the DMI in those systems, which compares well with experimental values. Subsequently, I will discuss how such systems can be employed for the stabilization of isolated magnetic skyrmions at room temperature and zero magnetic field via interlayer exchange coupling (IEC) [4,5]. By carefully adjusting the IEC in the system we can tune the size of the observed skyrmions. Those findings open up the possibility to develop cost-effective skyrmion-based spintronic devices suitable for general-user applications which go beyond modern nanoelectronics.

[1] N. Romming et al., Science 341, 636 (2013).
[2] C. Moreau-Luchaire et al., Nat. Nanotechnol. 11, 444 (2016).
[3] A. Soumyanarayanan et al., Nat. Mater. 16, 898 (2017).
[4] G. Chen et al., Appl. Phys. Lett. 106, 242404 (2015).
[5] R. Lo Conte et al., Nano Lett. 20, 4739 (2020).   

Live

Magnetic skyrmions are promising candidates for achieving low-power, next-generation synaptic computing due to their particle-like nature, strong coupling to electrical and spin currents, and room-temperature (RT) stability in multilayer films¹. However, in most homogeneous multilayers, the predominant magnetic textures at zero-field (ZF) are labyrinthine stripes². External applied magnetic field or geometric confinement³ are typically used to stabilize isolated skyrmions.

In this work, we design heterogeneous multilayers with two adjoining functional stacks with tunable interlayer exchange coupling (IEC)^4,5. Using magnetic force microscopy (MFM) and Lorentz transmission electron microscopy (LTEM), we investigate the evolution of magnetic textures with varying IEC as well as applied external field. Crucially, we show that isolated magnetic skyrmions could be stabilized within a finite IEC window. This work establishes and extends the use of IEC as a means of stabilizing ZF skyrmions and paves the path to skyrmion-based devices.

1. Fert, A. et al. Nat. Rev. Mater. 2, (2017).
2. Soumyanarayanan, A. et al. Nat. Mater. 16, (2017).
3. Ho, P. et al. Phys. Rev. Appl. 11, 024064 (2019).
4. Chen, G. et al. Appl. Phys. Lett. 106, 242404 (2015).
5. Legrand, W. et al. Nat. Mater. 1–9 (2019)   

Live

We investigate the prospects for stable, compact 10 nm sized skyrmions in ferromagneically and antiferromagneticially coupled Co/Pt-based multilayers from first-principles calculation of the spin-stiffness, the Dzyaloshinskii-Moriya interaction (DMI), magnetic anisotropy and the interlayer coupling. Employing a multiscale approach, we transfer the microscopic parameters determined from ab initio to a micromagnetic energy functional and determine the skyrmion radius as function of temperature and film thickness. We found an analytical expression for the skyrmion radius that covers our numerical results and is valid for a large regime of micromagnetic parameters. We focus on monatomic layer thick fcc-stacked (111)-oriented metallic Z/Co/Pt (Z = 4d series: Y–Pd, the noble metals: Cu, Ag, Au, post-noble metals: Zn and Cd) and on systems of thicker layers.   

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Magnetic skyrmions are quasiparticles with potential applications in memory and logic devices. Controlling the interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in manipulating skyrmion properties. We present a systematic analysis of the DMI variation in ferrimagnetic Pt/GdCo/Pt_1-xW_x multilayer as a function of Tungsten (W) composition (x) using Density Functional Theory (DFT). We find that ~10% W in the capping layer produces a non-zero DMI, and the DMI increases as the W composition increases but saturates at higher W composition in agreement with the experiment. We show that the vanishing of spin-orbit coupling (SOC) energy to the adjacent metal layers of the top interface and the simultaneous constancy of the bottom interface is responsible for such a saturating behavior of the DMI. Furthermore, we investigate Pt/GdCo/X (X = Ta, W, Ir) to demonstrate the impact of capping layer heavy metals on the DMI. We predict that W in the capping layer favors a higher DMI than Ta and Ir that shows a matching trend with the experimentally measured Spin Hall angle. Our results open up exciting combinatorial possibilities for controlling the DMI in ferrimagnets to nucleate and manipulate ultrasmall high-speed skyrmions.   

