Spin dynamics in the manganese tetramer compound α-MnMoO(4).

α-MnMoO(4) is a tetrameric magnetic cluster system which undergoes a transition to three-dimensional antiferromagnetic order at T(N) = 10.7 K. In the ordered state the Mn(2+) spins (s(ν) = 5/2) are ferromagnetically aligned within the tetramer, resulting in a total cluster spin S = 10. The magnetic excitation spectrum consists of eight excitation modes of tetramer origin propagating in the whole reciprocal space. From single-crystal inelastic neutron scattering investigations the three-dimensional coupling scheme is rationalized in the framework of a tetramer-based dispersion model. The tetramer states and the energy dispersion of all the magnetic excitations are described by Heisenberg-like intra- and intercluster exchange interactions, respectively, thus α-MnMoO(4) is a suitable tetrameric model system to study the interplay of these interactions.