Molecular spin resonance in the geometrically frustrated MgCr2O4 magnet by inelastic neutron scattering

TL;DR

This study uses inelastic neutron scattering to identify discrete magnetic modes in MgCr2O4, revealing spin molecules as quasiparticles in a geometrically frustrated magnet, and suggests a suppression of lattice distortion by spin-lattice coupling.

Contribution

First direct observation of spin molecules as elementary excitations in a frustrated magnet using neutron scattering.

Findings

Identified two magnetic modes as antiferromagnetic hexamer and heptamer

Demonstrated spin molecules behave as quasiparticles with discrete energies

Indicated suppression of lattice distortion by spin-lattice coupling

Abstract

We measured two magnetic modes with finite and discrete energies in an antiferromagnetic ordered phase of a geometrically frustrated magnet MgCr2O4 by single-crystal inelastic neutron scattering, and clarified the spatial spin correlations of the two levels: one is an antiferromagnetic hexamer and the other is an antiferromagnetic heptamer. Since these correlation types are emblematic of quasielastic scattering with geometric frustration, our results indicate instantaneous suppression of lattice distortion in an ordered phase by spin-lattice coupling, probably also supported by orbital and charge. The common features in the two levels, intermolecular independence and discreteness of energy, suggest that the spin molecules are interpreted as quasiparticles (elementary excitations with energy quantum) of highly frustrated spins, in analogy with the Fermi liquid approximation.