Electron doping evolution of the anisotropic spin excitations in BaFe _2-xNi _xAs ₂

We use inelastic neutron scattering to systematically investigate the Ni-doping evolution of the low-energy spin excitations in BaFe _2-xNi _xAs ₂ spanning from underdoped antiferromagnet to overdoped superconductor (0.03≤x≤0.18). In the undoped state, BaFe ₂As ₂ changes from paramagnetic tetragonal phase to orthorhombic antiferromagnetic (AF) phase below about 138 K, where the low-energy (≤∼80 meV) spin waves form transversely elongated ellipses in the [H,K] plane of the reciprocal space. Upon Ni doping to suppress the static AF order and induce superconductivity, the c-axis magnetic exchange coupling is rapidly suppressed and the momentum distribution of spin excitations in the [H,K] plane is enlarged in both the transverse and longitudinal directions with respect to the in-plane AF ordering wave vector of the parent compound. As a function of increasing Ni-doping x, the spin excitation widths increase linearly but with a larger rate along the transverse direction. These results are in general agreement with calculations of dynamic susceptibility based on the random phase approximation (RPA) in an itinerant electron picture. For samples near optimal superconductivity at x≥0.1, a neutron spin resonance appears in the superconducting state. Upon further increasing the electron doping to decrease the superconducting transition temperature T _c, the intensity of the low-energy magnetic scattering decreases and vanishes concurrently with vanishing superconductivity in the overdoped side of the superconducting dome. Comparing with the low-energy spin excitations centered at commensurate AF positions for underdoped and optimally doped materials (x≤0.1), spin excitations in the overdoped side (x=0.15) form transversely incommensurate spin excitations, consistent with the RPA calculation. Therefore, the itinerant electron approach provides a reasonable description to the low-energy AF spin excitations in BaFe _2-xNi _xAs ₂.