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

Abstract

We present a unified magnetoelastic framework describing ultrasound velocity anomalies in antiferromagnetic FeGe under low magnetic fields applied along the $c$ axis. A global multi-field analysis reveals the pronounced low-temperature anomaly near 35 K originates from hybridization between longitudinal acoustic phonons and a field-dependent magnetic mode associated with the exchange-driven conical spin structure, while the shoulder near 100 K arises from a field-independent charge density wave (CDW) susceptibility channel. The fitted parameters exhibit strong internal scaling relations, allowing the data to collapse onto universal magnetic and CDW scaling curves. By explicitly connecting the magnetic scaling variable to the cone angle measured by neutron diffraction, we establish a quantitative link between ultrasound softening and the evolution of the transverse spiral component of the double-cone structure. Our results therefore unify elastic and neutron-scattering observations within a single phenomenological framework and demonstrate that $Δv/v$ provides a sensitive probe of coupled magnetic and electronic instabilities in FeGe.

Figures (3)

• Figure 1: (a) The hexagonal $P6/mmm$ crystal structure of FeGe. Canted arrows illustrate the low-temperature magnetic moment orientations with in-plane ferromagnetic (FM) and antiferromagnetic (AFM) ordering along the $c$ axis. Transparent cones indicate the opening angle of the conical spin structure. (b) Schematic illustration of the low-temperature double cone-structure with helical propagation vector $\mathbf{q}\parallel c$ and the field-modulated AFM double-cone state. Dashed vertical lines indicate the field-induced change of the in-plane projection of the ordered moment (cone angle) (i.e., the cone angle) for $H\parallel c$. (c) Temperature dependence of the zero-field cooled magnetic susceptibility $\chi$ measured in 1 T for $H\parallel a$ and $c$.
• Figure 2: Temperature dependence of the relative sound-velocity change, $\Delta v/v$, measured at magnetic fields of 0, 3, and 5 T. Data were obtained for the longitudinal acoustic mode with $\mathbf{k} \parallel c$ and $\mathbf{u} \parallel c$, with the magnetic field applied along the $c$ axis. Dashed vertical lines mark the CDW transition temperature ($T_{\mathrm{CDW}}$) and the hybridization temperature ($T_{\mathrm{Hyb}}$), associated with the evolution of the magnetic cone half-angle. The green line corresponds to the fit using Eq. \ref{['eq:fit']}.
• Figure 3: Representative scaling collapse of (a) magnetic [eq. \ref{['eq:magnetic']}], and (inset) CDW [eq. \ref{['eq:CDW']}] channels. (b) Schematic two-parameter scaling space spanned by $(x_m, x_c)$, illustrating independent relevant directions associated with magnetic and CDW fluctuations.