Quantitative two-dimensional strain mapping of small core-shell FePt@Fe₃O₄ nanoparticles

We report a facile one-pot chemical synthesis of colloidal FePt@Fe₃O₄ core-shell nanoparticles (NPs) with an average diameter of 8.7 ±0.4 nm and determine their compositional morphology, microstructure, two-dimensional strain, and magnetic hysteresis. Using various state-of-the-art analytical transmission electron microscopy (TEM) characterization techniques - including high resolution TEM imaging, TEM tomography, scanning TEM-high angle annular dark field imaging, and scanning TEM-energy dispersive x-ray spectroscopy elemental mapping - we gain a comprehensive understanding of the chemical and physical properties of FePt@Fe₃O₄ NPs. Additional analysis using x-ray photoelectron spectroscopy, x-ray diffraction, and superconducting quantum interference device magnetometry distinguishes the oxide phase and determines the magnetic properties. The geometric phase analysis method is effective in revealing interfacial strain at the core-shell interface. This is of fundamental interest for strain engineering of nanoparticles for desirable applications.