Jiwen Yang, Bingbing Qiu, Nai Shi, Jun Huang, Yue Gao, Qiang Deng, Yangkai Wang, Bing Xiong, Jianlin Wang, Zhangzhang Cui, Qiuping Huang, Hao Cheng, Zhengping Fu, Bin Xiang, Qingyou Lu, Ranran Peng, Yalin Lu
Layered perovskite oxides offer exceptional structural tunability, providing an ideal platform for designing materials with finely controlled electronic and magnetic properties. Although water incorporation, hydration reactions, and proton insertion have been investigated in layered oxides, directly correlating lattice-confined water species with real-space structural reconstruction and reversible magnetic regulation remains unexplored. Here, by introducing a simple, highly controllable vapor-modulation strategy, we directly probe and trigger profound structural and magnetic transitions in Ruddlesden-Popper layered perovskite oxides. Such brief vapor-treatment of La0.25Sr2.75Fe0.8Co1.2O7-δ at 100 °C induces a dramatic 1257% enhancement in magnetic sensitivity and enables reversible weak-field magnetization switching. Through atomic-resolution imaging and theoretical simulations, we reveal that water molecules selectively intercalate into the rock-salt layers, causing c-axis expansion, lattice slip, and fundamentally overcoming the competing antiferromagnetic interactions to favor ferromagnetic coupling. This reversible water intercalation process, coupled with water-induced lattice sliding, provides a facile, low-cost, and reversible method for active magnetic control. More importantly, by directly visualizing the lattice-confined hydration process, our findings establish a real-space picture of water intercalation and storage in RP oxide lattices. This work opens avenues for designing advanced spintronic devices and high-performance humidity sensors and provides microscopic insights into water accommodation in layered oxide frameworks.