Pressure-Induced Inhomogeneous Chiral-Spin Ground State in FeGe

${}^{57}\mathrm{Fe}$ nuclear forward scattering on the chiral magnet FeGe reveals an extremely large precursor phase region above the helimagnetic ordering temperature $T_C(p)$ and beyond the pressure-induced quantum phase transition at 19 GPa. The decrease of the magnetic hyperfine field $⟨B_{\text{hf}}⟩$ with pressure is accompanied by a large increase of the width of the distribution of $⟨B_{\text{hf}}⟩$, indicating a strong quasistatic inhomogeneity of the magnetic states in the precursor region. Hyperfine fields of the order of 4 T (equivalent to a magnetic moment ${\mu }_{\mathrm{Fe}}\approx 0.4{\mu }_{\mathit{B}}$) persist up to 28.5 GPa. No signatures of magnetic order have been found at about 31 GPa. The results, supported by ab initio calculations, suggest that chiral magnetic precursor phenomena, such as an inhomogeneous chiral-spin state, are vastly enlarged due to increasing spin fluctuations as FeGe is tuned to its quantum phase transition.

Figure 1

Measured (open circles) and calculated (lines) ${}^{57}\mathrm{Fe}$ NFS spectra of cubic FeGe at (a) $p=0$, (b) 14.8 GPa and selected temperatures, and (c) $T=3\mathrm{K}$ at selected pressures.

Figure 2

Temperature dependence of the averaged hyperfine field $⟨B_{\text{hf}}⟩$ (top panel), of the relative width $\mathrm{\Delta }{B}_{\text{hf}}/⟨B_{\text{hf}}⟩$ of the hyperfine-field distribution (middle panel) and of the NFS count rate (bottom panel) at selected pressures (a) $p<10\mathrm{GPa}$ and (b) $p>10\mathrm{GPa}$. The count-rate curves are all normalized to unity at room temperature and then offset for clarity when needed. The positions of $T_C(p)$, $T_2(p)$, and $T_0(p)$ are indicated in all panels by the symbols explained in the legend of the middle panel in (a).

Figure 3

(a) ($T$, $p$) phase diagram of cubic FeGe including data from transport experiments (open symbols) [27]. The long-range helical magnetic order (LRMO) sets in at $T_C$ (black circles). The helix propagation along $⟨100⟩$ changes to $⟨111⟩$ below $T_2$ (blue squares); the point at $p=0$ (open square) is taken from Ref. [13]. $T_0$ (red triangles) is the upper boundary of the phase characterized by short range magnetic correlations, which is anticipated to be an inhomogeneous chiral-spin (ICS) state. The value of $T_0$ at ambient pressure is taken from Ref. [17]. (b) Pressure dependence of the experimental (bold symbols, $T=3\mathrm{K}$) and calculated (open symbols, $T=0\mathrm{K}$) average magnetic hyperfine field. $⟨B_{\text{hf}}⟩$ values for $p\le p_c$ and in the helical ground state are shown as black circles. Above $p_c$, $⟨B_{\text{hf}}(p)⟩$ (red triangles) is almost constant before it vanishes at $p_0=28.5(20)\mathrm{GPa}$.