Magnetic cluster excitations in the antiferromagnetic phase of a-MnMoO4

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 S510 cluster ground state. The weaker antiferromagnetic (AFM) intercluster interactions, Jint = -4.5*10-3 meV, are treated in a molecular-field model below the AFM transition temperature TN= 10.7 K. INS and susceptibility are in quantitative agreement with this approach.