Magnetic core-shell nanoparticles have the potential for numerous applications, such as in magnetic recording media, magnetic resonance imaging, drug delivery or hyperthermia, and spin valves. Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to the tunability of their magnetic properties. NiO is typically antiferromagnetic in nature and has a Néel temperature of 523 K. Our primary objective in this project is to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixMn1-xO (FM/FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixMn1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixMn1-xO CSNs have the rock salt cubic crystal structure throughout. SEM-EDS data indicates the presence of Mn in the CSNs. SQUID magnetic measurements show that the CSNs exhibit AFM/FM or AFM/FiM characteristics with a coercivity field of 425 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a significant exchange bias effect between the AFM NiO core and FM/FiM NixMn1-xO shell of the CSNs. The results of additional TEM and magnetic characterization are discussed.