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Topological magnetic states, such as skyrmions, experience rapid recent development due to their potential application in advanced spintronics. Such topics become even more intriguing and promising, considering the recent progresses in 2D magnets, such as Fe₃GeTe₂ with near room-temperature Curie temperature. However, despite quite a few studies on Fe₃GeTe₂, it still lacks a reliable model to describe the skyrmionics in Fe₃GeTe₂, especially the elusive Néel-type vs Bloch-type. Here we perform theoretical studies and report our newly developed first-principles-based effective Hamiltonian, which not only reproduces the complex topological states in Fe₃GeTe₂, but also sheds light on the elusive topics.   

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Noncollinear spin textures in low-dimensional magnetic systems have been studied for decades because of their extraordinary properties and promising applications derived from their chirality and topological nature. However, material realizations of topological spin states are still limited. Employing first-principles and Monte Carlo simulations, we propose that monolayer CrCl₃, can be a promising candidate for observing the vortex/antivortex types of topological defects, so-called merons. Beyond the typical bimerons picture, higher-order states which involves more than two merons, are also identified in simulations. By perturbing with external magnetic field, we further show the robustness of these meron pairs and reveal a rich phase space to tune the hybridization between the ferromagnetic order and meron-like defects. The signatures of topological excitations under external magnetic field also provide crucial information for experimental justifications. Our study predicts that two-dimensional magnets with weak spin-orbit coupling can be a promising family for realizing meron-like spin textures.   

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Magnetic skyrmions in two-dimensional (2D) chiral magnets are often stabilized by a combination of
Dzyaloshinskii-Moriya interaction and external magnetic field. Here, we show that skyrmions can also be
stabilized in twisted moire superlattices with Dzyaloshinskii-Moriya interaction in the absence of an external
magnetic field. Our setup consists of a 2D ferromagnetic layer twisted on top of an antiferromagnetic substrate.
The coupling between the ferromagnetic layer and the substrate generates an effective alternating exchange
field. We find a large region of skyrmion crystal phase when the length scales of the moire periodicity and
skyrmions are compatible. Unlike chiral magnets under magnetic field, skyrmions in moire superlattices show
enhanced stability for the easy-axis (Ising) anisotropy which can be essential to realize skyrmions since most
van der Waals magnets possess easy-axis anisotropy.   

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Antiskyrmions can potentially be supported by C2_v thin films which permit both Rashba- and Dresselhaus-type Dzyaloshinskii-Moriya interactions (DMI). This work presents C2_v Ag/Pt/Co-based films as potential antiskyrmion hosts. X-ray diffraction 2θ-ω scans of Ag/Pt/Co/Pt films grown epitaxially on Si(110) single crystal substrates show that Co adopts a (10.0) hexagonal closed-packed (hcp) orientation. These findings were corroborated by selected area electron diffraction patterns and further supported by a strong in-plane uniaxial anisotropy measured by alternating gradient field magnetometry. Crosstie domain walls in these films were directly observed by Lorentz transmission electron microscopy, which are also predicted from micromagnetic simulations. To promote perpendicular magnetic anisotropy and a net DMI field in these films, we fabricate thinner, asymmetric Ag/Pt/[Co/Ni]_x/Pt films. X-ray diffraction analysis suggest that the hcp structure is preserved throughout the Ni layers. Structural and magnetic observations of this lower symmetry film stack are compared to micromagnetic simulations. Finally, we discuss approaches to enhancing the expected Dresselhaus-type contribution.   

