# Metastable Crystalline Cobalt Iron Oxide Nano‐Flakes with Antiferromagnetic/Ferrimagnetic Composition Mosaicity

## TL;DR

Scientists created a new cobalt iron oxide material with unique magnetic properties by forming nano-flakes with alternating magnetic domains.

## Contribution

A novel metastable cobalt iron oxide material with antiferromagnetic/ferrimagnetic domains and exchange bias was synthesized.

## Key findings

- The material exhibits an exchange bias effect due to its unique microstructure.
- The iron-rich domains have a compressed lattice parameter caused by compressive strain from mosaicity.
- Heating above 450°C breaks down the microstructure and restores thermodynamic stability.

## Abstract

By thermal decomposition of a crystalline hydroxycarbonate precursor with a Co:Fe ratio of 2:1, crystals with alternating ferrimagnetic and antiferromagnetic nano‐domains were synthesized using a facile synthetic approach that combined bottom‐up co‐precipitation of the precursor with a self‐assembled top‐down nano‐structuring during spinel formation. Due to the miscibility gap of the spinel phase diagram at this composition, a topotactic segregation into CoFe2O4‐like and Co3O4‐like domains takes place at 400 °C, giving rise to porous crystalline nano‐flakes with spatial compositional fluctuations on a scale of approximately 5 nm. Experimental methods and density functional theory showed that the metastable nature of this interface‐rich material is manifested in the unexpectedly low lattice parameter of the iron‐rich domains, which can be explained by the compressive strain executed on this phase due to mosaicity. Investigations of the magnetic properties revealed an exchange bias effect, due to this unique microstructure, which is typically known for thin films or core/shell nanoparticles. Treatment at temperatures higher than 450 °C causes this microstructure to break down, the lattice strain to relax, and finally leads to properties expected for the thermodynamically stable phases according to the phase diagram.

Through precursor co‐precipitation and mild calcination, a metastable interface‐rich cobalt iron oxide with unique magnetic properties was synthesized. Due to compression of the lattice parameter in the iron‐rich phase and the multitude of interfaces, an exchange bias was created, usually not known for such powder samples.