Transition from Van Vleck Paramagnetism to Induced Antiferromagnetism inTbζY1−ζSb

Abstract

Low-field susceptibility and high-pulsed-field magnetization experiments have been performed on single crystals of ${\mathrm{Tb}}_{\zeta }{\mathrm{Y}}_{1-\zeta }\mathrm{Sb}$ across the entire composition range from YSb to TbSb. As the terbium concentration increases the magnetic behavior shows a transition from Van Vleck paramagnetism to antiferromagnetism at low temperatures at a Tb concentration of 40.3%. We attribute this antiferromagnetic ordering to an induced-moment effect in the ${\Gamma }_1$ singlet crystal-field ground state pertinent to ${\mathrm{Tb}}^{3+}$ in an octahedral crystal field. The crystal-field strength remains essentially constant, while the average exchange field varies approximately linearly with changing terbium concentration. We show that the behavior for small magnetic fields in the paramagnetic regime ($\zeta \le 0.403$) and at or near the Néel temperature in the antiferromagnetic regime ($\zeta \ge 0.403$) can be understood on the basis of a linear molecular-field theory involving one crystal field and one exchange parameter. Analysis of the small-moment data gives an energy splitting, in the absence of exchange, of 11.9°K from the ${\Gamma }_1$ crystal-field singlet ground state to the ${\Gamma }_4$ triplet first excited state of the ${\mathrm{Tb}}^{3+}$ ion. We show that high-field anisotropic magnetization measurements at 1.5°K in the paramagnetic regime ($\zeta \le 0.403$) indicate a significant contribution at large magnetization from higher-order anisotropic effective exchange; and we discuss how such measurements can be used to study the orbital contribution to effective exchange interaction.