Probing strongly exchange coupled magnetic behaviors in soft/hard Ni/CoFe ₂ O ₄ core/shell nanoparticles

Exchange coupling in a model core-shell system is demonstrated as a step on the path to 3d exchange spring magnets. Employing a model system of Ni@CoFe₂O₄, high quality core-shell nanoparticles were fabricated using a simple two-step method. The microstructural quality was validated using TEM, confirming a well-defined interface between the core and the shell. A strongly temperature dependent two-phase magnetic hysteresis loop was measured, wherein an analysis of step heights indicates coupling of roughly 50% between the core and the shell. Element-specific XMCD hysteresis confirms the presence of exchange coupling, suppressing the superparamagnetism of the Ni core at room temperature, and reaching a coercivity of >6 kOe at 80 K. These results provide a pathway to the development of heterostructured metal-oxide exchange coupled nanoparticles with improved maximum energy product.