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

TL;DR

This study investigates frustrated magnetism in Yb-based chalcogenide spinels with pyrochlore lattice, revealing antiferromagnetic order at very low temperatures, gapless excitations, and strong quantum fluctuations.

Contribution

It provides the first detailed analysis of the magnetic properties and quantum nature of Yb-based pyrochlore antiferromagnets, highlighting their frustrated magnetism and gapless excitations.

Findings

Antiferromagnetic order at 1.4-1.8 K much lower than exchange scale

Gapless excitations indicated by T^3 specific heat dependence

Reduced ordered magnetic moment suggesting strong quantum fluctuations

Abstract

Our polycrystalline sample study on the Yb-based chalcogenide spinels $A$Yb$_2$$X_4$ ($A =$ 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 $\sim 10$ K. The magnetic specific heat $C_{\rm M}$ 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 N\'{e}el temperature and the small local magnetic field estimated from the $\mu$SR measurements strongly suggest the significantly reduced size of the ordered moment in comparison with the bare moment size 1.33 $\mu_{\rm B}$/Yb supporting strong fluctuations in the commensurate and incommensurate ordered states in CdYb$_2$S$_4$ and MgYb$_2$S$_4$, respectively.