Spin trimers in Ca₃Cu₂Ni(PO₄)₄

The nature of spin trimer and 3d spin dynamics in Ca₃Cu₂Ni(PO₄)₄ is studied using ³¹P NMR measurements. The data are obtained by replacing one of the Cu ions by a Ni ion in the one dimensional spin trimer compound Ca₃Cu₃(PO₄)₄. The NMR spectrum suggests the presence of two types of magnetically inequivalent phosphorous atoms, as in the parent compound. This reveals the presence of only one type of trimer, Cu2-Ni-Cu2, rather than of three types, Cu2-Cu1-Cu2, Cu2-Cu1-Ni and Ni-Cu1-Ni, as indicated by neutron scattering. Hence, the ground state of Ca₃Cu₂Ni(PO₄)₄ appears to be a quintet. These results also explain the magnetic susceptibility behavior, which indicates no reduction in the net spin of the trimer in Ca₃Cu₂Ni(PO₄)₄, as expected for J_Ni-Cu = −0.85 meV compared to the value J_Cu-Cu = −4.74 meV derived from neutron scattering. The spin lattice relaxation rate suggests that the three magnon mediated scattering process dominant in Ca₃Cu₃(PO₄)₄ is reduced in Ca₃Cu₂Ni(PO₄)₄.