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Magnetic skyrmions are nanoscale spin textures in magnetic systems with strong Dzyaloshinskii–Moriya interaction (DMI). Recently, magnetic skyrmions have been observed in a two-dimensional (2D) van der Waals ferromagnetic materials, such as Fe₃GeTe₂, which is promising for atomic-scale spintronic devices. The electric control of skyrmions in these materials is an important functionality for the potential application. Here, we predict that the DMI and the related skyrmion behaviors in a Fe₃GeTe₂ monolayer can be controlled by ferroelectric polarization of an adjacent ferroelectric 2D van der Waals In₂Se₃ monolayer. Based on first-principles density functional theory calculations and micromagnetic modeling, we find that the symmetry breaking induced by the ferroelectric polarization of In₂Se₃ generates a sizable DMI in this 2D van der Waals heterostructure. The magnitude of DMI can be further controlled by switching the ferroelectric polarization, leading to notable changes in the skyrmion formation. The predicted electrically controlled skyrmions can be useful for spintronic devices based on the proposed van der Waals heterostructure.   

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Van der Waals magnets have drawn great attention since the discovery of intrinsic 2D ferromagnetism. Due to the extraordinary correlations among charge, spin, orbital and lattice degrees of freedom, layered magnetic transition metal chalcogenides (TMCs) is a rich area for uncovering exotic topological phenomena. Particularly, magnetic skyrmions have high potential for ultra-dense memory and logic devices while skyrmions are yet to be experimentally found in TMC monolayers (MLs) despite theoretical predictions. Here we report the realization of quasi-2D TMC Cr₂Te₃ down to 1 ML on SrTiO₃ (111) by molecular beam epitaxy (MBE). The spin-glass behavior in thick films has enabled the investigation of magnetic skyrmions in the ML limit, while robust ferromagnetism survives in insulating ML film (T_c =17 K). Topological Hall effect and unusual hysteresis observed in transport/magnetometry measurements unveil the emergence of stable magnetic skyrmions, down to 1 ML. The unique concomitant occurrence of 2D ferromagnetism and skyrmionic behavior in ultrathin Cr₂Te₃ films widens TMC-based material possibilities for various quantum hybrid studies.   

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We propose to study new physical effects based on electron scattering on magnetic skyrmions and vortexes distributed in a 2D ferromagnetic material. We show that the topological spin Hall effect (TSHE) can be efficiently employed for the filtering, switching, and separation of spin currents. For some values of the parameters (conduction electron concentrations, and skyrmion/vortex sizes) it is possible to separate Hall currents for different electron spin projections as it is like for different carrier charges (electrons and holes) in the normal Hall effect. The calculations are performed using the Boltzmann kinetic equation for the nonequilibrium distribution function and the Lippmann-Schwinger equation for the transition matrix in the whole range of the adiabaticity parameter. The spin filtering due to the skyrmion/vortex scattering can be several orders of magnitude more efficient in the narrow range of the electron concentrations than that of the ordinary ferromagnetic spin polarization in spintronics   

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Magnetic skyrmions – nanoscale topological spin structures stabilised by chiral interactions – are promising candidates for next-generation computing. As such devices may operate at room temperature and beyond, it is critical to study the role of temperature on skyrmions. Skyrmions can form from magnetic stripes with increasing field via two distinct mechanisms – the reversible shrinking of a stripe into a skyrmion or the irreversible fission of a stripe into multiple skyrmions [1]. In this work, we investigate Ir/Fe/Co/Pt multilayers [2] over temperatures of 100 - 350K using first order reversal curve (FORC) magnetometry [3] and Lorentz transmission electron microscopy (LTEM). We demonstrate an increased tendency for fission, resulting in increased skyrmion densities with increasing temperatures. With theoretical modelling, we interpret our results as the consequence of the temperature dependence of the fission energy barrier. These findings establish temperature as an important tuning parameter for skyrmion formation and stability, which is crucial to the realisation of skyrmionic devices.

[1] Tan et al., Physical Review Materials (2020), In Press.
[2] Soumyanarayanan, A. et al., Nature Materials 16, 898-904 (2017).
[3] J. E. Davies et al., Physical Review B 70, 224434 (2004).