Near-Room Temperature Ferromagnetic Insulating State in Highly Distorted LaCoO2.5 with CoO5 Square Pyramids

TL;DR

This study demonstrates that compressive strain can induce a novel LaCoO2.5 phase with ferromagnetic insulating properties at near-room temperature by oxygen vacancy ordering, revealing new pathways for functional oxide design.

Contribution

It uncovers a new LaCoO2.5 phase with oxygen vacancy ordering in compressively strained films, enabling ferromagnetic insulation at near-room temperature.

Findings

LaCoO2.5 phase with zigzag oxygen vacancies formed under compressive strain.

The phase exhibits ferromagnetism with a Curie temperature of ~284 K.

The structure involves distorted CoO5 pyramids affecting electronic properties.

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, atomically 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 {\mu}B/Co. These results demonstrate that extracting targeted oxygen from a compressive-strained oxide provides an opportunity for creating unexpected crystal structures and novel functionalities.