“…[ 38,39 ] Up to now, pursuing ultralow roughness to improve device performance has an important technical task for classical spintronic materials (Co, Y 3 Fe 5 O 12 ). [ 40,41 ] Furthermore, the ultrasmooth interface is promising for constructing original devices and exploring emergent physics, as the boom of van der Waals spintronics in recent years. [ 42,43 ] Even for other optoelectronic applications (e.g., photovoltaic solar cells, [ 44,45 ] lasers, [ 46 ] PeLED, [ 47 ] field‐effect transistors [ 48 ] ), numerous efforts have been paid to improving film quality to avoid the adverse impact from defects.…”