Magnetic Interactions in Tb and Tb-10% Ho from Inelastic Neutron Scattering

The magnon dispersion relations and lifetimes have been measured in Tb and a Tb‐10% Ho alloy by inelastic neutron scattering, in regions of both ferromagnetic and spiral ordering. In the ferromagnetic phase, the magnon energy is generally finite at zero wavevector and rises quadratically at low $q$ . The magnon energies scale approximately with the magnetization. In the spiral phase the magnon energy rises linearly from zero at low $q$ . The Fourier‐transformed exchange parameter $\mathrm{J(}\mathrm{q})$ has pronounced peaks in the $c$ direction, which are ascribed to transitions between states close to the Fermi surface. These peaks are less pronounced in the ferromagnetic phase. The primary mechanism limiting the magnon lifetimes appears to be interaction with the conduction electrons. In the alloy, the lifetime for magnons propagating in the $c$ direction in the ferromagnetic phase falls abruptly at about $\mathrm{q}{\text{\hspace{0.17em}=\hspace{0.17em}0.35\hspace{0.17em}Å}}^{\text{−1}}$ , and this may be due to the exchange splitting of the conduction‐electron energy bands. The dispersion curve for magnons propagating in the $a$ direction is considerably perturbed around 4 meV, and this is probably due to resonant scattering on the Ho impurities. A strong coupling occurs over the same energy range in the alloy between magnons and transverse phonons propagating in the $c$ direction. This effect is considerably smaller in pure Tb.