Phase Diagram and Magnetic Excitations of BaFe2 xNixAs2: A Neutron Scattering Study Introduction Iron based superconductors Neutron scattering Phase diagram and incommensurate antiferromagnetic order of BaFe2 xNixAs2 Origin of the incommensurate magnetic order in BaFe2 xNixAs2 Nematic spin correlations in detwinned BaFe2 xNixAs2 Magnetic excitations in overdoped BaFe2 xNixAs2 Summary and outlook References Publications Resume Acknowledgements. This book studies the structural magnetic and electronic properties of as well as magnetic excitations in high temperature BaFe2 xNixAs2 superconductors using neutron diffraction and neutron spectroscopic methods. It describes the precise determination of the phase diagram of BaFe2 xNixAs2 which demonstrates strong magnetoelastic coupling and avoided quantum criticality driven by short range incommensurate antiferromagnetic order showing cluster spin glass behavior. It also identifies strong nematic spin correlations in the tetragonal state of uniaxial strained BaFe2 xNixAs2. The nematic correlations have similar temperature and doping dependence as resistivity anisotropy in detwinned samples which suggests that they are intimately connected. Lastly it investigates doping evolution of magnetic excitations in overdoped BaFe2 xNixAs2 and discusses the links with superconductivity. This book includes detailed neutron scattering results on BaFe2 xNixAs2 and an introduction to neutron scattering techniques making it a useful guide for readers pursuing related research.