Session A23: Chiral Spin Textures and Dynamics, Including Skyrmions I: Skyrmions

Skyrmions in synthetic antiferromagnets and their fast current induced dynamics without skyrmion Hall effect

Skyrmions are topological spin textures which hold great promise as nanoscale bits of information in memory and logic devices [1]. The recent demonstration of room temperature skyrmions [2,3] as well as their current induced motion in industry compatible sputtered thin films have lifted important roadblocks toward the realization of skyrmion based devices. However, their development is impeded by a too low current induced velocity (about 100 m/s) [4] as well as the skyrmion Hall effect, namely a motion transverse to the current direction due to their topological charge which can lead to their annihilation in tracks. Antiferromagnetic (AF) skyrmions allow these limitations to be lifted owing to their vanishing magnetization and net zero topological charge, promising fast dynamics without skyrmion Hall effect. In this presentation, I will address the stabilization and current induced manipulation of skyrmion in compensated synthetic antiferromagnetic (SAF). I will first show that skyrmions can be stabilized at room temperature in Pt/Co/Ru based compensated SAFs and nucleated using local current injection or ultrafast laser pulses [5]. I will then show that SAF skyrmions can be moved by current at velocities over 900 m/s without skyrmion Hall effect. Micromagnetic simulations and analytical models using experimental parameters show that this enhanced skyrmion velocity can be explained by the compensation of the topological charges as well as an enhanced spin orbit torque in the synthetic antiferromagnet. I will conclude the talk with recent results on the electrical nucleation and detection of a skyrmion in magnetic tunnel junctions, which is another important milestone for skyrmion based devices [6]. Our results open important paths toward the realization of logic and memory devices based on the fast manipulation of skyrmions.   

Spontaneous skyrmion beyond room temperature in a boron-doped CrTe

Magnetic skyrmions, vortex-like spin textures with great potential for applications in spintronics and memory devices, are typically observed in a variety of magnetic materials that include centrosymmetric and non-centrosymmetric systems [1-3]. The magnetic ground state of these materials often consists of periodic helices or stripe domains, while skyrmions are observed only in the presence of an externally applied magnetic field. Achieving skyrmion stabilization without a magnetic field is challenging and typically requires extrinsic conditions such as geometrical confinement or sweeping the magnetic field in a decreasing mode starting from the skyrmion lattice state [4,5]. Therefore, potential uses for skyrmions are currently limited by the necessity of an external magnetic field, which makes the device design more complex. Consequently, the quest for spontaneously emerging magnetic skyrmions without an external magnetic field is of utmost importance, as it holds the potential to significantly reduce the complexity of device design and eliminate the need for a magnetic field or the current source to write the memory bits. Our study, using the state-of-the-art real-space Lorentz transmission electron microscopy technique, enabled the direct observation of spontaneous skyrmions beyond room temperature in a centrosymmetric Cr_0.9B_0.1Te crystal without the presence of external magnetic fields. We delve into the role of dipole-dipole interactions in the stabilization of these zero-field skyrmions by comparing results for different sample thicknesses. 

[1] S. Mühlbauer et. al., Science 323, 915 (2009).

[2] S. Parkin et. al., Science 320, 190 (2008).

[3] A. Fert et. al., Nat. Nanotechnol. 8, 152 (2013).

[4] J. Jena et. al., Sci. Adv. 6, eabc0723 (2020).

[5] J. Jena et. al., Nat. Commun. 11, 1115 (2020).   

Spin Dynamics of the Centrosymmetric Skyrmion Material GdRu2Si2

Magnetic skyrmion crystals are traditionally associated with non-centrosymmetric crystal structures; however, it has been demonstrated that skyrmion crystals can be stabilized by competing interactions in centrosymmetric crystals [1]. To understand and optimize the physical responses associated with topologically-nontrivial skyrmion textures, it is important to quantify their magnetic interactions by comparing theoretical predictions with spectroscopic data. Here, we present neutron diffraction and spectroscopy data on the centrosymmetric skyrmion material GdRu₂Si₂ [2,3], and show that the key spectroscopic features can be explained by the magnetic interactions obtained using density-functional theory calculations [4]. We further discuss how the agreement with our experimental data differs between the initially-proposed helical ground state and the double-q "topological charge stripe" ground state determined experimentally [5], and how the magnetic structure evolves with temperature.

