Route to form skyrmions in soft magnetic films

Abstract

Magnetic skyrmions which are topologically nontrivial magnetization configurations have attracted much attention recently due to their potential applications in information recording and signal processing. Conventionally, magnetic skyrmions are stabilized by chiral bulk or interfacial Dzyaloshinskii-Moriya interaction (DMI) in noncentrosymmetric B20 bulk crystals (at low temperatures) or ultrathin magnetic films with out-of-plane magnetic anisotropy (at room temperature), respectively. The skyrmion stability in the ultrathin films relies on a delicate balance of their material parameters that are hard to control experimentally. Here, we propose an alternate approach to stabilize a skyrmion in ferromagnetic media by modifying its surroundings in order to create strong dipolar fields of the radial symmetry. We demonstrate that artificial magnetic skyrmions can be stabilized even in a simple media such as a continuous soft ferromagnetic film, provided that it is coupled to a hard magnetic antidot matrix by exchange and dipolar interactions, without any DMI. Néel skyrmions, either isolated or arranged in a 2D array with a high packing density, can be stabilized using antidot as small as 40 nm in diameter for soft magnetic films made of Permalloy. When the antidot diameter is increased, the skyrmion configuration transforms into a curled one, becoming an intermediate between the Néel and Bloch skyrmions. In addition to skyrmions, the considered nanostructure supports the formation of nontopological magnetic solitons that may be regarded as skyrmions with a reversed core.

Helicity of magnetic vortices and skyrmions in soft ferromagnetic nanodots and films biased by stray radial fields

“…The main idea of the stabilization of magnetic skyrmions and unconventional vortices in soft magnetic films and dots is the application of magnetic fields having radial symmetry [43,44]. Here we describe one possible approach, which, with some modifications, allows us to achieve stabilization of both vortices and skyrmions.…”

“…Finally, by placing a continuous soft magnetic film underneath a hard layer with an antidot [ Fig. 2(d)], we create the nanostructure, in which magnetic skyrmions and nontopological solitons can be stabilized [44]. In this structure, the interlayer exchange coupling ensures that the magnetization of the soft layer is perpendicular to the plane in the area of direct contact to the hard layer (except for a small area near the antidot edge).…”

“…For the case of exchange-coupled soft magnetic film, the anisotropy of the hard layer should overcome demagnetization energy of both hard and soft films, and the uniaxial anisotropy constant was chosen larger (see details in Ref. [44]). The minimal required anisotropy constant, in a general case, depends on the antidot lattice geometry [51,52].…”

“…Recently, we have proposed an alternative method for the stabilization of magnetic skyrmions and vortices [43,44]. It is based on the exploiting of the radial magnetic field, which could be created, e.g., by an antidot matrix.…”

“…It is based on the exploiting of the radial magnetic field, which could be created, e.g., by an antidot matrix. This approach allows one to stabilize magnetic skyrmions and their nontopological counterparts in soft ferromagnetic films having neither DMI nor perpendicular magnetic anisotropy, just by dipolar and interlayer exchange coupling to a perpendicularly magnetized hard antidot matrix [44]. Simultaneously, in the application to soft magnetic nanodots, radial stray fields from an antidot matrix allow for significant reduction of the minimal size and thickness of dots, where magnetic vortices can be stabilized in Ref.…”