Magnetoelastic signatures of the conical state and charge density waves in antiferromagnetic FeGe

Abstract

Kagome systems host intertwined spin, charge, and lattice degrees of freedom that drive emergent collective states. Here, we identify two distinct energy scales in the noncollinear kagome magnet FeGe: a field-tunable magnetic fluctuation channel at $\sim 35$~K associated with the conical state, and a field-independent channel at $\sim 100$~K linked to charge-density-wave fluctuations. Ultrasound measurements provide direct access to the exchange-renormalized magnetic stiffness, whose softening governs the acoustic anomaly and follows a symmetry-constrained quadratic field dependence. We further predict a linear temperature dependence of neutron diffraction intensities at fixed field and a quadratic field suppression at fixed temperature. These results identify magnetic stiffness as a key control parameter of spin-lattice dynamics and establish ultrasound as a sensitive probe of coupled collective modes.