Annealing-Tunable Charge Density Wave in the Magnetic Kagome Material FeGe (2024)

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  Figure 1

  (a)Temperature-dependent in-plane magnetic susceptibilities of FeGe samples annealed at different temperatures for 48h, measured under a 0.1T magnetic field using zero-field cooling (ZFC) mode. The curves in (a)have been shifted vertically for clarity. The AFM, CDW, and canting transitions are marked by red squares, gray triangles, and blue circles. The inset in (a)shows the temperature derivative of ${\chi }_{\perp }(T)$ near $T_{\mathrm{N}}$. (b)Phase diagram of various transition temperatures tuned by the annealing temperature. (c)${\chi }_{\perp }(T)$ of a FeGe crystal annealed in the sequence of $560\text{−}320\text{−}560\text{−}320°\mathrm{C}$ (48h each) alongside a sample annealed at $320°\mathrm{C}$ for 8day (red curve). (d)Correlation of $T_{\mathrm{CDW}}$ and $T_{\mathrm{N}}$ with the $c$-axis lattice parameters. Lattice parameters were obtained from powder x-ray diffraction measurements with a ${\mathrm{LaB}}_6$ standard (Fig.S2 in Supplemental Material [32]). Dashed lines are guides to the eye. Note: Error bars in panels (b) and (d) are identical, but are not visible in (b)due to large symbol sizes.

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  Figure 2

  (a)Neutron diffraction pattern of a FeGe crystal annealed at $320°\mathrm{C}$, captured in the $[H,0,L]$ plane. (b)Integrated intensities of the AFM peak (1, 0, 0.5), incommensurate magnetic peak (1, 0, $0.5+\delta$), and CDW peak (1.5, 0, 0.5) derived from panel (a)data. Both $y$ axes in panel (b)share the same scale. (c) and (d)Temperature-dependent in-plane and out-of-plane magnetic susceptibilities, respectively. (e) and (f)Magnetostriction coefficient $\mathrm{d}\lambda /\mathrm{d}H$ and the corresponding $H-T$ phase diagrams, with black squares indicating the spin-flop transition field $H_{\mathrm{sf}}$.

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  Figure 3

  (a)Crystal structure of pristine FeGe at 300K. In addition to the established Ge1a (the Ge site in the ${\mathrm{Fe}}_3\mathrm{Ge}$ plane), a disordered Ge1b site is found shifted along the crystallographic $c$ axis as indicated by the light blue arrows. The degree of disorder strongly depends on the annealing conditions and is reflected in the occupancy of the disordered site and its distance to Ge1a as shown in (f). (b)$2\times 2\times 2$ CDW crystal structure of FeGe at 80K realized only for samples annealed at $320°\mathrm{C}$. To form the Ge1a-Ge1a dimers, the Ge1a atoms are moved upwards (downwards) from their initial positions. However, also for the low-$T$ structure a disordered Ge1b site is found at the position where the Ge1a atoms initially reside at 300K. (c),(d), and (e)Illustrate the RL view along the $c^{*}$ direction measured at 80K for samples annealed at 560, 480, and $320°\mathrm{C}$, respectively. The high-$T$ RL of all investigated samples looks essentially identical to the one in (c). (f)Distance at 300K between Ge1b and Ge1a which is a reasonable indicator for the amount of disorder.

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