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 s
pin excitations are expected to have a Lorentzian function centered at zero energy transfer, the high-energy ($hbaromega>100$ 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$_2$.
We use inelastic neutron scattering to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe$_{0.6}$Se$_{0.4}$ ($T_c=14$ K). In the low-temperature superconducting state, the imaginary part of
the dynamic susceptibility at the electron and hole Fermi surfaces nesting wave vector $Q=(0.5,0.5)$, $chi^{primeprime}(Q,omega)$, has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing $hbaromega$ for energies above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above $T_c$, the energy of the mode is weakly temperature dependent and vanishes concurrently above $T_c$. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe$_{2-x}$(Co,Ni)$_x$As$_2$, the mode in FeTe$_{0.6}$Se$_{0.4}$ is not directly associated with the superconducting electronic gap.
