Skyrmion phases in thin films: a finite-temperature Monte Carlo study

We present the results of large-scale Monte Carlo simulations of the field-temperature phase diagram of thin films of chiral magnets with ferromagnetic exchange and Dzyaloshinskii-Moriya interaction. We will show results for the specific heat, spin structure factor S(Q), average skyrmion density, and skyrmion density structure factor S_sk(Q) to characterize the various phases: helical, skyrmion crystal, skyrmion liquid and field-polarized ferromagnet. The melting of the skyrmion crystal to a liquid state is characterized using the orientational order parameter. A significant portion of the phase diagram at high temperatures harbors a skyrmion liquid which has a non-zero average skyrmion density, but exhibits short-range orientational order and "rings" in both S(Q) and S_sk(Q). This is distinct from a state with fluctuating helices, at say H = 0, which only exhibits a "ring" in S(Q), but not in S_sk(Q). We will show that the phase diagram changes in interesting ways with increasing number of layers as the system evolves from the strict 2D limit of a monolayer.