Direct neutron-diffraction-based measurement of magnetic order in brownmillerite SrCoO_2.5 and La_0.5Sr_0.5CoO_2.5 thin films

Epitaxial cobaltites have emerged as exemplary materials for electrochemical gating, in large part due to their topotactic perovskite (P) ↔ brownmillerite (BM) transformations. SrCoO_3−δ, for example, can be cycled between metallic ferromagnetic P SrCoO₃ and insulating BM SrCoO_2.5, 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 SrCoO_2.5. Little is understood about magnetism in thin-film BM SrCoO_2.5, however, meaning that the true magnetic property modulation is unclear. We address this here through a neutron diffraction study of BM La_1−xSr_xCoO_2.5 films at x = 0.5 and 1.0. Lightly compressively strained SrCoO_2.5 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 SrCoO_3-δ P ↔ BM transformation. At x = 0.5, however, BM La_0.5Sr._0.5CoO_2.5 films are found to exhibit no detectable G-type AF order but instead weak F order (Curie temperature ∼115 K), unveiling a La_0.5Sr._0.5CoO_3−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.