Stefan T. Ochsenbein, Grégory Chaboussant, Andreas Sieber, Hans U. Güdel, Stefan Janssen, Albert Furrer and J. Paul Attfield
Magnetic cluster excitations in the antiferromagnetic phase of a-MnMoO₄
Phys. Rev. B 68, 092410/1-4 (2003)                 

Abstract: The tetramer-based compound a-MnMoO₄ exhibits four prominent peaks in the inelastic neutron scattering (INS) spectrum between 0.5 and 2.0 meV below 10 K. They are assigned to magnetic excitations of the (Mn²⁺)₄ rhombus shaped cluster, with resulting values of the exchange parameters J = +0.051 meV and J' = –0.019 meV along the edges and the short diagonal, respectively. The interactions within the tetramer are treated exactly in an isotropic quantum mechanical model leading to an S = 10 cluster ground state. The weaker antiferromagnetic (AFM) intercluster interactions, J_int = –4.5×10^–3 meV, are treated in a molecular-field model below the AFM transition temperature T_N = 10.7 K. INS and susceptibility are in quantitative agreement with this approach.