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Epitaxial cobaltites have emerged as exemplary materials for electrochemical gating, in large part due to their topotactic perovskite (P) ↔ brownmillerite (BM) transformations. SrCoO3−δ, for example, can be cycled between metallic ferromagnetic P SrCoO3 and insulating BM SrCoO2.5, realizing exceptional modulation of electronic, thermal, and optical properties. It is often presumed that such cycling also generates ferromagnetic–antiferromagnetic (F-AF) modulation due to the G-type AF order in bulk SrCoO2.5. Little is understood about magnetism in thin-film BM SrCoO2.5, however, meaning that the true magnetic property modulation is unclear. We address this here through a neutron diffraction study of BM La1−xSrxCoO2.5 films at x = 0.5 and 1.0. Lightly compressively strained SrCoO2.5 films are shown to retain G-type AF order, albeit with suppressed Néel temperature (∼340 K). Of high interest for AF spintronics, room-temperature F–AF cycling is thus possible across the SrCoO3-δ P ↔ BM transformation. At x = 0.5, however, BM La0.5Sr0.5CoO2.5 films are found to exhibit no detectable G-type AF order but instead weak F order (Curie temperature ∼115 K), unveiling a La0.5Sr0.5CoO3−δ phase diagram with two distinct F phases. These results thus uncover new, unanticipated magnetic phase behavior in these materials, in addition to being directly relevant to cobaltite-based magnetoionics. 

Abstract Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr_1-xCa_xCoO₃). Here, we show that in thin films of the complex perovskite (Pr_1-yY_y)_1-xCa_xCoO_3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.