Origin of Higher Order Magnetic Exchange: Evidence for Local Dimer Exchange Striction in CsMn_0.28Mg_0.72Br₃ Probed by Inelastic Neutron Scattering

The origin of higher-order exchange interactions in localized S-state systems has been the subject of intensive investigations in the past. In particular, it has been suggested that a biquadratic exchange term may arise from the magnetoelastic energy. Here we report on the pressure and temperature dependence of the excitation spectra of magnetic Mn²⁺ dimers in CsMn_0.28Mg_0.72Br₃ probed by inelastic neutron scattering. Biquadratic exchange and a strong distance dependence of the bilinear exchange are observed. It is shown that the mechanism of local exchange striction may explain the occurrence of biquadratic exchange in accordance with the elastic properties of the compound.