Magnetic excitations in the spin-trimer compounds Ca3 Cu3-x Nix (P O4) 4 (x=0,1,2)

Inelastic neutron scattering experiments were performed for the spin-trimer compounds Ca3 Cu3-x Nix (P O4) 4 (x=0,1,2) in order to study their 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±0.02 meV, which is enhanced for x=1 to JCu-Cu =-4.92±0.06 meV. For x=1 and x=2, we find JCu-Ni =-0.85±0.10 meV and an axial single-ion anisotropy parameter DNi =-0.7±0.1 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 λ=0.63±0.12 meV derived by neutron spectroscopy.