Abstract
Pairs of electron-spin vortices and antivortices have been created in thin magnetic films at low temperatures. They have been visualized using cryogenic Lorentz transmission electron microscopy in a quasi-2D ferromagnet K2CuF4. These studies have validated the theoretical predictions for the topological phase transition known as the Berezinskii–Kosterlitz–Thouless (BKT) phase transition.