Excitations in Magnetic Systems with a Singlet Ground State

Abstract

We have studied the magnetic properties of a system with a crystal-field-only singlet ground state and a triplet first excited state in the presence of exchange interactions by using a pseudospin formalism, in the molecular-field and the random-phase approximations. By using the molecular-field results as a first approximation, we obtain the low-lying collective excitation spectra at zero temperature. The spontaneous magnetization and the correlation functions at finite temperatures can then be deduced. As a practical application of the model, we extend the calculations to a face-centered-cubic antiferromagnet and calculate the excitation spectra of the rare-earth compound TbSb. An estimate of the validity of this application is given and further studies and possible improvements of the present calculations are also suggested.