Anisotropic itinerant magnetism and spin fluctuations in BaFe2As2: A neutron scattering study

TL;DR

This neutron scattering study reveals the anisotropic spin dynamics and low-energy excitations in BaFe2As2, highlighting its two-dimensional magnetic behavior and the evolution of spin fluctuations across the magnetic transition.

Contribution

First detailed neutron scattering analysis of magnetic excitations and spin fluctuations in BaFe2As2, elucidating its anisotropic and two-dimensional magnetic properties.

Findings

Low energy spin-wave gap of 9.8 meV in the ordered state

Anisotropic high-energy scattering indicating 2D spin dynamics

Quasi-elastic scattering above T_N showing short-range fluctuations

Abstract

Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide BaFe2As2, 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 of 9.8(4) 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) meV A, 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) K, the gap closes, and quasi-elastic 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.