Spin Dynamics of a Model Singlet Ground-State System

Inelastic neutron scattering has been used to study the spin dynamics of $\mathrm{Li}{\mathrm{Tb}}_p{\mathrm{Y}}_{1-p}{\mathrm{F}}_4$, an induced-moment ferromagnet in which the lowest crystal-field levels of the ${\mathrm{Tb}}^{3+}$ ions are two singlets. For $p=0.38\mathrm{}$, magnetic excitons are observed which obey the dispersion relation given by mean-field theory. In addition, there is a resolution-limited peak at zero energy transfer which is not accounted for within conventional mean-field theory. For $p=0.97\mathrm{}$, this central peak is also observed, but no magnetic excitons are seen.