Fabricating room-temperature ferromagnetic insulators, which are crucial candidates for next-generation dissipation-free quantum and spintronic devices, remains a significant challenge. In this Letter, we report on the epitaxial synthesis of novel room-temperature ferromagnetic insulating thin films created through the precise construction of (111)-oriented $3\mathrm{d}/5\mathrm{d}$ interfaces. Our analysis indicates that, unlike conventional doping methods, the (111)-oriented ${\mathrm{SrIrO}}_3/{\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ ($\mathrm{SIO}/\mathrm{LSMO}$) interfaces exhibit markedly enhanced spin-orbit coupling. This enhanced interfacial spin-orbit coupling strengthens the electron-phonon coupling in LSMO, thereby shortening the electronic mean free path. As a result, the intrinsic metallicity of LSMO is suppressed, giving rise to a new ferromagnetic insulating phase that emerges between the ferromagnetic metal and paramagnetic insulator regimes of the LSMO phase diagram. Furthermore, the temperature window of the ferromagnetic insulating phase can be tuned by precisely controlling the thickness of the LSMO layers. Our Letter reveals a new strategy for developing ferromagnetic insulators by engineering $3\mathrm{d}/5\mathrm{d}$ interfaces and orientations, paving a way for the development of novel dissipation-free quantum and spintronic devices.

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• Metal-insulator transition
• Phase transitions
• Spin-orbit coupling
• Weak antilocalization
• Magnetic insulators