Unified picture for the spin resonance mode in Cr-based spinel oxides using inelastic neutron scattering
18 Nov 2021
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- Evan Jones

 

 

Neutron scattering measurements of the spinel oxide LiGaCr4O8 reveal the coexistence of a nearly dispersionless resonance mode and dispersive spin-wave excitations in the magnetically ordered state, suggesting a unified picture for spin resonances in Cr

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​(a) Crystal struture of the spinel oxide LiGaCr4O8. (b) Measured magnetic excitaion spectra of LiGaCr4O8 at 5 K obtained from MERLIN time-of-flight (TOF) spectrometer. (c) The total simulated neutron scattering spectra combining the contributions from (d) calculated spin-wave spectra and (e) calculated hexagonal loops' excitations from the quantum spin model.

 

Geometrical frustration is a phenomenon present in lattices that are based on triangles or tetrahedra. It may prevent long-range magnetic order and lead to highly degenerated ground states with exotic spin-spin correlations and emergent excitations. The spinel oxide ACr2O4 is a canonical geometrically frustrated magnet. Transition metal chromium (Cr) atoms, which are arranged into a crystal structure with an octagonal base, form a pyrochlore lattice with corner-sharing tetrahedra. The pyrochlore lattice consists of alternating stacked kagome and triangle planes along the [111] direction and is highly frustrated. These Cr spinel oxides become ordered at a lower temperature than that the Curie-Weiss model predicts; this is to do with them being highly frustrated.

The Cr spinel oxide ACr2O4 exhibit exotic excitation spectra dominated by a dispersionless sharp resonance mode instead of conventional dispersive spin waves in the magnetically ordered states. Currently there is not a unified description of the resonance mode. Furthermore there is also not a clear causation for the underlying magnetic exchange interaction that induces the resonance mode among different types of Cr spinel oxides.

The spinel oxide LiGaCr4O8 has potential to provide opportunity to study the nature of spin excitations and magnetic interactions within spinel oxides. The research group lead by Prof. Jun Zhao from Fudan University has investigated the spin excitations in LiGaCr4O8 using inelastic neutron scattering experiments, performed on the MERLIN direct geometry chopper spectrometer at the ISIS Neutron and Muon Source.

Their data reveal both a dispersionless resonance mode and dispersive spin-wave excitations existing simultaneously in the magnetically ordered state. It was found that the strong spin resonance is associated with the hexagonal loops’ excitations based on the quantum spin loop model, while weak spin-wave excitations can be well reproduced using the same set of magnetic interaction parameters extracted from the spin loop model.

Furthermore, the researchers have shown that the resonance mode’s energy and lattice constant (Cr-Cr bond distance) follow a linear relationship among different classes of Cr spinel oxides, indicating that the resonance modes in these compounds have the same microscopic origin. The creation of a correct and accurate quantum spin model and the determination of the dominant magnetic interactions provide a strong knowledge base from which other exotic properties in similar systems can be deduced and ultimately understood.


Contact: Jones, Evan (STFC,RAL,ISIS)