Ultrafast Ratchet Dynamics of Skyrmions by Defect Engineering in Materials with Poor Conductivity Under Gigahertz Magnetic Fields

Abstract

The novel ratchet motion of magnetic skyrmions driven by microwave magnetic fields, with the motion direction and speed tunable by field parameters, provides a promising route to drive magnetic skyrmions in materials with poor conductivity. However, as an indirect motion, skyrmion ratchet motion speed is generally low in comparison with the direct motions driven by current. Toward practical applications, it is important to ask if there are mechanisms to realize ultrafast ratchet motion of magnetic skyrmions and how such novel motion can be integrated into racetrack-type skyrmion devices. In this work, we explore the effects of defects and edges on the ratchet motion of magnetic skyrmions under time-varying magnetic fields in GHz. We demonstrate that the ratchet motion of skyrmion is not only guided along the defect tracks or edges, but also with a remarkable speed-up (with a factor over ten) compared with that in the bulk region. The skyrmion ratchet motion speed reaches 100 m/s along a straight defect track/edge and 10 9 rad/s along a circular edge under a field of ~50 mT, comparable to those direct motions driven by currents. Moreover, the skyrmion ratchet motion along the defect track/edge can be facilely controlled by the field and defect parameters. Analysis based on time-averaged Thiele's equation of skyrmion verifies that such a speed-up effect is due to the increased time-averaged driving force perpendicular to the skyrmion motion when it approaches the defect track/edge, analogous to that discovered in direct motions driven by currents.The generality of our conclusions has been examined for the ratchet motion of Bloch and Néel-type skyrmions driven by a variety of time-varying magnetic fields, and for systems with open edges or defect tracks with modified Dzyaloshinskii-Moriya or exchange interactions and anisotropy.

Spontaneous skyrmion conformal lattice and transverse motion during dc and ac compression

“…There is increasing interest in identifying ways to control individual and collective skyrmion motion. Possible methods include periodic pinning [21][22][23][24][25][26], ratchet effects [27][28][29][30][31][32], interface guided motion [33,34], strain, magnetic or temperature gradients [35][36][37][38], one-dimensional potential wells [39], curvature of the sample [40][41][42], and skyrmion-vortex coupling using a ferromagnet-superconductor heterostructure [43]. Skyrmions can also be manipulated by being compressed against a wall or linear obstacle, such as by applying a drive that forces the skyrmions to move toward an interface or extended nanostructure.…”

Skyrmion Jellyfish in Driven Chiral Magnets

“…Generally, a skyrmion will start to move in the presence of external forces if enough symmetries are broken in the system. One option is to break the translational symmetry, for instance by subjecting the skyrmion to magnetic [7,8] or temperature [9,10] field gradients, electric or spin currents [11,12], by driving it with an oscillating magnetic field near a wall [13,14], or even by firing at it with magnons [15,16]. Another possibility is to break FIG.…”

Skyrmion Ratchet in Funnel Geometries

“…The ratchet effect can also be understood in terms of a diode effect, where the asymmetry produces different depinning forces in different directions, yielding a preferential or "easy" direction of motion 1 . This effect has been investigated in several systems such as protein motors [2][3][4] , molecular motors [5][6][7] , colloids 8,9 , type II superconducting vortices [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] , electrons 28 , active matter [29][30][31][32] and recently in skyrmions [33][34][35][36][37][38][39] . Reversals of motion from the easy to the hard direction may occur as a function of the applied magnetic field or other variables 12,[17][18][19][40][41][42][43] due...…”