[1] Okubo et al., Phys. Rev. Lett. 108, 017206 (2012); [2] Yasui et al., Nat. Commun. 11, 5925 (2020); [3] Khanh et al., Nat. Nanotech. 15, 444-449 (2020); [4] Bouaziz et al., Phys. Rev. Lett. 128, 157206 (2022); [5] Wood et al., Phys. Rev. B 107, L180402 (2023).   

Room-temperature magnetic field stability of skyrmion bubbles in ultrathin films with Dzyaloshinskii-Moriya interaction

The Dzyaloshinskii-Moriya interaction (DMI) at heavy-metal/ferromagnet interfaces can stabilize chiral spin textures, such as magnetic skyrmions. Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviors and have potential in highly energy efficient spintronic device applications. However, the room temperature stability of skyrmion bubbles has not been quantified experimentally. Here, we show that when the ratio of the DMI effective field to the perpendicular anisotropy field is large (domain wall energy is low), expanding bubble domains leave behind fine-scale magnetic dendritic structures. These dendritic structures can be manipulated to form stable skyrmion bubbles, consisting of chiral Néel domain walls. To quantify the stability of the skyrmion bubbles, we imaged skyrmion bubbles in Pt/Co/GdO_x films using wide-field Kerr microscopy as a function of in-plane and out-of-plane field.  Histograms of the skyrmion annihilation fields were obtained for several in-plane fields. We show that in-plane fields reduce the annihilation threshold of the skyrmions, lowering the barrier to annihilation. The skyrmion annihilation field becomes deterministic at large in-plane fields above the DMI effective field. Micromagnetic simulations qualitatively confirm these measurements and suggest an enhanced skyrmion stability due to the DMI.   

Machine learning for crystallization dynamics of skyrmions in itinerant chiral magnets

Itinerant magnets exhibit complex magnetization textures due to the long-range electron-mediated spin-spin interactions, which depend intimately on the underlying electron Fermi surface. The resultant effective spin Hamiltonian is often highly frustrated, giving rise to non-coplanar spin structures that endow the electrons with a nontrivial Berry phase. Of particular interest is magnetic skyrmions that play a crucial role in the emerging field of spintronics. However, accurate large-scale Landau-Lifshitz-Gilbert (LLG) dynamics simulation of itinerant magnets is computationally highly demanding due to the electron degrees of freedom, which have to be integrated out on-the-fly at every time-step. Here we present a general and scalable machine learning (ML) model for efficient and accurate prediction of local electron-induced effective fields. To demonstrate our approach, we incorporate the ML model, which is trained from small-size exact solutions, into large-scale LLG simulations to systematically study the phase-ordering dynamics of skyrmion crystals in the well-known s-d model of itinerant magnets. Our work opens a new avenue for multi-scale dynamical modeling of metallic spin systems.   

Antiskyrmions and anisotropic magnetic domain structures in S4 symmetry magnets

Antiskyrmions, new topological spin textures with topological numbers of opposite sign to those of skyrmions, have attracted much attention in the research field of topological magnetism. Antiskyrmions are stabilized by anisotropic Dzyaloshinskii–Moriya interaction and magnetic dipolar interaction in non-centrosymmetric magnets with D_2d and S₄ symmetry. Thus far, however, antiskyrmion studies have been restricted to Heusler alloys with D_2d symmetry. Recently, we have discovered a new antiskyrmion-host material with S₄ symmetry, Pd-doped schreibersite (Fe,Ni,Pd)₃P. Antiskyrmions were observed in thin plates over a wide temperature region, including room temperature, using Lorentz transmission electron microscopy [1]. Furthermore, antiskyrmions and elliptic skyrmions were found to be interconverted by varying magnetic field and lamella thickness. Systematic compositional tuning studies have revealed that the stability of antiskyrmions is governed by uniaxial magnetic anisotropy and demagnetization energy, as well as anisotropic Dzyaloshinskii–Moriya interaction [2]. Because of the important role of magnetic dipolar interaction, the magnetic texture size increases and the magnetic domain pattern becomes more complex with increasing crystal thickness. Magnetic force microscopy has shown that sawtooth-shaped anisotropic fractal magnetic domain patterns appear near the surface of thick crystals. In addition, small-angle neutron scattering measurements have revealed a three-dimensional fractal structure of the magnetic domain walls in bulk single crystals [3].   

