Quantum magnets with large single-ion easy-plane anisotropy in magnetic field
A. V. Sizanov*, A. V. Syromyatnikov*†
*Petersburg Nuclear Physics Institute, Gatchina, 188300 St. Petersburg, Russia
†Department of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia
Abstract
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)
in the first nonvanishing orders of the perturbation theory. An expression is
found for the boundary of the paramagnetic phase Hc1(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).