Terbium³⁺ Coupling Parameters and the Phase Diagram of the Singlet Ground State Terbium- Monopnictides

Abstract

The Terbium monopnictides undergo phase transitions to type-11 antiferromagnetic order between 7 and 18K. In the heavier pnictides, TbBi and TbSb, the phase changes take place continuously, while first order transitions occur in the lighter compounds TbP and TbAs (see review by Hulliger¹). It is the aim of this paper to elucidate to some extent the physical mechanisms behind this trend, that appears to be present also in other rare earth pnictides¹. For this purpose we first summarize some results of recent investigations on the microscopic interactions in the Tb³⁺-monopnictides^2,3 among them the magnetic exchange, quadrupole-quadrupole and crystalline field coupling parameters.We then investigate their effect on the phase transitions within a general phase diagram, from where one realizes the strong influence of the crystalline field due to the singlet ground state of the Tb³⁺, and of the quadrupolar forces on the nature of the transitions.