Columnar magnetic structure coupled with orthorhombic distortion in the antiferromagnetic iron arsenide ${\text{SrFe}}_2{\text{As}}_2$

Neutron powder-diffraction experiments were carried out on polycrystalline ${\text{SrFe}}_2{\text{As}}_2$ in order to determine the magnetic structure and its relationship with the crystallographic one. Below $T_0=205\text{K}$, magnetic reflections appear simultaneously with the onset of the orthorhombic distortion. The magnetic propagation vector of ${\text{SrFe}}_2{\text{As}}_2$ is determined to be $\mathit{q}=\left(101\right)$; the coupling of Fe moments is antiferromagnetic along the longer $a$ direction within the Fe-As layer, and the interlayer coupling is antiferromagnetic as well. The size of the Fe magnetic moment is deduced to be $1.01\left(3\right){\mu }_B$ with an orientation parallel to the $a$ axis. The temperature dependence of the magnetic moment shows excellent agreement with not only that of the muon precession frequency but also with that of the structural distortion, revealing the strong coupling of the magnetic order and the structural distortion in ${\text{SrFe}}_2{\text{As}}_2$. The discrepancy in the deduced size of the ordered moment between neutron and Mössbauer and $\mu \text{SR}$ may imply an unusual magnetism in ${\text{SrFe}}_2{\text{As}}_2$.

Figure 1

(Color online) Neutron powder-diffraction patterns measured on E6 at (a) $220\text{K}\left(T>T_0\right)$ and (b) $1.5\text{K}\left(T<T_0\right)$. The result of the Rietveld analysis, residual curves, and the calculated peak positions indicated by tick marks are included in each panel. The inset shows the temperature variation in the peak profile around $2\theta =57.5°$.

Figure 2

(Color online) Temperature dependence of the 1 0 3 magnetic reflection intensity normalized to that at 1.5 K, plotted as $\sqrt{I/I_0}$, which is proportional to the size of the ordered moment. Closed circles and triangles correspond to the temperature dependence of normalized lattice distortion and the muon precession frequency $f_1$, respectively, of ${\text{SrFe}}_2{\text{As}}_2$ taken from Ref. 11. The inset shows the 1 0 3 magnetic peak profile taken at 50, 201, and 210 K.

Figure 3

(Color online) Magnetic Bragg peak profiles at (1 0 1), (1 0 3), and (1 2 1). Red (gray), dotted and solid lines are the calculated intensity assuming the respective magnetic structure model: (i) $\mathit{q}=\left(101\right)$, $\mu \parallel a$, (ii) $\mathit{q}=\left(011\right)$, $\mu \parallel b$, (iii) $\mathit{q}=\left(101\right)$, $\mu \parallel b$. Shaded arrows illustrate the orthorhombic distortion concomitantly occurs at $T_0$.

Figure 4

(Color online) Magnetic and crystal structure of ${\text{SrFe}}_2{\text{As}}_2$ in the ground state. The AFM coupling is along the $a$ direction having the longer Fe-Fe distance, corresponding to $\mathit{q}=\left(101\right)$. The Fe magnetic moment lies within the Fe plane and is aligned parallel to the $a$ axis.