Anisotropic itinerant magnetism and spin fluctuations inBaFe2As2: A neutron scattering study

Abstract

Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide ${\text{BaFe}}_{2}{\text{As}}_{2}$, a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy $\ensuremath{\Delta}=9.8(4)\text{ }\text{meV}$. The in-plane spin-wave velocity ${v}_{ab}$ and out-of-plane spin-wave velocity ${v}_{c}$ measured at 12 meV are 280(150) and $57(7)\text{ }\text{meV}\text{ }\text{\AA{}}$, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimensional spin dynamics, which possibly exist inside the Stoner continuum. At ${T}_{N}=136(1)\text{ }\text{K}$, the gap closes and quasielastic scattering is observed above ${T}_{N}$, indicative of short-range spin fluctuations. In the paramagnetic state, the scattering intensity along the $L$ direction becomes ``rodlike,'' characteristic of uncorrelated out-of-plane spins, attesting to the two-dimensionality of the system.