Temperature dependence of magnetic excitations in the frustrated antiferromagnetic spinel ZnMn2 O4

We report the temperature-dependent evolution of spin ordering and excitations of the frustrated tetragonal (c>a) spinel ZnMn2O4 across the two-dimensional antiferromagnetic transition. Muon spin relaxation indicates full development of ordered magnetic moments immediately below TN=62.7(2)K in spite of the apparent low-dimensional ordering. Using inelastic neutron scattering, we obtained the spin Hamiltonian accounting for the temperature-dependent spin excitations. The damped spin waves at high temperature exhibit a continuous increase in their lifetime on cooling across TN. In contrast, the finite anisotropy gap appears suddenly below TN indicating that single-ion anisotropy stabilizes the antiferromagnetic chains. We also observed the frustrated out-of-plane exchange contributing to the high-energy modes.