The effect of quantum fluctuations on the ferromagnet-helix transition

Abstract

The authors study the effect of quantum zero-point motion on the zero-temperature phase diagram of a Heisenberg system with competing interactions on a hexagonal lattice. Within the basal plane, first-neighbour interactions J₁ are ferromagnetic while second- and third-neighbour interactions J₂ and J₃ can have either sign. The inter-plane interaction J' is taken to be small. They find that quantum fluctuations enhance the tendency towards a first-order transition from ferromagnetic to helical order. Even a very small single-ion anisotropy suffices to explain the small first-order jump observed in NiBr₂.