Effect of quadrupolar interactions on the magnetic transitions of the terbium-monopnictides

The thermodynamic properties below the Neél temperatures of the single-groundstate magnets TbP, TbAs, TbSb and TbBi are analyzed within the mean-field approximation. If, in addition to the magnetic interaction, also quadrupole-quadrupole couplings between the Tb³⁺ are taken into account, the self-consistent calculations describe experimental data on the spontaneous dipolar and quadrupolar moments and the specific heat quite satisfactorily up toT_N. In a general phase diagram, the validity of the MFA is traced to the quadrupolar coupling, pushing the transitions to first or almost first order. Going from TbP to TbBi, a systematic strengthening of the exchange coupling is observed which is probably due to the conduction electrons. The ferrodistortive quadrupolar interaction, decreasing from TbP to TbBi, is also dominated by exchange interactions.Project of the SFB 65 Festkörperspektroskopie