Molecular spin-liquid state in spin-3/2 frustrated spinel HgCr$_2$O$_4$

TL;DR

This study reveals a unique spin-3/2 molecular singlet state in HgCr₂O₄, differing from other chromates, due to distinct exchange interactions, advancing understanding of spin-liquid-like states in frustrated magnets.

Contribution

It identifies a new molecular spin excitation in HgCr₂O₄ and explains its origin through comparative analysis and model Hamiltonians, highlighting the role of exchange interactions.

Findings

Discovery of a molecular spin-3/2 singlet in HgCr₂O₄

Different molecular excitations linked to exchange interactions

Comparison with other chromates clarifies excitation origins

Abstract

A hexamer-type spin excitation seen in spinel chromates $A$Cr$_2$O$_4$ ($A$ = Mg, Zn, or Cd) is the representative spin-liquid-like state caused by geometrical frustration. To clarify an origin of the state, we comparatively studied spin excitations in an isomorphic material HgCr$_2$O$_4$ by inelastic neutron scattering, and observed a different molecular-type excitation. Numerical analyses performed using model Hamiltonians suggest that these two types of spin excitations originate from a spin-3/2 molecular singlet hidden in a magnetically ordered phase. The difference between the molecular types is explained by the difference in the kind of exchange interactions occurring in the chromates.