Realization of Fully High‐Spin State and Strong Ferromagnetism in LaCoO₃ Monolayer

Abstract Perovskite LaCoO 3 is a subject of extensive and ongoing investigation due to the delicate competition between high‐spin (HS) and low‐spin (LS) states of Co 3+ . On the other hand, their indistinct free energy boundary poses a significant challenge to annihilate the magnetically/electrically inert LS Co 3+ for yielding fully HS state. Here, electronic transformation from the conventional isovalent mixed HS/LS state () into an unprecedented aliovalent fully HS state () is demonstrated in monolayer LaCoO 3 confined by 5d SrIrO 3 slabs via atomically constructing SrIrO 3 /LaCoO 3 superlattices. Excitingly, this emergent fully HS monolayer exhibits not only remarkable 2D ferromagnetism beyond the Mermin–Wagner restriction, but also larger magnetization (≈1.8µ B /Co) and higher Curie temperature (above 100 K) than that of conventional thick film and any previously reported oxide‐based monolayer ferromagnets. Furthermore, Ir/Co hybridization driven orbital reconstruction with polarization beyond standard crystal field expectations is observed, which is supported by DFT calculations. The findings not only expand the electronic phase domains of LCO into fully HS state, but also provide a fresh platform for investigating the 2D magnetic physics under strongly spin‐orbit coupled regime and developing new 2D spintronic devices.