Ionic-Liquid-Gating-Induced Hydrogenation in Epitaxial Strontium Ferrite

Incorporating hydrogen into complex oxides holds promise for discovering exotic phenomena and novel functionalities by promoting couplings between ion and lattice/charge/spin/orbital degrees of freedom. Here, electrolyte gating-driven hydrogenation is investigated in epitaxial brownmillerite SrFeO2.5 thin films in which the hydrogenation-induced lattice distortion is qualitatively different from its counterpart SrCoO2.5. The achievable lattice expansion in hydrogenated SrFeO2.5 is weaker than in SrCoO2.5 and primarily occurs along the normal of the stacked octahedral FeO6 and tetrahedral FeO4 layers. Upon the substitution of Fe with Co, the lattice expansion monotonically increases with increase of Co/Fe ratio, implying an intrinsic difference in accommodating hydrogen between Fe- and Co-based brownmillerites. Moreover, a net magnetic moment in hydrogenated SrFeO2.5 films is observed above room temperature, which gradually weakens with the increase of Co substitution, suggesting a stronger canted magnetism in Fe-based hydrogenated brownmillerites. This work clarifies the electrolyte gating-driven hydrogenation mechanisms in brownmillerite SrFeO2.5 epitaxial thin films and those with Co substitution, particularly the deterministic role of Co/Fe ratio in the evolution of structure and properties upon hydrogenation. The hydrogenation induced by ionic liquid gating in epitaxial SrFeO2.5 demonstrates distinctive dependences of structural evolutions on crystallographic orientations and Co substitutions, and promotes a ferromagnetic order above room temperature in SrFeO2.5, which gradually weakens with Co substitution increases. This work reveals the mechanisms for hydrogenation in BM-SFO, and provides insight into optimizing the hydrogenation-related functionalities by designing the Co/Fe compositions. image