In recent years, few-layer or even monolayer ferromagnetic materials have drawn a great deal of attention due to the promising integration of two-dimensional (2D) magnets into next-generation spintronic devices. The SrRuO₃ monolayer is a rare example of stable 2D magnetism under ambient conditions, but only weak ferromagnetism or antiferromagnetism has been found. The bi-atomic layer SrRuO₃ as another environmentally inert 2D magnetic system has been paid less attention heretofore. Here we study both the bi-atomic layer and monolayer SrRuO₃ in (SrRuO₃)_n/(SrTiO₃)_m (n = 1, 2) superlattices in which the SrTiO₃ serves as a non-magnetic and insulating space layer. Although the monolayer exhibits arguably weak ferromagnetism, we find that the bi-atomic layer exhibits exceedingly strong ferromagnetism with a T_c of 125 K and a saturation magnetization of 1.2 µ_B/Ru, demonstrated by both superconducting quantum interference device (SQUID) magnetometry and element-specific X-ray circular dichroism. Moreover, in the bi-atomic layer SrRuO₃, we demonstrate that random fluctuations and orbital reconstructions inevitably occurring in the 2D limit are critical to the electrical transport, but are much less critical to the ferromagnetism. Our study demonstrates that the bi-atomic layer SrRuO₃ is an exceedingly strong 2D ferromagnetic oxide which has great potentials for applications of ultracompact spintronic devices.

This work presents an in-situ technique to quantify the layer-by-layer roughness of thin films and heterostructures by measuring the spectral profile of the reflection high-energy electron diffraction (RHEED). The characteristic features of the diffraction spot, including the vertical to lateral size ratio c/b and the asymmetrical ratio c₁/c₂ along the vertical direction, are found to be quantitatively dependent on the surface roughness. The quantitative relationships between them are established and discussed for different incident angles of high-energy electrons. As an example, the surface roughnesses of LaCoO₃ films grown at different temperatures are obtained using such an in-situ technique, which are confirmed by the ex-situ atomic force microscopy. Moreover, the in-situ measured layer-by-layer roughness oscillations of two LaCoO₃ films are demonstrated, revealing drastically different information from the intensity oscillations. The experiments assisted with the in-situ technique demonstrate an outstanding high resolution down to ~ 0.1 Å. Therefore, the new quantitative RHEED technique with real-time feedbacks significantly escalates the thin film synthesis efficiency, especially for achieving atomically smooth surfaces and interfaces. It opens up new prospects for future generations of thin film growth, such as the artificial intelligence-assisted thin film growth.