Frustrated magnetism in the Heisenberg pyrochlore antiferromagnets $A$Yb$_2$$X$$_4$ ($A$ = Cd, Mg, $X$ = S, Se)

Abstract

Our polycrystalline sample study on the Yb-based chalcogenide spinels AYb2X4 (A = Cd, Mg, X = S, Se) has revealed frustrated magnetism due to the antiferromagnetically coupled Heisenberg spin on the pyrochlore lattice. Our crystal electric field analysis indicates the Yb ground state has nearly Heisenberg spins with a strong quantum character of the ground doublet. All the materials exhibit an antiferromagnetic order at 1.4-1.8 K, much lower temperature than the antiferromagnetic exchange coupling scale of ∼ 10 K. The magnetic specific heat CM shows a T 3 dependence, indicating the gapless feature in the Yb-based chalcogenide spinels. The magnetic entropy change much smaller than R ln 2 below the Néel temperature and the small local magnetic field estimated from the µSR measurements strongly suggest the significantly reduced size of the ordered moment in comparison with the bare moment size 1.33 µB/Yb supporting strong fluctuations in the commensurate and incommensurate ordered states in CdYb2S4 and MgYb2S4, respectively.