Finite-temperature phase transition to the m=1/2 plateau phase in the spin-1/2 XXZ model on the Shastry-Sutherland lattices

We study the finite-temperature transition to the m=1/2 magnetization plateau in a model of interacting S=1/2 spins with longer-range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. This model was shown to capture the qualitative features of the field-induced magnetization plateaus in the rare-earth tetraboride, TmB4. Our results show that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, whereas a single phase transition takes place in the purely Ising limit. To better understand these behaviors, we perform Monte Carlo simulations of the classical generalized four-state chiral clock model and compare the phase diagrams of the two models. Finally, we estimate a parameter set that can explain the magnetization curves observed in TmB4. The magnetic properties and critical behavior of the finite-temperature transition to the m=1/2 plateau state are also discussed.