Excitations in the spin-1 trimer chain compound CaN i3 P4 O14: From gapped dispersive spin waves to gapless magnetic excitations

Magnetic excitations and the spin Hamiltonian of the spin-1 trimer chain compound CaNi3P4O14 have been investigated by inelastic neutron scattering. The trimer spin chains in CaNi3P4O14 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 (TC=16K), and gapless magnetic excitations above the TC 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 CaNi3P4O14 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 CaNi3P4O14 gives a conventional 3D-type magnetic ordering behavior below the TC with full ordered moment at 1.5 K, however, retains its 1D character above the TC. The present experimental study also reveals a sharp change of single-ion anisotropy across the TC, indicating that the stability of the 3D magnetic ordering in CaNi3P4O14 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 TC is also discussed.