Skyrmions with attractive interactions in an ultrathin magnetic film

TL;DR

This paper models and analyzes skyrmions in an ultrathin Fe/Pd(111) film with alloy overlayers, revealing attractive skyrmion interactions and cluster formation driven by frustrated exchange interactions.

Contribution

It introduces a classical spin Hamiltonian derived from ab initio calculations that highlights the role of frustrated exchange in skyrmion attraction and clustering.

Findings

Frustrated isotropic exchange interactions induce attractive skyrmion interactions.

Skyrmions form stable clusters at finite temperature.

The system transitions from ferromagnetic to spin spiral state with Ir intermixing.

Abstract

We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111) surface with a Pt_{1-x}Ir_{x} alloy overlayer from ab initio calculations. While the ground state of the system is ferromagnetic for x=0.00, it becomes a spin spiral state as Ir is intermixed into the overlayer. Although the Dzyaloshinsky-Moriya interaction is present in the system, we will demonstrate that the frustrated isotropic exchange interactions play a prominent role in creating the spin spiral state, and these frustrated couplings lead to an attractive interaction between skyrmions at short distances. Using spin dynamics simulations, we show that under these conditions the individual skyrmions form clusters, and that these clusters remain stable at finite temperature.