  Finding and designing ferromagnets that operate above room temperature is
crucial in advancing high-performance spintronic devices. The pioneering van
der Waals (vdW) ferromagnet Fe$_3$GaTe$_2$ has extended the way for spintronic
applications by achieving a record-high Curie temperature among its analogues.
However, the physical mechanism of increasing Cuire temperature still needs to
be explored. Here, we propose a practical approach to discovering
high-temperature ferromagnetic materials for spintronic applications through
flat band engineering. We simulate the magnetic transition directly from
strongly correlated calculations, reconciling the dual nature of $d$-electrons
with both localization and itinerant characters. Significantly, our systematic
studies unveil the emergence of quasi-particle flat bands arising from
collective many-body excitations preceding the ferromagnetic phase transition,
reinforcing magnetic stability through a positive feedback mechanism. This
research provides a promising pathway for exploring next-generation spintronic
devices utilizing low-dimensional vdW flat band systems.
