A research team from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences has discovered that specific grain boundaries in magnetite (Fe₃O₄) exhibit substantially higher electrical conductivity than the surrounding grain interiors, overturning the long-held belief that grain boundaries inevitably degrade electrical transport.

The study, led by Prof. CHEN Chunlin, demonstrates for the first time that specific oxide grain boundaries can be more conductive than the grain interior. The findings were published in Science Advances on May 1.

Grain boundaries are ubiquitous in polycrystalline materials and play a decisive role in determining mechanical strength and physical properties. In structural applications, they impede dislocation motion, leading to the well-known Hall-Petch strengthening effect. In functional materials, their unique atomic arrangements often give rise to novel phenomena, making grain boundary engineering a powerful strategy for property optimization. However, when it comes to electrical conductivity, grain boundaries typically act as barriers. Atomic disordering and chemical discontinuities at boundaries scatter charge carriers and block current, severely limiting the performance of many electronic devices.

The researchers have now turned this conventional wisdom on its head. Using pulsed laser deposition, they epitaxially grew high-quality Fe₃O₄ bicrystal thin films containing single, well-defined Σ5 and Σ13 grain boundaries. Nano- to macro-scale electrical measurements revealed that both types of grain boundaries exhibited significantly higher electrical conductivity than the grain interiors.

To understand this counterintuitive behavior, the researchers combined atomic-resolution aberration-corrected scanning transmission electron microscopy with first-principles calculations. They found that electrons accumulate at the grain boundaries, reducing some Fe³⁺ ions to Fe²⁺.

More importantly, the tetrahedrally coordinated Fe sublattice at the interface forms a spin-up conduction channel, triggering a transition from the material's native half-metallic state to a truly metallic electronic structure. This half-metallic–to–metallic transition is the atomistic origin of the enhanced conductivity.

The findings deepen fundamental understanding of grain boundary physics and open new avenues for designing functional grain boundaries in half-metallic systems where spin-polarized transport is key. The strategy of engineering a half-metal–to-metal transition may be extendable to other half-metallic functional materials.

Structural characterization of the Fe₃O₄ Σ5 bicrystal thin film. (Image by IMR)

Structural characterization of theFe₃O₄ Σ13 bicrystal thin film. (Image by IMR)

Conductivity measurements of the Fe₃O₄ bicrystal thin films. (Image by IMR)

First‑principles calculations of the electronic structure of the Σ5 grain boundary. (Image by IMR)

First‑principles calculations of the electronic structure of the Σ13 grain boundary. (Image by IMR)