Magnetically self-assembled SrFe12O19/Fe–Co core/shell particles

• A new synthesis of core/shell particles by magnetic self-assembly is reported.
• The limits of large (∼100%) lattice mismatch on epitaxial growth are thus avoided.
• The magnetically hard core and the magnetically soft shell exhibit exchange coupling that improves the magnetic properties.
• The synthesis provides a viable route to rare-earth-free permanent magnets.

Epitaxial growth to synthesize core/shell-structured materials is limited because large lattice mismatches are common between materials. Magnetically hard/soft, core/shell-structured materials can be potentially used for rare-earth free permanent magnets, but their synthesis presents a challenge. We report a wet chemistry method to synthesize core/shell structured particles consisting of a magnetically hard SrFe12O19 core and a soft Fe–Co shell, with a lattice mismatch of ∼100%, which cannot be achieved by conventional epitaxial growth or other alternative methods. When decreasing the size of the magnetically soft Fe–Co nanoclusters to below 5 nm, we show that they can be magnetically attracted by the hard SrFe12O19 to form core/shell structured particles. An AC demagnetization experiment demonstrates the formation mechanism of the core/shell particles, and their magnetic hysteresis loop shows potential for use as rare-earth free permanent magnets.