Instability of skyrmions in magnetic fields

In this combined experimental and theoretical work, we report on the evolution of the skyrmion radius and its destruction in the system Co/Ru(0001) when an out-of-plane magnetic field is applied. At low fields, skyrmions are metastable and display an elliptical instability in which along the short axis, the spin texture approaches that of the spin-spiral phase and the long axis expands in order to go back to the spin-spiral ground state. At high fields, we observe round skyrmions of finite size up to the collapse field Bc, where they are destroyed and the topological charge is annihilated. We estimate Bc via numerical methods based on magnetization dynamics simulations parametrized by density functional theory calculations and compare it to experimental scanning tunneling microscopy observations obtained at ≈ 30 mK.