P. S. Häfliger, S. T. Ochsenbein, B. Trusch, H.-U. Güdel and A. Furrer
Spin dynamics in the manganese tetramer compound a-MnMoO₄
J. Phys.: Condens. Matter 21, 026019/1-7 (2009)                 

Abstract: a-MnMoO₄ 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 Mn²⁺ spins (s_n = 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 a-MnMoO₄ is a suitable tetrameric model system to study the interplay of these interactions.