Copper nickel magnetism

Ni is ferromagnetic. Cu is not. When Cu is added to Ni, they form an isomorphous substitutional alloy. Some of the Ni atoms are replaced by Cu atoms.

To put it simpistically, the interaction between a Ni-atom and a Cu-atom is not a ferromagnetic interaction. For a ferromagnetic interaction between 2 atoms, you must have the right extent of orbital overlap - and this depends on the shape of the valence wavefunctions as well as the separation between atoms. In a substitutional alloy, you are not changing the interatomic spacing, but by changing the atoms, you are changing the valence orbital shapes/sizes. By making some of the near-neighbor interactions be paramagnetic instead of ferromagnetic, you are reducing the magnetization.