Magnon Interactions in Terbium

Magnon energies and lifetimes have been studied in Tb and Tb-10% Ho single crystals by inelastic neutron scattering. The lifetimes of magnons propagating in the c-direction have been measured in the ferromagnetic phase of Tb, and are found to decrease with increasing temperature and wave-vector, probably principally due to magnon-magnon interactions. The interaction of magnons with phonons has also been observed and the effect of Ho impurities on this interaction studied. In addition, excitations which are ascribed to local modes associated with the Ho ions have been observed. The dependence of the indirect exchange interaction on temperature in the alloy gives information on the mechanisms responsible for the transition from the helical to ferromagnetic structures. The dependence of the magnon energies on magnetic field at low temperatures gives detailed information on the role of magnetoelastic effects in the magnetic anisotropy. It is found that the cylindrically symmetric magnetoelastic effect gives a large contribution to the q=0 magnon energy, in accordance with the ``frozen lattice'' model, and that the hexagonal anisotropy is probably principally due to crystal field effects. ©1970 The American Institute of Physics