Magnetic and lattice coupling in single-crystal ${\text{SrFe}}_2{\text{As}}_2$: A neutron scattering study

A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal ${\text{SrFe}}_2{\text{As}}_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_{\text{O}}=T_{\text{N}}=\left(201.5\pm 0.25\right)\text{K}$. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1–2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than $T_{\text{O}}$, we suggest that the T-O transition may be an order-disorder transition.

Figure 1

(Color online) Crystal $\left(Fmmm\right)$ and AFM structure of $A{\text{Fe}}_2{\text{As}}_2$ ($A=\text{Ca}$, Sr, and Ba) below $T_{\text{O}}$.

Figure 2

(Color online) (a) Rocking curve of ${\left(008\right)}_{\text{O}}$ indicating the mosaic spread in our crystal. (b) Coexistence of O and T structures at 201.5 K and splitting of ${\left(040\right)}_{\text{O}}/{\left(400\right)}_{\text{O}}$ due to twins in $Fmmm$ symmetry at 120 K upon warming. O and T stand for orthorhombic and tetragonal, respectively.

Figure 3

(Color online) Temperature variations (warming: solid symbols; cooling: void symbols) of (a) the lattice parameters $a_{\text{O}}$, $b_{\text{O}}$, and $c_{\text{O}}$, and $a_{\text{T}}$ (circles) and $c_{\text{T}}$, (b) the integrated intensity of AFM ${\left(103\right)}_{\text{O}}$ of a single-crystal ${\text{SrFe}}_2{\text{As}}_2$. (${\mathrm{a}}^{\prime }$) Coexistence region of the O and T phases. (${\mathrm{b}}^{\prime }$) Enlarged (b) near the transition temperature. O and T stand for orthorhombic and tetragonal, respectively. The lattice parameter $a_{\text{T}}$ in $I4/mmm$ symmetry was multiplied by $\sqrt{2}$ for comparison to the $Fmmm$ lattice parameters $a_{\text{O}}$ and $b_{\text{O}}$.

Figure 4

(Color online) Temperature variation in the O distortion in the crystalline $a\text{−}b$ plane, namely, $S=\left(a_{\text{O}}-b_{\text{O}}\right)/\left(a_{\text{O}}+b_{\text{O}}\right)$. Inset (a) is the enlarged figure near the transition.

Figure 5

(Color online) The change in position of the AFM ${\left(103\right)}_{\text{O}}$ relative to the alignment at 20 K follows the change in the position of the ${\left(400\right)}_{\text{O}}$ related to the temperature dependence of the lattice parameter $a_{\text{O}}$. This temperature dependence is evidence that the direction of the AFM propagation vector is along the long O $a$ axis consistent with the observation of the complete absence of ${\left(100\right)}_{\text{O}}$ peak in our experiment. O stands for orthorhombic.

Figure 6

(Color online) Differences of neutron scattered intensities at the ${\left(hh0\right)}_{\text{T}}$ and ${\left(00l\right)}_{\text{T}}$ positions in the tetragonal phase (T stands for tetragonal) with temperature up to 600 K. The residual intensities at the ${\left(00l\right)}_{\text{T}}$ position suggest that the thermal effect is neglectable. The residual peak shapes at the ${\left(hh0\right)}_{\text{T}}$ position suggest a remnant orthorhombic phase is present above the O-T transition. The residual intensity at 300 and 450 K was shifted along its positive axis for clarity.