Incommensurate Magnetism in FeAs Strips: Neutron Scattering from ${\mathrm{CaFe}}_4{\mathrm{As}}_3$

Magnetism in the orthorhombic metal ${\mathrm{CaFe}}_4{\mathrm{As}}_3$ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate [${\mathit{q}}_m\approx (0.37–0.39)\times {\mathit{b}}^{*}$] and predominantly longitudinally ($\parallel b$) modulated order develops through a 2nd order phase transition at $T_N=89.63(6)\mathrm{K}$ with a 3D Heisenberg-like critical exponent $\beta =0.365(6)$. A 1st order transition at $T_2=25.6(9)\mathrm{K}$ is associated with the development of a transverse component, locking ${\mathit{q}}_m$ to $0.375(2){\mathit{b}}^{*}$, and increasing the moments from 2.1(1) to $2.2(3){\mu }_B$ for ${\mathrm{Fe}}^{2+}$ and from 1.3(3) to $2.4(4){\mu }_B$ for ${\mathrm{Fe}}^{+}$. The ab initio Fermi surface is consistent with a nesting instability in cross-linked FeAs strips.

Figure 1

Crystal (a),(b) and magnetic structures of ${\mathrm{CaFe}}_4{\mathrm{As}}_3$ at (c),(d) $T=50\mathrm{K}$ and (e),(f) 5 K in $ac$ plane (left) and along $b$ axis (right). Solid squares indicate a unit cell. The vector $\widehat{\mathit{d}}$ ($\widehat{\mathit{a}}·\widehat{\mathit{d}}=\cos 30°$) is used for right-hand panels.

Figure 2

A portion of the ${\mathrm{CaFe}}_4{\mathrm{As}}_3$ neutron diffraction pattern at 100, 50, and 5 K with Rietveld refinement (solid lines). The black bars and gray (green) bars with representative ($hkl$) indices show the calculated positions of nuclear and magnetic peaks, respectively.

Figure 3

(a) Color plot of the neutron scattering intensity versus temperature and wave vector transfer along the $(0k1)$ direction. The temperature dependence of the inferred integrated intensity and peak position are shown in frames (b) and (c), respectively. The vertical bar in (c) represents a systematic instrumental uncertainty. The insets of (b) show warming intensity against the reduced temperature near $T_N$ and coexistence of two peaks near $T_2$. The insets of (c) emphasize thermal hysteresis of $I$ and $\rho$.

Figure 4

(a) Fermi surface of ${\mathrm{CaFe}}_4{\mathrm{As}}_3$. Unlike in the 122 family, planelike Fermi surfaces are dominant. (b) Viewing along $k_c$ reveals tubelike pockets as well as planelike Fermi surfaces. The nesting vector $k=3/8$ is indicated.

Figure 5

Observed ($x$ axis) and calculated ($y$ axis) values for the squared magnetic structure factors within representative irreps at (a) $T=50\mathrm{K}$ and (b) 5 K.