FePt nanoparticles have practical potential in fields as diverse as catalysis and magnetic storage media. But far from being pristine crystalline materials, these nanoparticles are structurally heterogeneous with grain boundaries and other crystal defects. In this paper, Jianwei Miao and colleagues reveal the complex atomic-scale structure of a single FePt nanoparticle containing more than 22,000 atoms. They do this by generating a high-resolution tomographic tilt series of 68 images of the nanoparticle and reconstructing it using a new algorithm, achieving resolution with 22 picometre precision. The resulting structure reveals the complexity of the nanoparticle, and the chemistry and crystal structure of the grains within the material. When analysing the order/disorder character, the authors find that the grains are more ordered towards the core of the nanoparticle and less ordered towards the surface. They use data from the boundary between two grains to calculate local magnetocrystalline anisotropy energies using density functional theory, revealing how these energies vary across the grain with order parameter and across a grain boundary.
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