Skyrmions and hopfions in frustrated ferromagnetics

Full text: Magnetic skyrmion is a two-dimensional 'hedgehog-like' spin texture, characterized by non-zero topological number. Stability of skyrmions is topologically protected, therefore they can be potentially used as information carriers in magnetic memory devices. Skyrmions have been experimentally observed in several materials, such as chiral magnets and thin magnetic films with long-range dipolar interaction. Recent theoretical works proposed frustrated ferromagnets (FFM) on triangular lattice as new skyrmion materials, that can host isolated skyrmions, skyrmion lattice and other exotic magnetic phases. In the present work we show that stable skyrmions can exist in the FFM on the simple square lattice with spin anisotropy. This finding broadly enhances the scope of possible skyrmion materials. We also found attraction of skyrmions with opposite helicity, that leads to existence of stable skyrmions with very high topological numbers. Hopfion is a three-dimensional topological object, which is similar to torroidal vortex. Hopfions have been predicted in various physical systems such as Bose-Einstein condensates, liquid He³, and vortex-like dynamical spin precession in a collinear uniaxial ferromagnet. Extending our skyrmion work and considering 3D FFM on simple cubic lattice, we for the first time predict existence of a static magnetic hopfion.(author)