Near-room temperature ferromagnetic insulating state in highly distorted LaCoO2.5 with CoO5 square pyramids

Abstract

Dedicated control of oxygen vacancies is an important route to functionalizing complex oxide films. It is well-known that tensile strain significantly lowers the oxygen vacancy formation energy, whereas compressive strain plays a minor role. Thus, atomic reconstruction by extracting oxygen from a compressive-strained film is challenging. Here we report an unexpected LaCoO2.5 phase with a zigzag-like oxygen vacancy ordering through annealing a compressive-strained LaCoO3 in vacuum. The synergetic tilt and distortion of CoO5 square pyramids with large La and Co shifts are quantified using scanning transmission electron microscopy. The large in-plane expansion of CoO5 square pyramids weaken the crystal field splitting and facilitated the ordered high-spin state of Co2+, which produces an insulating ferromagnetic state with a Curie temperature of ~284 K and a saturation magnetization of ~0.25 μB/Co. These results demonstrate that extracting targeted oxygen from a compressive-strained oxide provides an opportunity for creating unexpected crystal structures and novel functionalities.

Dynamics of Anisotropic Oxygen-Ion Migration in Strained Cobaltites

“…The evolution behavior of OVCs in LCO x films under compressive strain is dramatically different from that of a film under tensile strain. At T a = 500 °C, we observe a new V ö ordered phase with diagonal-aligned dark-stripes, which can be called n-LCO 2.5 as reported in our recent work . The diagonal V ö stripes first appear randomly in the 3 c -LCO 2.67 films as indicated by the inclined purple arrows in Figure e.…”

“…At T a = 500 °C, we observe a new V oö rdered phase with diagonal-aligned dark-stripes, which can be called n-LCO 2.5 as reported in our recent work. 32 The diagonal V ös tripes first appear randomly in the 3c-LCO 2.67 films as indicated by the inclined purple arrows in Figure 2e. At increasing annealing times, the n-LCO 2.5 domains propagate laterally toward two sides in the moving speed of the domain wall of ∼7 nm/min, accompanied by the disappearance of the horizontal V ös tripes (Figure 2f and g).…”

“…31 Recently, an unexpected zigzag-like V öo rdering in a compressively strained LaCoO cannot be formed through annealing a tensile-strained LaCoO 3 (LCO 3 ) film. 32 Previously, OVC formation under tensile strain has been studied by high-dose electron beam irradiation. 33,34 The dynamic process in differently strained LaCoO x (LCO x ) films toward further oxygen reduction to the metastable phases has not been visualized directly so far.…”

“…For instance, the periodicity and orientation of V ö layers are parallel (perpendicular) to the interface when the films are under compressive (tensile) strain . Recently, an unexpected zigzag-like V ö ordering in a compressively strained LaCoO 2.5 film has been found, whereas the unique structure cannot be formed through annealing a tensile-strained LaCoO 3 (LCO 3 ) film . Previously, OVC formation under tensile strain has been studied by high-dose electron beam irradiation. , The dynamic process in differently strained LaCoO x (LCO x ) films toward further oxygen reduction to the metastable phases has not been visualized directly so far.…”

Origin of insulating ferromagnetism in iron oxychalcogenide Ce$_2$O$_2$FeSe$_2$

“…However, insulators exhibiting robust FM states that overwhelm AFM tendencies have been observed experimentally, such as perovskite BiMnO 3 with orbital order [9], double perovskite Sr 3 OsO 6 [10], 90 • bond CrSiTe 3 [11] and others [12][13][14][15][16][17][18][19][20][21]. Some of these materials with simple 180 • and 90 • bond case could be qualitatively understood by the semiempirical Goodenough-Kanamori-Anderson (GKA) rules [1][2][3][22][23][24][25].…”