Nickel based magnetic nanocrystals have been widely applied in magnetic and catalytic facilities. Tunable magnetic properties of nickel can be easily obtained via non-magnetic doping or phase transformation. However, phase transformation from face centered cubic (fcc) to hexagonal close packed (hcp) induced magnetism adjustment of Ni are always confused with nickel carbide (Ni₃C), due to the similar atomic structures of hcp-Ni and Ni₃C. Here, we present series of Au@Ni-carbide magnetic materials achieved from the controlled carbonation of Au@Ni core–shell structures, whose magnetism is tunable by adjusting the amount of carbon in the Ni layer. Ex-situ hard X-ray absorption spectroscopy (XAS) at the metal K edge and soft XAS at both metal L edge and carbon K edge provide solid evidence for the carbonation process from fcc-Ni to Ni_xC, rather than phase transformation to hcp-Ni. Further investigation reveals that the magnetism of the hybrids is mainly contributed from the residual fcc-Ni. The result represents an accurate and effective way to distinguish hexagonal Ni₃C from hcp-Ni, and provides the pathway to control magnetism of Ni-based materials for applications.