Quasi-one-dimensional magnet NiCl₂·4SC(NH₂)₂ denoted as DTN remains disordered in zero magnetic field down to T = 0: the S_z = 0 ground state is separated from S_z =±1 excitations by a gap caused by strong single-ion easy-plane anisotropy acting on the Ni²⁺ ions. When a magnetic field is applied along the principal axis of anisotropy, the gap closes in a field above B_c1 = 2.18 T and the field-induced antiferromagnetic order arises. There are two excitation branches in this field-induced phase, one of which should be the Goldstone mode. Recent studies of the excitation spectrum in the field-induced ordered phase of the DTN magnet (T. Soldatov et al., Phys. Rev. B 101, 104410 (2020)) have revealed that the Goldstone mode acquires a gap in the excitation spectrum of the field-induced phase at a small deviation of the applied magnetic field from the tetragonal axis of the crystal. In this work, a simple description of both magnetic resonance branches in the ordered phase of a quasi-one-dimensional quantum S = 1 magnet with strong single-ion anisotropy is proposed. This approach is based on a combination of an effective strong coupling model for an anisotropic spin chain and the classical antiferromagnetic resonance theory. This description reproduces the experimental results semi-quantitatively without additional parameters.

表示为DTN的准一维磁体NiCl ₂ ·4SC（NH ₂）₂在零磁场下直至T = 0时都保持无序：S _z = 0的基态与S _z =±1的励磁因间隙而分开Ni ²⁺离子具有很强的单离子易平面各向异性。当沿着各向异性主轴线施加磁场时，在B _c1以上的磁场中，间隙会闭合= 2.18 T，并且产生了场感应的反铁磁序。在该场感应相中有两个激发分支，其中之一应该是Goldstone模式。在DTN磁体（T.索尔达托夫等人，物理评论B的场致有序相的激发光谱的最近的研究101，104410（2020））已经揭示，戈尔德斯顿模式获取在激发光谱的间隙在磁场与晶体的四边形轴有很小偏差的情况下，磁场感应相位的变化。在这项工作中，对准一维量子S的有序相中的两个磁共振分支的简单描述提出一种具有强单离子各向异性的磁体= 1。该方法基于各向异性自旋链的有效强耦合模型和经典反铁磁共振理论的结合。此描述不需额外参数即可半定量地复制实验结果。