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1. Inducing a tunable skyrmion-antiskyrmion system through ion beam modification of FeGe films

   https://doi.org/10.1038/s44306-024-00013-8  

   Venuti, M B; Zhang, Xiyue S; Lang, Eric J; Addamane, Sadhvikas J; Paik, Hanjong; Allen, Portia; Sharma, Peter; Muller, David; Hattar, Khalid; Lu, Tzu-Ming; et al (June 2024, npj Spintronics)

   Abstract Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic noncentrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals—determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au⁴⁺ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and antiskyrmions as storage bits, and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals. 

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2. Single skyrmion true random number generator using local dynamics and interaction between skyrmions

   https://doi.org/10.1038/s41467-022-28334-4  

   Wang, Kang; Zhang, Yiou; Bheemarasetty, Vineetha; Zhou, Shiyu; Ying, See-Chen; Xiao, Gang (February 2022, Nature Communications)

   Abstract Magnetic skyrmions are of great interest to both fundamental research and applications in post-von-Neumann computing devices. The successful implementation of skyrmionic devices requires functionalities of skyrmions with effective controls. Here we show that the local dynamics of skyrmions, in contrast to the global dynamics of a skyrmion as a whole, can be introduced to provide effective functionalities for versatile computing. A single skyrmion interacting with local pinning centres under thermal effects can fluctuate in time and switch between a small-skyrmion and a large-skyrmion state, thereby serving as a robust true random number generator for probabilistic computing. Moreover, neighbouring skyrmions exhibit an anti-correlated coupling in their fluctuation dynamics. Both the switching probability and the dynamic coupling strength can be tuned by modifying the applied magnetic field and spin current. Our results could lead to progress in developing magnetic skyrmionic devices with high tunability and efficient controls. 

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3. A model heterostructure with engineered Berry curvature

   https://doi.org/10.1063/5.0151126  

   Schreiber, Nathaniel_J; Miao, Ludi; Goodge, Berit_H; Kourkoutis, Lena_F; Shen, Kyle_M; Schlom, Darrell_G (June 2023, APL Materials)

   Molecular-beam epitaxy enables ultrathin functional materials to be combined in heterostructures to create emergent phenomena at the interface. Magnetic skyrmions are an example of an exciting phase found in such heterostructures. SrRuO3 and SrRuO3-based heterostructures have been at the center of the debate on whether a hump-like feature appearing in Hall resistivities is sufficient evidence to prove the presence of skyrmions in a material. To address the ambiguity, we synthesize a model heterostructure with engineered Berry curvature that combines, in parallel, a positive anomalous Hall effect (AHE) channel (a Sr0.6Ca0.4RuO3 layer) with a negative AHE channel (a SrRuO3 layer). We demonstrate that the two opposite AHE channels can be combined to artificially reproduce a “hump-like” feature, which closely resembles the hump-like feature typically attributed to the topological Hall effect and the presence of chiral spin textures, such as skyrmions. We compare our heterostructure with a parallel resistor model, where the inputs are the AHE data from individual Sr0.6Ca0.4RuO3 and SrRuO3 films. To check for the presence of skyrmions, we measure the current dependence, angle dependence, and minor loop dependence of Rhump in the heterostructure. Despite the clear hump, no evidence of skyrmions is found. 

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4. Surface anchoring as a control parameter for stabilizing torons, skyrmions, twisted walls, fingers and their hybrids in chiral nematics

   Tai, Jung-Shen; Smalyukh, Ivan I. (January 2020, Physical review)

   Chiral condensed matter systems, such as liquid crystals and magnets, exhibit a host of spatially localized topological structures that emerge from the medium’s tendency to twist and its competition with confinement and field coupling effects. We show that the strength of perpendicular surface boundary conditions can be used to control the structure and topology of solitonic and other localized field configurations. By combining numerical modeling and threedimensional imaging of the director field, we reveal structural stability diagrams and intertransformation of twisted walls and fingers, torons and skyrmions and their crystalline organizations upon changing boundary conditions. Our findings provide a recipe for controllably realizing skyrmions, torons and hybrid solitonic structures possessing features of both of them, which will aid in fundamental explorations and technological uses of such topological solitons. Moreover, with limited examples, we discuss how similar principles can be systematically used to tune stability of twisted walls versus cholesteric fingers and hopfions versus skyrmions, torons and twistions. 

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5. An atomically tailored chiral magnet with small skyrmions at room temperature

   https://doi.org/10.1038/s42005-023-01444-1  

   Liu, Tao; Selcu, Camelia M.; Wang, Binbin; Bagués, Núria; Wu, Po-Kuan; Hartnett, Timothy Q.; Cheng, Shuyu; Pelekhov, Denis; Bennett, Roland A.; Corbett, Joseph Perry; et al (November 2023, Communications Physics)

   Abstract Creating materials that do not exist in nature can lead to breakthroughs in science and technology. Magnetic skyrmions are topological excitations that have attracted great attention recently for their potential applications in low power, ultrahigh density memory. A major challenge has been to find materials that meet the dual requirement of small skyrmions stable at room temperature. Here we meet both these goals by developing epitaxial FeGe films with excess Fe using atomic layer molecular beam epitaxy (MBE) far from thermal equilibrium. Our atomic layer design permits the incorporation of 20% excess Fe while maintaining a non-centrosymmetric crystal structure supported by theoretical calculations and necessary for stabilizing skyrmions. We show that the Curie temperature is well above room temperature, and that the skyrmions have sizes down to 15 nm as imaged by Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The presence of skyrmions coincides with a topological Hall effect-like resistivity. These atomically tailored materials hold promise for future ultrahigh density magnetic memory applications. 

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