Observation of non-linear effects in a quasi-one-dimensional antiferromagnet: magnetic excitations in

The spin dynamics of the hexagonal -type quasi-one-dimensional antiferromagnet is investigated by means of an inelastic neutron scattering technique. In good qualitative agreement with a recent spin-wave calculation including higher-order terms, a large scattering cross-section arising from two-magnon excitations is observed at the one-dimensional antiferromagnetic zone centre. In addition, we measured spin-wave excitations between the chains precisely and revealed that the spin-wave dispersion curves are modified in energy and in intensity on account of the anticrossing between the one-magnon branches and two-magnon continuum. These results demonstrate that anharmonic terms are important in the spin dynamics of even at low temperatures. We also measured the temperature dependence of the magnetic excitations and found that far above the Néel temperature the two-magnon process gives a considerable contribution to the inelastic spectrum.