Phase transitions and iron-ordered moment form factor in LaFeAsO

Abstract

Elastic neutron scattering studies of an optimized LaFeAsO single crystal reveal that upon cooling, an onset of the tetragonal (T)-to-orthorhombic (O) structural transition occurs at $T_{\text{S}}\approx 156$ K, and it exhibits a sharp transition at $T_{\text{P}}\approx 148$ K. We argue that in the temperature range $T_{\text{S}}$ to $T_{\text{P}}$, T and O structures may dynamically coexist possibly due to nematic spin correlations recently proposed for the iron pnictides, and we attribute $T_{\text{P}}$ to the formation of long-range O domains from the finite local precursors. The antiferromagnetic structure emerges at $T_{\text{N}}\approx 140$ K, with the iron moment direction along the O \emph{a} axis. We extract the iron magnetic form factor and use the tabulated $⟨j_0⟩$ of Fe, Fe${}^{2+}$ and Fe${}^{3+}$ to obtain a magnetic moment size of $\sim$0.8 $\hat{I}{¼}_{\text{B}}$ at 9.5 K.

7 pages, 6 figures, 3 tables