Route to form skyrmions in soft magnetic films

D Navas,B A Ivanov,A O Adeyeye,S A Bunyaev,K Y Guslienko,R V Verba,X Zhou,A Hierro-Rodriguez,O V Dobrovolskiy,G N Kakazei

doi:10.1063/1.5093371

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.

Highlights

The development of new data storage technologies that combine ultrahigh areal density and low power consumption is one of the highest priorities of modern nanoscience
Here we focused our attention on the study of the magnetic behavior of patterned nanostructures with a hard layer of large saturation magnetization: we used the material parameters of Fe/Pt multilayers with MHL = 1000 kA/m, exchange stiffness AHL = 2 × 10−11 J/m, and perpendicular anisotropy constant Ku = 1 × 106 J/m3.34 The thickness of the Permalloy layer was fixed at 3 nm, while the matrix thickness, antidot diameter, and distance between them were systematically varied
We have demonstrated a novel method to achieve magnetic skyrmion configurations in soft ferromagnetic films without Dzyaloshinskii-Moriya interaction

Summary

INTRODUCTION

The development of new data storage technologies that combine ultrahigh areal density and low power consumption is one of the highest priorities of modern nanoscience. The room temperature skyrmion stability in the ultrathin films relies on a delicate balance of the exchange interaction, uniaxial magnetic anisotropy, and DMI. These parameters are poorly controlled for layer thicknesses of and below 1 nm. Skyrmion lattices at room temperature in the absence of external magnetic field, stabilized by a competition between the intrinsic exchange, magnetocrystalline anisotropy, and dipolar interaction, were experimentally observed in a multiferroic Ni2MnGa single crystals with inversion symmetry.. Our main idea is to create skyrmions by means of a strong stray dipolar field generated by a patterned hard magnetic layer near the soft magnetic film in a hybrid bilayer structure. The antidot can be isolated, or a two-dimensional antidot lattice can be formed in the film plane

Model and magnetic energy of the nanostructure

Soliton structure

MICROMAGNETIC SIMULATIONS

CONCLUSIONS