Magnetic field-induced softening of spin waves and hard-axis order in Kondo-lattice ferromagnet CeAgSb$_{2}$

TL;DR

This study investigates the spin wave dynamics and magnetic field effects in the hard-axis ferromagnet CeAgSb2 using neutron scattering, revealing field-induced softening, gap suppression, and a transition to a polarized state, advancing understanding of anisotropic Kondo-lattice ferromagnets.

Contribution

The paper provides the first detailed inelastic neutron scattering analysis of CeAgSb2's spin dynamics, modeling the dispersion with linear spin-wave theory and elucidating the effects of magnetic fields on its magnetic excitations.

Findings

Magnetic field suppresses the magnon gap and enhances specific heat near critical field.

Linear spin-wave theory accurately models the observed spin dispersion and field effects.

CeAgSb2 exhibits well-localized moments with weak Kondo coupling, evidenced by narrow magnon modes and moderate electronic specific heat.

Abstract

A significant number of Kondo-lattice ferromagnets order perpendicular to the easy magnetization axis dictated by the crystalline electric field. The nature of this phenomenon has attracted considerable attention, but remains poorly understood. In the present paper we use inelastic neutron scattering supported by magnetization and specific heat measurements to study the spin dynamics in the hard-axis ferromagnet CeAgSb2. In the zero field state we observed two sharp magnon modes, which are associated with Ce ordering and extended up to $\approx 3 meV with a considerable spin gap of 0.6 meV. Application of a magnetic field perpendicular to the moment direction reduces the spectral intensity and suppresses the gap and significantly enhances the low-temperature specific heat at a critical field of Bc ~ 2.8 T via a mean-field-like transition. Above the transition, in the field polarized state, the gap eventually reopens due to the Zeeman effect. We modeled the observed dispersion using linear spin-wave theory (LSWT) taking into account the ground state Gamma 6 doublet and exchange anisotropy. Our model correctly captures the essential features of the spin dynamics including magnetic dispersion, distribution of the spectral intensity as well as the field-induced behavior, although several minor features remain obscure. The observed spectra do not show significant broadening due to the finite lifetime of the quasiparticles. Along with a moderate electronic specific heat coefficient gamma = 46 mJ/mol K2 this indicates that the Kondo coupling is relatively weak and the Ce moments are well localized. Altogether, our results provide profound insight into the spin dynamics of the hard-axis ferromagnet CeAgSb2 and can be used as solid ground for studying magnetic interactions in isostructural compounds including CeAuSb2, which exhibits nematicity and unusual mesoscale magnetic textures.