Formation and stability of metastable skyrmionic spin structures with various topologies in an ultrathin film

Abstract

We observe metastable localized spin configurations with topological charges ranging from Q = −3 to Q = 2 in a (Pt 0.95 Ir 0.05 )/Fe bilayer on a Pd(111) surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The DzyaloshinskyMoriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with Q = −1, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.

Spin-polarized scanning tunneling microscopy characteristics of skyrmionic spin structures exhibiting various topologies

“…Formation and stability of the metastable structures are extensively discussed in Ref. [36]. Note that the configurations shown in Figs.…”

“…[36] that due to the presence of the Dzyaloshinsky-Moriya interaction in the system, not all of the observed skyrmionic objects possess a circular shape. The integral over ϕ in Eq.…”

“…8. These values were selected to ensure similar diameters of the skyrmionic structures, and because the field-polarized state where the objects are metastable is reached at higher field values for higher Ir concentrations [36]. Note that the magnitude and direction of the B field can drastically modify the size and shape of skyrmions [29].…”

“…[36], our work establishes a connection between SP-STM contrasts and skyrmionic topologies, most importantly through an investigation of the variation of the in-plane angle between the spins along the perimeter of the structures, which can be characterized by the local vorticity or linear density of the winding number. Our findings are expected to be applicable to the topological characterization of skyrmionic spin structures based on experimentally measured SP-STM images using a series of in-plane tip magnetization orientations.…”

“…In Ref. [36], other types of metastable skyrmionic structures were also investigated in the (Pt 1−x Ir x )Fe/Pd(111) ultrathin magnetic film, and it was discussed how their shapes become distorted due to the interplay between the DzyaloshinskyMoriya and the frustrated Heisenberg exchange interactions. These observations enable the generalization of the findings of Dupé et al [35] for the SP-STM contrasts of topologically distinct skyrmionic structures.…”