Instability of magnetic skyrmion strings induced by longitudinal spin currents

One of the fascinating aspects of two-dimensional topological spin textures, so-called magnetic skyrmions, is their interplay with spin currents. It is well established that spin-transfer torques exerted by in-plane spin currents give rise to a motion of magnetic skyrmions resulting in a skyrmion Hall effect. In films of finite thickness or in three-dimensional bulk samples the skyrmions extend in the third direction forming a string. This string is aligned with the applied magnetic field and nonreciprocal spin waves can propagate along skyrmion strings. In this work, we investigate the influence of spin currents that flow parallel to the skyrmion string. We show that, remarkably, such a current component immediately destabilizes the string in a clean system. This instability is caused by the longitudinal current leading to the emission of translational Goldstone modes with finite wavevectors along the string, in contrast to the transversal current that couples only to the Goldstone mode with zero momentum. As a result, helix-shaped deformations develop, whose amplitudes grow with time and eventually break the string. Employing an analytical stability analysis complemented by micromagnetic simulations, we demonstrate that both a single string and skyrmion string lattice are destabilized by this mechanism.   

Title: Quantum Skyrmions in Ferromagnets via Local Magnetic Field Control

Our proposed model suggests a way to create quantum skyrmions on a ferromagnet by locally adjusting the magnetic field. This is achieved by using a triangular lattice and taking into account the Heisenberg exchange and Dzyaloshinskii-Moriya interactions. These interactions play an important role in stabilizing the skyrmion structure and in creating the appropriate ground state. The model provides a theoretical framework to understand the behavior of skyrmions on a ferromagnet, however, further experimental research is necessary to verify it. This approach can lead to the discovery of new materials and techniques for skyrmion manipulation, with potential applications in various fields.   

Skyrmion dynamics and applications in neuromorphic computing

Skyrmionics and neuromorphics are among the most promising fields of physics with the perspective of creating future devices and technologies. Magnetic skyrmions are nanoscale magnetic whirls that are topologically protected and can be moved by currents, leading to the prediction of several applications. Its topological charge leads to high stability; however, it also leads to the skyrmion Hall effect. From memory storage devices, like the racetrack memory, to computing devices, like artificial neurons, this shortcoming is one of the primary reasons why skyrmion-based spintronic devices have yet to be achieved. Here, we study the motion of skyrmions with different topological charges and helicities. Using an effective center-of-mass description of these magnetic quasiparticles, namely, the Thiele equation, we analyze their dynamics under different gradient landscapes and interactions aiming to suppress or take advantage of the skyrmion Hall effect. Following a neuroscience approach, we also discuss possible applications in neuromorphic computing.   

Magnetoelectric Cavity Magnonics in Skyrmion Crystals

Recently, a strong coupling between magnons and microwave photons in a cavity has attracted much attention. Previous works have focused on the magnetic coupling between magnons and photons via the Zeeman effect. In comparison, multiferroic materials host electromagnons that can be excited by oscillating electric fields. Here, we present a theory of magnetoelectric magnon-photon coupling in cavities hosting noncentrosymmetric magnets. Analogously to nonreciprocal phenomena in multiferroics, the magnetoelectric coupling is time-reversal and inversion asymmetric. This asymmetry establishes a means for exceptional tunability of magnon-photon coupling, which can be switched on and off by reversing the magnetization direction.

Taking the multiferroic skyrmion-host Cu₂OSeO₃ as an example, we reveal the electrical activity of skyrmion eigenmodes and propose it for magnon-photon splitting of ``magnetically dark'' elliptic modes. Furthermore, we predict a cavity-induced magnon-magnon coupling between magnetoelectrically active skyrmion excitations. We discuss applications in quantum information processing by proposing protocols for all-electrical magnon-mediated photon quantum gates and a photon-mediated SPLIT operation of magnons. Our study shows magnetoelectric cavity magnonics as a novel platform for realizing a quantum-hybrid system.