Low-energy excitations in DTN below T_c: ESR studies

NiCl₂-4SC(NH₂)₂ (known as DTN) is an S = 1 system with an easy-plane anisotropy dominating over the exchange interaction and exhibiting a field-induced antiferromagnetic ordering with critical fields B_c1 = 2.1 T, B_c2 = 12.6 T and temperature T^max _c = 1.2 K. A systematic study of the low-energy excitation spectrum of DTN in the ordered phase at temperatures down to 0.45 K is presented. It is showed that two observed gapped modes can be consistently interpreted within a four-sublattice antiferromagnet model with a weak isotropic corner-center interaction of magnetic ions in the body-centered tetragonal lattice.