Induced moment magnetism in TbAsO4: Constant coupling approximation

The constant-coupling approximation (CCA) is applied to the Ising model with a transverse field which is then used to study the induced-moment magnetism in TbAsO₄. We first show that in contrast to previous work, the CCA is valid throughout the entire temperature range; there exist no anomalies at low temperatures. A comparison of the phase boundaries in the Δ (transverse field)-T plane as predicted by the molecular-field approximation (MFA), CCA, and the high temperature series expansion (HTE), shows that the CCA predicts a transition temperature which is much closer to the HTE value for all Δ. The temperature dependence of the sublattice magnetization is also shown for the CCA and the MFA. A fitting to the experimentally observed energy level splitting is shown for TbAsO₄. By using a single parameter, the exchange integral, a good agreement is found between the theory and the experiment for the whole range of temperature.