  The burgeoning fields of spintronics and topological electronics require
materials possessing a unique combination of properties: ferromagnetism,
metallicity, and chemical stability. SrRuO3 (SRO) stands out as a compelling
candidate due to its exceptional combination of these attributes. However,
understanding its behavior under tensile strain, especially its
thickness-dependent changes, remains elusive. This study employs
machine-learning-assisted molecular beam epitaxy to investigate SRO films with
thicknesses from 1 to 10 nm. This work complements the existing focus on
compressive-strained SRO, opening a new avenue for exploring its hitherto
concealed potential. Using soft X-ray magnetic circular dichroism, we uncover
an intriguing interplay between film thickness, electronic structure, and
magnetic properties. Our key findings reveal an intensified localization of Ru
4d t2g-O 2p hybridized states at lower thicknesses, attributed to the weakened
orbital hybridization. Furthermore, we find a progressive reduction of magnetic
moments for both Ru and O ions as film thickness decreases. Notably, a
non-ferromagnetic insulating state emerges at a critical thickness of 1 nm,
marking a pivotal transition from the metallic ferromagnetic phase. These
insights emphasize the importance of considering thickness-dependent properties
when tailoring SRO for next-generation spintronic and topological electronic
devices.
