Compact Skyrmions, Merons and Bimerons in Thin Chiral Magnetic Films

A meron is a controversial topological excitation because it carries just one half of the topological charge unit. It is believed that it is tightly binded to another meron and cannot be observed by isolating it. We present a counter example, investigating the 2-dimensional nonlinear sigma model together with the Dzyaloshinskii-Moriya interaction, where topological excitations are merons, bimerons and skyrmions. They behave as if they were free particles since they are electrically neutral. A prominent feature is that the topological charge density is strictly confined within compact domains. We propose an analytic approach for these compact excitations, and construct a phase diagram. It is comprised of the helix, meron, skyrmion-crystal, skyrmion-gas and ferromagnet phases. It explains quite well the experimental data recently performed in chiral magnets such as MnSi and FeCoSi thin films, as verifies that merons are surely basic topological excitations in the system.