Excitations in the spin-1 trimer chain compound CaNi3P4O14 : From gapped dispersive spin waves to gapless magnetic excitations

Abstract

Magnetic excitations and the spin Hamiltonian of the spin-1 trimer chain compound CaNi 3 P 4 O 14 have been investigated by inelastic neutron scattering. The trimer spin chains in CaNi 3 P 4 O 14 are resulted from the crystal structure that provides a special periodicity of the exchange interactions (J 1-J 1-J 2) comprised of intratrimer (J 1) and intertrimer (J 2) 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 J 1 and J 2 interactions within the chains, and an antiferromagnetic interchain interaction J 3 between chains. The strengths of the J 1 and J 2 are found to be closer (J 2 /J 1 ~ 0.81), and J 3 is determined to be weaker (J 3 / J 1 ~ 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 CaNi 3 P 4 O 14 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 DFT calculations, whereas, their strengths are found to be significantly different. The relatively strong value of the J 3 in CaNi 3 P 4 O 14 gives a conventional 3D type magnetic ordering behaviour 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 CaNi 3 P 4 O 14 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.