Magnetic excitations in the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2)

Inelastic neutron scattering experiments were performed for the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2) in order to study the dynamic magnetic properties. The observed excitations can be associated with transitions between the low-lying electronic states of linear Cu-Cu-Cu, Cu-Cu-Ni, and Ni-Cu-Ni trimers which are the basic constituents of the title compounds. The exchange interactions within the trimers are well described by the Heisenberg model with dominant antiferromagnetic nearest-neighbor interactions J. For x=0 we find JCu-Cu=-4.74(2) meV which is enhanced for x=1 to JCu-Cu=-4.92(6) meV. For x=1 and x=2 we find JCu-Ni=-0.85(10) meV and an axial single-ion anisotropy parameter DNi=-0.7(1) meV. While the x=0 and x=1 compounds do not exhibit long-range magnetic ordering down to 1 K, the x=2 compound shows antiferromagnetic ordering below TN=20 K, which is compatible with the molecular-field parameter 0.63(12) meV derived by neutron spectroscopy.