Highly Robust Room-Temperature Interfacial Ferromagnetism in Ultrathin Co₂Si Nanoplates

The reduced dimensionality and interfacial effects in magnetic nanostructures open the feasibility to tailor magnetic ordering. Here, we report the synthesis of ultrathin metallic Co₂Si nanoplates with a total thickness that is tunable to 2.2 nm. The interfacial magnetism coupled with the highly anisotropic nanoplate geometry leads to strong perpendicular magnetic anisotropy and robust hard ferromagnetism at room temperature, with a Curie temperature (T_C) exceeding 950 K and a coercive field (H_C) > 4.0 T at 3 K and 8750 Oe at 300 K. Theoretical calculations suggest that ferromagnetism originates from symmetry breaking and undercoordinated Co atoms at the Co₂Si and SiO₂ interface. With protection by the self-limiting intrinsic oxide, the interfacial ferromagnetism of the Co₂Si nanoplates exhibits excellent environmental stability. The controllable growth of ambient stable Co₂Si nanoplates as 2D hard ferromagnets could open exciting opportunities for fundamental studies and applications in Si-based spintronic devices.