Monte Carlo studies of (T, H) phase diagram of quasi-2D skyrmion systems: Role of magnetic anisotropy

Recent topological Hall effect experiments [1] and MFM imaging in thin heavy metal/rare-earth iron garnet (Pt/TmIG) bilayers show that skyrmions are stable in a very specific temperature-field (T, H) regime, which is quite distinct from the much-studied skyrmion phase diagram in B20 thin plates and thin films. We show using Monte Carlo (MC) simulations that the main cause of this striking difference is the magnetic anisotropy: Pt/TmIG has easy-axis anisotropy while the B20 systems have a negligible uniaxial anisotropy. We present detailed MC results comparing these two cases, focusing on topological charge density, skyrmion size,
and orientational order of the 2D skyrmion crystal phase. We also discuss how aspects of our MC results for the (T, H) phase diagrams can be qualitatively understood in terms of T=0 phase diagram with T-dependent “micromagnetic” parameters.

[1] A. S. Ahmed et al., Nano Lett. 19, 5683 (2019)