Route to form skyrmions in soft magnetic films

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. Neel 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 Neel 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. (C) 2019 Author(s).The Portuguese team acknowledges the Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through Project Nos. NORTE-01-0145-FEDER-022096, MIT-EXPL/IRA/0012/2017, POCI-0145-FEDER-030085 (NOVAMAG), PTDC/FIS-MAC/31302/2017, EXPL/IF/00541/2015 (S.A.B.), and Grant No. SFRH/BPD/90471/2012(A.H.-R.). Work at IMag was supported by the Ministry of Education and Science of Ukraine (Project No. 0118U004007). K.Y.G. acknowledges support from IKERBASQUE (the Basque Foundation for Science) and the Spanish MINECO, Grant No. FIS2016-78591-C3-3-R. R.V.V. B.A.I., K.Y.G., and O.V.D. acknowledge the support from the European Union Horizon 2020 Research and Innovation Programme under Marie Sklodowska-Curie, Grant Agreement No. 644348. A.H.-R. acknowledges the support from Spanish MINECO under Project Ref. No. FIS2016-76058-C4-4-R and from European Union's Horizon 2020 research and innovation program under the Marie Skodowska-Curie Action Reference No. H2020-MSCA-IF-2016-746958. B.A.I. was supported by the Program of NUST "MISiS" (Grant No. K2-2017-005), implemented by a governmental decree dated 16th of March 2013, No. 211. G.N.K. and O.V.D. acknowledge the support from European Cooperation in Science and Technology (COST), Project No. CA16218 "NANOCOHYBRI". A.O.A. was supported by the Ministry of Education, Singapore, under Research Project No. R-263-000-C61-112. A.O.A. is a member of Singapore Spintronics Consortium (SG-SPIN)