Manipulation of skyrmions in nanodisks with a current pulse and skyrmion rectifier

Abstract

A skyrmion in a nanosized disk of a chiral magnet can be used as a bit of information. To this end, it is desirable to control the creation and removal of a skyrmion only by currents without using external magnetic fields. Here we propose to create a skyrmion by applying a current pulse to a nanodisk. The skyrmion can be removed from the disk by applying a dc current. We show that the dynamics of the created skyrmion can lead to a rectification effect, in which a dc voltage is generated by the motion of skyrmion in the presence of an ac current. PACS numbers: 75.10.Hk, 75.30.Kz, A new spin texture called skyrmion has been found in certain chiral magnets without inversion symmetry, such as MnSi and Fe 0.5 Co 0.5 Si. [1-7] A skyrmion can be described in terms of spins which wrap a sphere. When a conduction electron passes through the skyrmion, its spin is fully polarized by the spin texture of the skyrmion, yielding a quantized Berry phase 4π that corresponds to a quantized magnetic flux Φ = hc/e. This emergent electromagnetic field significantly affects the dynamics of the skyrmions and produces a Magnus force perpendicular to the skyrmion velocity. [8,9] Application of a spin polarized current exerts a spin transfer torque on the skyrmion, causing it to move at an angle to the current direction determined by the dissipation.[10] The resulting voltage is perpendicular to the velocity; thus, a Hall (transverse) voltage is induced, which has been measured experimentally. [11] Since skyrmions are stable topological excitations in chiral magnets, they have promising applications in spintronics. [12] Due to the presence of the Magnus force [8,9], the threshold current to drive the skyrmion from the pinning sites is extremely small, 4 − 5 orders of magnitude smaller than that for magnetic domain walls. [11,13,14] Thus, skyrmions have tremendous advantages for information storage applications, because the lower current implies less Joule heating. From a technological point of view, it is desirable to be able to manipulate, create, or destroy skyrmions by applying only a current without using an external magnetic field. This requires use of magnetic disk that is is typically the same size as the skyrmion, of the order of 100 nm, such that only one skyrmion can be accommodated in the disk. The goal of producing a single magnetic vortex in a nanodisk was recently achieved [15,16]. In most known chiral magnets, the skyrmion is not stable without an external magnetic field, and in the absence of a field a spiral structure is favored. Upon increasing the magnetic field, a skyrmion state becomes the ground state.[17] Two immediate questions regarding the dynamic creation of skyrmions arise: first, how to stabilize skyrmions without magnetic fields, and secondly, how to create a skyrmion using currents.The answer to the first question is to exploit the magnetic anisotropy in thin films. [18] Due to the reduced dimensionality, an out-of-plane anisotropy develops that is inversely proportional to the film thickness. Such anisotrop...

Direct imaging of magnetic field-driven transitions of skyrmion cluster states in FeGe nanodisks

“…Magnetic skyrmion is characterized as a nanoscale topological particle producing unconventional spin electronic phenomena (2, 3) that holds great promise for future spintronic devices, including racetrack memory (4), magnetic random access memory, and magnetic sensors (1). Essentially, such schemes rely on the controllable formation and manipulation of individual skyrmions at nanostructured elements with various shapes such as disks, stripes, or wires (5)(6)(7). Investigation of skyrmions in confined geometries has therefore become one of the major topics in the field of skyrmion physics (5)(6)(7)(8)(9)(10).…”

Internal modes of a skyrmion in the ferromagnetic state of chiral magnets

“…Single skyrmions can also be created in small samples where only one skyrmion can be accommodated. 29 Furthermore, a single skyrmion becomes a metastable solution in the ferromagnetic background of fully polarized spins that is stabilized at high magnetic fields. Consequently, the controlled manipulation of a single skyrmion in a ferromagnetic background is possible and directly relevant for applications 30 .…”

Comparing the dynamics of skyrmions and superconducting vortices

“…It may also be possible to create asymmetric pinning geometries in order to realize skyrmion ratchets or diodes, similar to the ratchet effects observed in vortex systems with asymmetric pinning [27][28][29][30]. There is already some theoretical work showing how the Magnus term can lead to rectification effects [44].…”