Role of Cu doping in promoting diffusion-assisted evolution of magnetic properties in equiatomic FeNi films

The increasing need for environment-friendly substitutes for rare-earth-based magnets has sparked interest in materials such as the L10-ordered FeNi (tetrataenite) phase, which possesses high magnetocrystalline anisotropy and saturation magnetization. Despite being a promising candidate, preparation of this ordered phase in the laboratory remains a challenge due to the slow diffusion kinetics that prevent atomic ordering under normal conditions. From the theoretical estimations and experimental results, Cu is known for accelerating the atomic interdiffusion and promoting chemical disorder, which may facilitate the grain boundary diffusion. In the present work, chemically homogeneous multilayers of equiatomic FeNi and Cu-doped FeNi (5 at.%) were studied to investigate the correlation between self-diffusion and magnetism. Nuclear resonance reflectivity and forward scattering measurements on as-deposited and annealed samples showed that Cu doping substantially increases self-diffusion, which is in agreement with significant changes in the local magnetic environment, as supported by conversion electron Mössbauer spectroscopy. Although the net magnetic moment remained nearly unchanged, an enhancement in the coercivity at 573 K was observed in the Cu-doped sample, as quantified by SQUID-VSM. These observations highlight the potential of Cu-assisted diffusion channels to facilitate the formation of ordered phases in FeNi systems as a strategic approach to the development of rare-earth-free permanent magnets.