Exotic magnetization curves in classical square-kagomé spin lattices

Abstract Classical spin systems with non-coplanar ground states typically exhibit nonlinear magnetization curves characterized by kinks and jumps. Our article briefly summarizes the most important related analytical results. In a comprehensive case study, we then address AF-square kagom'{e} and AF/FM-square kagom'{e} spin lattices equipped with additional cross-plaquette interactions. It is known that these systems have non-coplanar ground states that assume a cuboctahedral structure in the absence of a magnetic field. When a magnetic field $H$ is switched on, a rich variety of different phases develops from the cuboctahedral ground state, which are studied in their dependence on $H$ and a cross-plaquette coupling constant $J_3>0$. For the AF square-kagom'{e} spin lattice, we carefully identify and describe seven phases that appear in a phase diagram with five triple points. The transitions between these phases are predominantly discontinuous, although two cases exhibit continuous transitions. In contrast, the phase diagram of the AF/FM square-kagom'{e} model shows only four phases with a single triple point, but these also lead to exotic magnetization curves. Here, too, there are two types of phase boundaries belonging to continuous and discontinuous transitions.