Inelastic neutron scattering study of spin waves in the garnet Mn3Al2Ge3O12 with a triangular magnetic structure

The spin-wave spectrum of the antiferromagnet Mn3Al2Ge3O12 with the garnet structure has been studied by inelastic neutron scattering on the triple-axis spectrometers IN12 and IN14 at ILL. Magnon dispersion curves were measured along [xi xi 0] and [xi xi xi] directions up to energies of 0.3 THz. The spin-wave symmetry analysis, which reduced the number of independent exchange parameters to six, was performed to ensure correct description of the spin-wave branches. As determined by the fitting, five of the parameters are statistically significant and have been found to be J(1) = -0.70(2) K, J(2) = -0.08(2) K, J(3) = -0.25(2) K, J(4) = -0.20(2) K and J(5) = -0.07(2) K. This quite unusual sequence of exchange parameters clearly demonstrates the importance of the superexchange chain geometry.