Current-induced skyrmion motion on magnetic nanotubes

Abstract

Magnetic skyrmions are believed to be the promising candidate of information carriers in spintronics. However, the skyrmion Hall effect due to the nontrivial topology of skyrmions can induce a skyrmion accumulation or even annihilation at the edge of the devices, which hinders the real-world applications of skyrmions. In this work, we theoretically investigate the current-driven skyrmion motion on magnetic nanotubes which can be regarded as "edgeless" in the tangential direction. By performing micromagnetic simulations, we find that the skyrmion motion exhibits a helical trajectory on the nanotube, with its axial propagation velocity proportional to the current density. Interestingly, the skyrmion's annular speed increases with the increase of the thickness of the nanotube. A simple explanation is presented. Since the tube is edgeless for the tangential skyrmion motion, a stable skyrmion propagation can survive in the presence of a very large current density without any annihilation or accumulation. Our results provide a new route to overcome the edge effect in planar geometries.Ever since its experimental discovery, 1 the magnetic skyrmion, a chiral quasiparticle, 2,3 has been an active research area in condensed matter physics because of not only the potential for future spintronic applications such as skyrmion racetrack memories 4-6 and logic devices, 7,8 but also the fundamental interests. 9-13 In chiral magnets, skyrmions can be stabilized by the Dzyaloshinskii-Moriya interaction (DMI) of two types: 1,14-22 the bulk DMI and the interfacial one. The bulk DMI typically exists in noncentrosymmetric magnets, and can support the formation of Bloch-type (vortex-like) skyrmions, 1,15-18 while the latter one usually exists in inversion-symmetry breaking thin films, and can give rise to Néel-type (hedgehoglike) skyrmions. [19][20][21][22] Several methods have been proposed to drive the skyrmion motion, such as spin-polarized currents, 23 microwaves, 24 and thermal gradients, 25 to name a few. However, when the skyrmion is driven by an in-plane current via the spin transfer torque, the trajectory of its motion deviates from the current direction due to the intrinsic skyrmion Hall effect. 2,3,26-29 Furthermore, there exists a threshold current density above which skyrmions can annihilate at the film edge. 30 This edge effect strongly limits the speed of skyrmion propagation which is of vital importance for real applications. Several solutions have been proposed to overcome this problem. Zhang et al.proposed an antiferromagnetically exchange-coupled bilayer system, where the skyrmions move straightly along the current direction. 31 Upadhyaya et al. showed that the skyrmion can be guided in a desired trajectory by applying electric fields in a certain pattern. 32 More recently, Yang et al. discovered a novel twisted skyrmion state at the boundary of two antiparallel magnetic domains coupled antiferromagnetically, through which skyrmions with opposite polarities can transform mutually. 33 Under proper conditions, the doma...

The stability of a skyrmion in a nanotube

“…Various experiments [33][34][35][36][37] and theoretical studies [38][39][40][41] based on nanotubes have also show that this structure has great influence on material properties. More recently, Wang et al reported the current-induced skyrmion motion on magnetic nanotubes [42]. Based on this point, we design a nanotube made of chiral magnets and study the stability of skyrmion in a nanotube utilizing micromagnetic simulations.…”

SERS detection of food contaminants by means of portable Raman instruments

“…in order to increase the affinity of the analyte towards the substrate surface. In addition, it also possible to transform the analytes of interest by chemical reaction into new species with improved cross section and/or affinity to the metallic substrate: This method was used, for example, to detect nitrites and formaldehyde . A further development of these methodologies and their integration with plasmonic substrates is crucial for a widespread use of SERS.…”

Curvature effects on phase transitions in chiral magnets

“…we consider systems with quality factor Q = 2K/ µ 0 M 2 s 1 [38]. Examples of chiral magnets which satisfies these condition were recently studied in [32,39]. Additionally, for thin stripes the magnetostatic contribution can be reduced to an effective easy-surface anisotropy [23,[40][41][42][43], which simply results in a shift of the anisotropy constant K → K − µ 0 M 2 s /2.…”

“…Additionally, for thin stripes the magnetostatic contribution can be reduced to an effective easy-surface anisotropy [23,[40][41][42][43], which simply results in a shift of the anisotropy constant K → K − µ 0 M 2 s /2. This approximation is widely used for the description of equlibrium states on toroidal nanoshells [44], statics and dynamics of skyrmions [32,[45][46][47] and DWs [48] in curved nanoshells. Using a curvilinear reference frame we parametrize the magnetization in the following way m = sin θ cos φ e 1 + sin θ sin φ e 2 + cos θ n. Expressions for E X , E B D , and E N D for a general case of a local curvilinear basis were previously obtained in Refs.…”

“…An important question is, how the curvature modifies the critical DMI d 0 [5,7], which separates homogeneous and periodic magnetization structures. This is important for assessing the stability of skyrmions [31] and their motion [32] along the tubes and other curvilinear surfaces [33,34]. Here, we present a detailed study of equilibrium states of the ferromagnetic nanotubes with intrinsic DMI of different symmetries.…”