  Developing a comprehensive magnetic theory of correlated itinerant magnets is
a challenging task due to the difficulty in reconciling both local moments and
itinerant electrons. In this work, we investigate the microscopic process of
magnetic phase transition in ferromagnet metal Fe$_{3-\delta}$GeTe$_2$. A new
paradigm is proposed to describe the magnetic phase transition in correlated
metallic ferromagnets, where Hund's coupling dominates the spectral weight
transfer between different spin channels, rather than spin-splitting as
described by the Stoner model. We recognize that our theory should be universal
for itinerant magnets. Additionally, we reveal an efficient way to achieve
novel quantum states from various competing orders in multi-site crystal
structures. Our research shows that Fe1 are proximate to Mott physics, while
Fe2 exhibit Hund physics due to their distinct atomic environments. These
competing orders work together to produce heavy fermion behavior within
ferromagnetic long-range order through well-defined quasiparticle bands, which
are promoted by Hund's coupling and further hybridized with relative itinerant
bands. The complex interactions of competing orders drive correlated magnetic
metal to a new frontier for discovering outstanding quantum states and exotic
phenomena in condensed matter physics.
