Temperature dependence of the paramagnetic spin excitations in BaFe$_2$As$_2$

TL;DR

This study investigates how high-energy paramagnetic spin excitations in BaFe₂As₂ behave with temperature, revealing persistent spin-wave-like features and temperature-independent magnetic moments, indicating unconventional magnetism and strong correlations.

Contribution

It provides the first detailed temperature-dependent analysis of high-energy spin excitations in BaFe₂As₂ using inelastic neutron scattering, highlighting deviations from conventional local moment models.

Findings

High-energy spin excitations exhibit spin-wave-like features up to 290 K.

Magnetic moments remain essentially temperature independent from 0.05T_N to 2.1T_N.

Results suggest unconventional magnetism and strong electron correlations.

Abstract

We use inelastic neutron scattering to study temperature dependence of the paramagnetic spin excitations in iron pnictide BaFe$_2$As$_2$ throughout the Brillouin zone. In contrast to a conventional local moment Heisenberg system, where paramagnetic spin excitations are expected to have a Lorentzian function centered at zero energy transfer, the high-energy ($\hbar\omega>100$ meV) paramagnetic spin excitations in BaFe$_2$As$_2$ exhibit spin-wave-like features up to at least 290 K ($T= 2.1T_N$). Furthermore, we find that the sizes of the fluctuating magnetic moments $<m^2>\approx 3.6\ \mu^2_B$ per Fe are essentially temperature independent from the AF ordered state at $0.05T_N$ to $2.1T_N$, which differs considerably from the temperature dependent fluctuating moment observed in the iron chalcogenide Fe$_{1.1}$Te [I. A. Zaliznyak {\it et al.}, Phys. Rev. Lett. {\bf 107}, 216403 (2011).]. These results suggest unconventional magnetism and strong electron correlation effects in BaFe$_2$As$_2$.