Robust Ferromagnetically Insulating States in LaCoO ₃ Films through Hydrogen-Driven Multistate Topotactic Phase Transformations

One focal challenge in engineering low-power oxide spintronic devices lies in seeking room-temperature ferromagnetic insulators capable of generating pure spin currents free from Joule heating. Nevertheless, a robust ferromagnetic-insulating state in oxide materials is fundamentally restricted by a long-standing trade-off between ferromagnetic ordering and itinerant electrons in magnetic exchange interactions. Here, hydrogenation establishes a versatile paradigm for accessing exotic magnetoelectric states by precisely adjusting coupled ion-electron–lattice interplay, through which emergent room-temperature ferromagnetic insulators are stabilized in the LaCoO 3 system. The incorporation of hydrogen drives sequential LaCoO 3 –HLaCoO 2.67 -HLaCoO 2.5 topotactic phase transformations through oxygen vacancy ordering, progressively enhancing electron localization and ferromagnetic ordering. Benefiting from the double exchange interactions between high-spin Co 3+ and high-spin Co 2+, hydrogen-driven spin-state crossover results in the emergence of robust ferromagnetic ordering in LaCoO 3 above room temperature, beyond the traditional magnetoelectric phase diagram. Of particular note is the reversible but robust ferromagnetic-insulating state realized in LaCoO 3 through hydrogenation, which is attractive for low-power spintronic devices. Our present work demonstrates hydrogen-driven multistate magnetoelectric evolutions in LaCoO 3 through adjusting ion-electron–lattice interplay, fostering a ferromagnetic insulator above room temperature with great potential for low-power spintronics.