Fe, Co, and Ni exhibit ferromagnetism due to their unique electronic structures, particularly the presence of unpaired d-electrons that align their spins. The ferromagnetism in 3d metals is primarily attributed to the exchange interaction between these unpaired electrons, which stabilizes parallel spin alignment. In contrast, 4f metals, such as those in the lanthanide series, demonstrate ferromagnetism through a combination of localized f-electron interactions and their coupling with the crystal field. The origin of ferromagnetism differs between these groups, with 3d metals relying on band structure effects and 4f metals on localized magnetic moments. Understanding these distinctions is crucial for applications in magnetic materials and technologies.