L1₀-ordered FeNi (tetrataenite) exhibits a large saturation magnetization and a robust uniaxial magnetic anisotropy, making it a valuable rare-earth free candidate for permanent magnet applications. It has been predominantly identified in iron meteorite rocks, and its laboratory synthesis has been hindered due to sluggish diffusion occurring below the order-disorder temperature of about 600 K. Doping of Cu is anticipated to facilitate interdiffusion and therefore enabling the laboratory synthesis of the L1₀-ordered FeNi phase. This study investigates interdiffusion and magnetization in undoped and Cu-doped Fe/Ni thin film multilayers through a complementary usage of polarized neutron reflectivity (PNR) and SQUID-VSM (S-VSM). The PNR measurements were performed on the as-deposited (at 423 K) and isochronally annealed samples at 473, 503, 533, and 573 K. It was found that the magnetic moment of Fe/Ni multilayers was not significantly affected by Cu doping but a substantial increase in interdiffusion is clearly evident. The enhancement of coercivity and anisotropy field was further examined using S-VSM. These findings are expected to lead to a new pathway toward the laboratory synthesis of the L1₀ phase.