The unresolved issue of magnetism of the alloyed AuGe/Ni/Au Ohmic contacts in GaAs is investigated in the context of Hall magnetic field sensors. Magnetization measurements provided some interesting insights into changes taking place in the metallization layer before alloying with the substrate takes place. The results show that Ni undergoes solid-state, solubility-limited, dissolution into the AuGe layer during the course of anneal. This results in the metallization structures becoming non-magnetic before alloying with the substrate takes place. Solubility of Ni into AuGe increases with temperature and decreases with decreasing Ge content from the eutectic composition (88:12 wt%) of the AuGe alloy. The melting temperature of the AuGe layer increases as a result of the Ni dissolution into it. Formation of NiGe phase in the structure is observed after cooling from anneal temperature at which the magnetic to non-magnetic transformation is completed, but alloying has not yet occurred.