Excitations in the spin-1 trimer chain compound CaN i3P4O14 : From gapped dispersive spin waves to gapless magnetic excitations

Magnetic excitations and the spin Hamiltonian of the spin-1 trimer chain compound CaN i₃P₄O₁₄ have been investigated by inelastic neutron scattering. The trimer spin chains in CaN i₃P₄O₁₄ result from the crystal structure that provides a special periodicity of the exchange interactions (J₁-J₁-J₂ ) comprising intratrimer (J₁) and intertrimer (J₂) exchange interactions along the crystallographic b axis. Experimental data reveal gapped dispersive spin-wave excitations in the 3D long-range ordered magnetic state (T_C=16 K ), and gapless magnetic excitations above the T_C due to the low-dimensional spin-spin correlations within chains. Simulated magnetic excitations, by using the linear spin-wave theory, for a model of coupled trimer spin chains provide a good description of the observed experimental data. The analysis reveals both ferromagnetic J₁ and J₂ interactions within the chains, and an antiferromagnetic interchain interaction J₃ between chains. The strengths of the J₁ and J₂ are found to be closer (J₂/J₁∼0.81 ), and J₃ is determined to be weaker (| J₃/J₁|∼0.69 ), which is consistent with the spin-chain-type crystal structure. Presence of a weak single-ion anisotropy (D /J =0.19 ) is also revealed. The strengths and signs of exchange interactions explain why the 1/3 magnetization is absent in the studied spin-1 compound CaN i₃P₄O₁₄ in contrast to its S =5 /2 counterpart Mn-based isostructural compound. The signs of the exchange interactions are in agreement with that obtained from the reported density-functional theory calculations, whereas their strengths are found to be significantly different. The relatively strong value of the J₃ in CaN i₃P₄O₁₄ gives a conventional 3D-type magnetic ordering behavior below the T_C with full ordered moment at 1.5 K, however, retains its 1D character above the T_C. The present experimental study also reveals a sharp change of single-ion anisotropy across the T_C, indicating that the stability of the 3D magnetic ordering in CaN i₃P₄O₁₄ is ascribed to the local magnetic anisotropy in addition to interchain interactions. The present study divulges the importance of full knowledge of the exchange interactions in trimer spin-chain compounds to understand their exotic magnetic properties, such as 1/3 magnetization plateau. The importance of the observed gapless magnetic excitations of the strongly correlated spin chains above the T_C is also discussed.