Magnetic-field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce(1-x)La(x)B(6)

TL;DR

This study explores how magnetic field and La doping affect low-energy spin fluctuations in Ce(1-x)La(x)B(6), revealing a crossover from itinerant to localized spin behavior and emphasizing the role of heavy-fermion quasiparticles.

Contribution

It provides new insights into the evolution of spin excitations in CeB6 under doping and magnetic field, highlighting the crossover between itinerant and localized spin dynamics.

Findings

Exciton energy decreases with La doping proportional to T_N.

Magnetic field suppresses exciton intensity but not energy.

Two new modes emerge above the critical magnetic field, growing linearly with field.

Abstract

CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its magnetic properties being typically interpreted within localized models. More recently, the observation of strong and sharp magnetic exciton modes forming in its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors suggested a significant contribution of itinerant electrons to the spin dynamics. Here we investigate the evolution of the AFQ excitation upon the application of an external magnetic field and the substitution of Ce with non-magnetic La, both parameters known to suppress the AFM phase. We find that the exciton energy decreases proportionally to T_N upon doping. In field, its intensity is suppressed, while its energy remains constant. Its disappearance above the critical field of the AFM phase is preceded by the formation of two modes, whose energies grow linearly with magnetic field upon entering the AFQ phase. These findings suggest a crossover from itinerant to localized spin dynamics between the two phases, the coupling to heavy-fermion quasiparticles being crucial for a comprehensive description of the magnon spectrum.