Quantum magnets with large single-ion easy-plane anisotropy in magnetic field

We propose a theory describing low-temperature properties of magnets with integer spin and large single-ion easy-plane anisotropy D in magnetic field H directed parallel to the hard axis. Considering the exchange interaction between spins as a perturbation and using the bosonic spin representation proposed in our recent paper 1] we find thermal corrections to the elementary excitation spectrum, magnetization and specific heat in the vicinity of the quantum critical point (QCP) H = H _c1(0) ∼ D in the first nonvanishing orders of the perturbation theory. An expression is found for the boundary of the paramagnetic phase H _c1(T) in the H-T plane. The effective interaction between bosons is derived near the QCP. The proposed theory describes well experimental data obtained in NiCl₂-4SC(NH₂)₂ (DTN).