Monte Carlo and spin dynamics simulations of magnetic phases in PdFe bilayer on Ir(111) based on ab initio calculations
It has been demonstrated in the recent years that few atomic layer thick magnetic films may display different types of non-collinear magnetic order. The Dzyaloshinsky-Moriya interaction, which is a consequence of the spin-orbit coupling and the broken inversion symmetry at the surface, plays an important role in stabilizing these magnetic phases.
PdFe bilayer on the (111) surface of Ir has been previously investigated by spin-polarized scanning tunnelling microscopy experiments[1][2]. Besides the spin spiral ground state, the external magnetic field perpendicular to the surface may also stabilize magnetic skyrmions, possibly making this system applicable for future spintronics devices.
We performed ab initio calculations based on the screened Korringa-Kohn-Rostoker method to map the system onto a classical vector spin model with tensorial coupling between the spins[3]. Using the spin model, we performed Monte Carlo and spin dynamics simulations to examine the magnetic patterns as a function of external magnetic field and temperature. At low temperature, we found that increasing the external magnetic field drives the system from a cycloidal spin spiral state through a skyrmion lattice state into the ferromagnetic state, in agreement with the experimental results. At higher temperature the spin spiral and skyrmion lattice phases turn into a state exhibiting short-range order and a finite skyrmion lifetime, before reaching the completely disordered paramagnetic phase. Based on the similarities of the calculated neutron scattering intensity profiles and static magnetic susceptibility to available experimental data for the bulk system MnSi[4], we identify this intermediate regime as a fluctuation-disordered state. In this region, the time dependence of the topological charge was examined to obtain the skyrmion lifetime as a function of temperature, which follows the Arrhenius law.
[1] N. Romming et al., Science 341, 636 (2013).
[2] N. Romming et al., Phys. Rev. Lett. 114, 177203 (2015).
[3] E. Simon et al., Phys. Rev. B 90, 094410 (2014).
[4] M. Janoschek et al., Phys. Rev. B 87, 134407 (2013).