INELASTIC NEUTRON-SCATTERING AND OPTICAL-ABSORPTION SPECTROSCOPY OF THE TI2+(MN2+)6 SPIN CLUSTER IN TI2+-DOPED MNBR2 AND MNCL2

MnCl2 and MnBr2 were doped with Ti2+ in low concentrations. The Ti2+(Mn2+)6 spin clusters thus created were studied by single-crystal absorption spectroscopy in the region of 3A2g --> 1E2(g) Ti2+ and 6A1g --> 4E(g), 4A1g Mn2+ excitations (D3d notation) as well as by inelastic neutron scattering. Exchange splittings were determined both in the electronic ground state and in the singly excited 4E(g), 4A1g state. Biquadratic exchange significantly affects the ground-state splitting in Ti2+:MnBr2; 2J(Ti-Mn) = -7.16 cm-1, and j = 0.18 cm-1. Mn2+-Mn2+ exchange can be neglected to a good approximation. In the lowest spin levels of the ground state the ordering has a ferromagnetic appearance, despite the antiferromagnetic nature of the Ti2+-Mn2+ exchange. The "large" spins of the six nearest-neighbor Mn2+ ions are lined up parallel by the dominant coupling to the "small" Ti2+ spin.