Tuning the heating efficiency of hybrid core/shell nanoparticles by modulating the shell composition and thickness

Exchange-coupled ferrite nanostructures are promising candidates to optimize the heating power of magnetic colloids under radiofrequency magnetic fields. We studied Fe₃O₄/Co_xZn_1-xFe₂O₄ core/shell nanoparticles with 12 nm cores and variable shell composition (x=0-1) and thickness (t=0-4 nm). Due to the symmetry and lattice matching between both phases, the Zn-Co ferrite is epitaxially grown on the magnetite, and prevents its oxidation. By applying magnetic fields (f=309 kHz, H=100-800 Oe) to nanoparticles either dispersed in water or fixed in an agar matrix, we found the conditions that maximize the dissipated power in each case. While x=0.5 and t=3 nm are preferred for water colloids at large H, x=0 and t=2 nm provide an efficient heating for agar gel dispersions at low H. We discuss the influence of the overall effective anisotropy (determined by both x and t) on the heating mechanisms. While the Brown-relaxation governs the dissipation of water colloids with large anisotropies and a purely Néel process is observed for isolated particles in agar, interparticle interactions are responsible for large heating powers (above 2000 W/g) in water colloids with low anisotropies.