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Physics of Nanomagnets
Published in Klaus D. Sattler, 21st Century Nanoscience – A Handbook, 2020
Ralph Skomski, Balamurugan Balasubramanian, D. J. Sellmyer
A research area of renewed interest is magnetic skyrmions. By definition, skyrmions are solitonic solutions of nonlinear field equations originally used to describe nuclear matter [53]. Much of the interest in magnetic skyrmions comes from potential applications in date storage and processing, where miniaturization is a major consideration. An early example of magnetic skyrmions is magnetic bubbles, first described in 1967 [17]. Bubble skyrmions are stabilized by magnetic anisotropy and magnetostatic interactions, and DM interactions (Section 18.3.3) can be used to modify skyrmions, add functionality, and further improve stability [54]. The micro-magnetic description of skyrmions continues to be a challenge, aside from simple thin-film geometries [55,56], and often the DM vector is approximated by a scalar.
Spin-Orbit Torques
Published in Evgeny Y. Tsymbal, Žutić Igor, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
Aurélien Manchon, Hyunsoo Yang
Néel skyrmions have been experimentally observed at room temperature in a variety of transition metal bilayers [137–140]. In such systems, interfacial DMI creates an effective symmetry breaking in-plane field, HDMI, directed along the magnetization gradient. When it exceeds the magnetic anisotropy,|HDMI|>(2/π)HK, this field enables metastable skyrmions. These skyrmions have been limited to sizes well over 100 nm, making them ill-suited for practical applications. Furthermore, the total magnetic volume in these systems is below 1.5 nm, posing challenges for thermal stability. Recently, Moreau-Luchaire et al. [141], demonstrated sub-100 nm magnetic skyrmions in dipolar coupled Pt/Co/Ir multilayers, in which the asymmetric Pt and Ir layers result in an enhanced HDMI strong enough to generate skyrmions in films as thick as 6.6 nm. However, to date, no skyrmions have been observed in thick ferromagnetic multilayers in which the disparate magnetic layers are exchange coupled. The discovery of skyrmions in bulk non-centrosymmetric magnets and thin films has led to the proposal of a variety of skyrmionic devices, in which the skyrmion is used as the basic information carrier in next-generation logic and memory devices [145,146,149,150].
Magnetic Skyrmions on Discrete Lattices
Published in Evgeny Y. Tsymbal, Igor Žutić, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
Elena Y. Vedmedenko, Roland Wiesendanger
It is well known that multiple periodic states appear in a wide variety of physical systems due to competing exchange interactions [48]. The physical reason for such behavior is quite well known [46, 48]: the fundamental solutions of the Heisenberg exchange model on a periodic lattice are spiral spin structures. The reciprocal vector Q→ of a spin spiral depends on the strength of the exchange parameters and on the crystal structure. Recently, it has been shown that the addition of magnetic anisotropy and magnetic field to competing exchange interactions may lead to the formation of skyrmions [62, 63]. These systems with frustrated magnetic interactions constitute a third class of magnetic skyrmions (SC-III). In contrast to chiral skyrmions, which require the presence of the DM interaction, their counterparts originating from magnetic frustration arise in systems with ferro/antiferromagnetic exchange interactions, Zeeman energy, and perpendicular magnetic anisotropy. For a triangular lattice, the specific ratio of exchange constants between first and second nearest neighbors required for the appearance of modulated configurations is |J1|/|J2|=3. As the DM energy fixing the sense of magnetic rotation is not present in this case, the chirality of frustrated skyrmions can be changed by an external magnetic field.
Skyrmions in blue phases of chiral liquid crystals
Published in Liquid Crystals, 2023
J. Pišljar, M. Marinčič, S. Ghosh, S. Turlapati, Rao Nandiraju, A. Nych, M. Škarabot, A. Mertelj, A. Petelin, A. Pusovnik, M. Ravnik, I. Muševič
Skyrmions were originally proposed to describe the stability of a nucleon as topological solitons of a pion field [33] by Tony Skyrme in 1962. In later years, it has been realised that these vortex-like field configurations could exist in other continuous field systems as well [34]. Skyrmions have been observed as 2D spin textures of a quantum Hall effect [35], as coreless vortex formations in spinor Bose–Einstein condensates [36] and as configurations of the velocity field in a superfluid phase [37]. Skyrmions in solid state materials are most prominent in chiral magnetic systems [38–40], where the often called ‘baby’ – skyrmions are tube-like localised swirls of magnetisation vector stabilised by the Dzyaloshinskii–Moriya interaction (DMI) between the magnetic spins [41,42]. Magnetic skyrmions are interesting for application in memory devices thanks to their fast responsiveness to external electrical current, small size and compactness and most importantly their energetic stability [43]. The latter is a consequence of topology of their structure, which cannot be disentangled into a homogeneous structure by any smooth transformation. A skyrmion can only be disentangled by applying an energetically costly discontinuity of the magnetisation field. In this sense, it is often said that skyrmions are topologically protected structures [44].
Theory of elastic interaction between axially symmetric 3D skyrmions in confined chiral nematic liquid crystals and in skyrmion bags
Published in Liquid Crystals, 2023
S. B. Chernyshuk, E. G. Rudnikov
Skyrmions were predicted and observed in quantum Hall systems [5–7]. Later, Bose–Einstein condensates with spin degrees of freedom were shown to accommodate Skyrmions [8–11]. As well, great attention has been paid to Skyrmions in noncentrosymmetric ferromagnets [12–18] with the presence of Dzyaloshinskii-Moriya spin-orbit interaction, such as MnSi [7,13,14] and [15,16]. Chiral magnetic skyrmions are now considered as promising objects for applications in magnetic data storage technologies and in the emerging spin transport electronics (spintronics), because local twisted magnetic structures coupled to electric or spin currents could be used to manipulate electrons and their spins [17,18]. Theoretical aspects of skyrmions in chiral magnets were investigated in [19–30].
Formation and annihilation of magnetic skyrmions on a square lattice Heisenberg Ferromagnet: the role played by the pure and random anisotropy configurations
Published in Philosophical Magazine, 2021
Magnetic anisotropy plays an essential role in magnetisation process and formation of skyrmions in ferromagnetic materials. In the presence of single ion anisotropy, a rapid saturation of zero temperature magnetisation with increasing magnetic field is expected in the system. This behaviour can be clearly seen from Figure 6(a) where the influence of large antisymmetric exchange energy on the system decreases with increasing anisotropy values. In a two dimensional ferromagnet with magnetic anisotropy and non-negligible DM interaction, application of relatively weak magnetic field causes the total energy of the system to be minimised by the emergence of skyrmion pattern. By examining the chirality versus magnetic field curves, critical field values for which the skyrmion phase originates between them can also be determined for varying anisotropy values. As seen from Figure 6(b), as the magnetic anisotropy energy becomes enhanced, emergence of skyrmion state shifts towards weak magnetic field region. We note that the critical fields and both reduce with increasing magnetic anisotropy. This means that for a weak anisotropy, formation of skyrmions in the system requires more Zeeman energy. This observation agrees well with that reported by a recent work for a thin film of CeFeB [41].