Lattice and magnetic instabilities in ${\text{CaFe}}_2{\text{As}}_2$: A single-crystal neutron diffraction study

Neutron diffraction measurements of a high-quality single crystal of ${\text{CaFe}}_2{\text{As}}_2$ are reported. A sharp transition was observed between the high-temperature tetragonal and low-temperature orthorhombic structures at $T_S=172.5\text{K}$ (on cooling) and 173.5 K (on warming). Concomitant with the structural transition, the Fe moments order in a commensurate antiferromagnetic structure with a saturated moment of $0.80\left(5\right){\mu }_B/\text{Fe}$ directed along the orthorhombic $a$ axis. The hysteresis of both the structural and magnetic transitions is 1 K between cooling and warming, consistent with previous thermodynamic, transport, and single-crystal x-ray studies and provides clear evidence of the coupling between the structural and magnetic transitions in this material.

Figure 1

(Color online) (a) $Q$ scans through the positions of the tetragonal $\left(1-1-2\right)$ nuclear peak (referenced to the tetragonal unit cell) and the magnetic peak positions at 180 K. No intensity is observed at the positions of the magnetic reflections. (b) Below the tetragonal-to-orthorhombic transition, two twin domains are observed in longitudinal scans through the $\left(20-2\right)$ (indexed to the orthorhombic cell) nuclear peak position. The magnetic peak $\left(10-1\right)$ is associated with only one of these domains. (c) Two-dimensional plots showing the $Q$ and $\omega$ (sample rotation in the scattering plane) dependences of peaks in (b).

Figure 2

(Color online) Illustration of the antiferromagnetic structure of ${\text{CaFe}}_2{\text{As}}_2$ below $T_S$. The magnetic unit cell is the same as the orthorhombic chemical unit cell. Fe moments are oriented along the orthorhombic $a$ axis.

Figure 3

(Color online) (a) Temperature dependence of the orthorhombic splitting (red curves) and magnetic integrated intensity (blue symbols) normalized to the orthorhombic volume fraction upon warming (filled circles) and cooling (open circles) through the transition. Below 170 K, both the orthorhombic distortion and the magnetic peak intensity are saturated. The dashed lines are guides to the eye. (b) The raw integrated intensities of the magnetic $\left(-101\right)$ reflection (blue symbols) and the orthorhombic (400) nuclear peak.