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Spin-state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low-spin to high-spin transition in LaCoO3 thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen-vacancy formation, spin-state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3 thin films is systematically modulated by varying the oxygen pressure during thin-film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen-vacancy and spin-ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low-spin to high-spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation-off-stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3 thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation-stoichiometry engineering.

Spin‐state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low‐spin to high‐spin transition in LaCoO3 thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen‐vacancy formation, spin‐state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3 thin films is systematically modulated by varying the oxygen pressure during thin‐film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen‐vacancy and spin‐ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low‐spin to high‐spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation‐off‐stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3 thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation‐stoichiometry engineering. [ABSTRACT FROM AUTHOR]

Magnetic spinel oxides have attracted extensive research interest due to their rich physics and wide range of applications. However, these materials invariably suffer suppressed magnetization, due to structural imperfections (e.g., disorder, anti‐site defects, etc.). Herein, a dramatic enhanced magnetization is obtained with an increasement of 5 µB/u.c in CoFe2O4 (CFO) through ionic liquid gating induced hydrogen doping. The intercalated hydrogen ions lead to both distinct lattice expansion of ≈0.7% and notable Fe valence state reduction through electron doping, in which ≈17% Fe3+ is reduced into Fe2+. These facts collectively trigger a site‐specific spin‐flip on tetrahedrally coordinated Co2+ sites that enhances the net ferrimagnetic moment nearly to its theoretical maximum for perfect CFO. [ABSTRACT FROM AUTHOR]

Topotactic phase transition obtains extensive research attention due to its associated rich physics as well as a promising potential application. Particularly, the oxygen incorporation into brownmillerite oxides leads to the structural transition into perovskite oxides with distinct magnetic and electronic properties. Using brownmillerite SrCoO2.5 (BM‐SCO), two novel pathways are revealed to achieve the topotactic phase transition into its corresponding perovskite SrCoO3 (P‐SCO). It is demonstrated that by using aqueous alkali as the electrolyte during gating, the negative biased voltage triggers a rapid transition into P‐SCO, which is attributed to the presence of strong oxidizing hydroxyl radicals. While surprisingly, it is observed that the acid solution with rich protons can also trigger an unexpected phase transition from BM‐SCO into P‐SCO in a much faster manner. With density functional theory calculations, this transition is elucidated as a proton‐assist ionic disproportionation, in which the Co ions are simultaneously oxidized and reduced, while the latter one is dissolved within the solution. These case studies not only achieve a deep understanding of the structural phase transition in BM‐SCO but also shed new light on the topotactic phase transition of complex oxides. [ABSTRACT FROM AUTHOR]

Complex oxide heterointerfaces and heterostructures have demonstrated enormous emergent phenomena over the last decades, attributed to the reconstructions of mis‐matched crystalline structure, polarity, and spin ordering across the heterointerfaces. This work employs the heterostructures of La0.7Sr0.3MnO3 and CaFeO2.5 as model system to demonstrate an interface‐specific oxygen migration/reconstruction across the interfaces due to the mismatched chemical potential, which dramatically influences the ferromagnetic and electronic states of La0.7Sr0.3MnO3 layer. Specifically, the alternative stacking of octahedral (Oh) and tetrahedral (Td) layers in CaFeO2.5 are used to form two distinct heterointerfaces, namely the Oh‐Td and the Oh‐Oh interfaces with the adjacent La0.7Sr0.3MnO3 layer. Interestingly, the oxygen ion migrates toward opposite directions across the interface for these two cases, in which the CaFeO2.5 layer acts as an “oxygen pump” and manipulates the oxygen contents of its adjacent La0.7Sr0.3MnO3 layers. Such manipulation leads to a dramatically changed ferromagnetic transition temperature for the heterostructure with the Oh‐Td and Oh‐Oh interface. This work establishes a feasible and efficient strategy to control the oxygen ionic distribution through atomic‐scale interface design and opens up new opportunities to exploit emergent states at the complex oxide heterostructures through selective oxygen ion evolution. [ABSTRACT FROM AUTHOR]

Localized electron polarons formed through the coupling of excess electrons and ionic vibrations play a key role in the functionalities of materials. However, the mechanism of the coexistence of delocalized electrons and localized polarons remains underexplored. Here, the discovery of high-mobility 2D electron gas at the rutile TiO 2 surfaces through argon ion irradiation induced oxygen vacancies is reported. Strikingly, the electron gas forms localized electronic states at lower temperatures, resulting in an abrupt metal-insulator transition. Moreover, it is found that the low-temperature conductivity in the insulating state is dominated by excess free electrons with a high mobility of ≈10 3 cm 2 V -1 s -1 , whereas the carrier density is dramatically suppressed with decreasing temperature. Remarkably, it reveals that the application of an electric field can lead to a collapse of the localized states, resulting in a metallic state. These results reveal the strongly correlated/coupled nature between the localized electrons and high-mobility electrons and offer a new pathway to probe and harvest the exotic electron states at the complex oxide surfaces., (© 2023 Wiley-VCH GmbH.)