Spin and lattice structures of single-crystalline ${\text{SrFe}}_2{\text{As}}_2$

We use neutron scattering to study the spin and lattice structure of single-crystal ${\text{SrFe}}_2{\text{As}}_2$, the parent compound of the FeAs-based superconductor $\left(\text{Sr},\text{K}\right){\text{Fe}}_2{\text{As}}_2$. We find that ${\text{SrFe}}_2{\text{As}}_2$ exhibits an abrupt structural phase transition at 220 K, where the structure changes from tetragonal with lattice parameters $c>a=b$ to orthorhombic with $c>a>b$. At almost the same temperature, Fe spins develop a collinear antiferromagnetic structure along the orthorhombic $a$ axis with spin direction parallel to this $a$ axis. These results are consistent with earlier work on the $R\text{FeAsO}$ ($R=\text{rare}$ earth) families of materials and on ${\text{BaFe}}_2{\text{As}}_2$, and therefore suggest that static antiferromagnetic order is ubiquitous for the parent compounds of these FeAs-based high-transition temperature superconductors.

Figure 1

(Color online) Crystal and magnetic structures of ${\text{SrFe}}_2{\text{As}}_2$. (a) The three-dimensional AFM structure of Fe in ${\text{SrFe}}_2{\text{As}}_2$ as determined from our neutron-diffraction data. (b) The in-plane magnetic structure of Fe in the orthorhombic unit cell of ${\text{SrFe}}_2{\text{As}}_2$. The Fe moments are along the $a$ axis, and form an AFM collinear structure along the $a$-axis direction and ferromagnetic along the $b$-axis direction. The nearest-neighbor Fe spins along the $c$ axis are antiparallel, identical to that of LaFeAsO (Ref. 9); $J_{1a}$, $J_{1b}$, and $J_2$ indicate the effective exchange couplings. [(c) and (d)] Radial scans through the magnetic (1,0,1) and (1,0,3) magnetic Bragg peaks below and above the Néel temperature, showing clear resolution-limited magnetic peaks.

Figure 2

(Color online) Structural and magnetic phase transition as a function of temperature in single-crystal ${\text{SrFe}}_2{\text{As}}_2$. (a) Temperature dependence of the ${\left(2,2,0\right)}_T$ structural peak, showing that it abruptly splits below $220\pm 1\text{K}$. The data were collected using $10^{\prime }–{11}^{\prime }\text{−}S\text{−}10^{\prime }–80^{\prime }$ collimation. (b) Comparison of structural distortion and magnetic order parameter, both occurring at essentially the same temperature. The structural transition is first order, while the magnetic transition appears to be continuous.

Figure 3

(Color online) (a) Rocking curves of the (3,0,3) magnetic Bragg peak, and its comparison with structural Bragg peaks obtained from $\lambda /2$ of the nuclear (6,0,6) and (0,6,6) reflections. (b) Identical rocking curve for (0,3,3) magnetic peak position showing no magnetic scattering. This provides definitive evidence that the AFM order occurs along the $a$-axis direction.

Figure 4

(Color online) (a) Calculated and observed integrated magnetic Bragg-peak intensities for Fe spin direction along the $a$ axis. The agreement is excellent, demonstrating that the moment direction is along $a$. $\varphi$ is the angle between Fe spin direction and $a$ axis, which was found to be close to zero for the best fit of the experimental data. [(b) and (c)] Resolution-limited $\left[H,0,1\right]$ and $\left[1,0,L\right]$ scans through the (1,0,1) magnetic Bragg peak, indicating that the order is long range in nature with a minimum correlation length of $330\text{Å}$.