Tuning the size and magnetic properties of ZnxCo₁-xFe₂O₄ nanocrystallites.

Magnetically soft zinc-substituted cobalt ferrite ZnxCo1-xFe2O4 (x = 0.4, 0.5 and 0.6) nanocrystallites were successfully synthesized from cheap, abundant materials, using a mild, scalable hydrothermal route. The partial substitution of zinc by cobalt was generally observed to reduce the resulting crystallite sizes and the saturation magnetization. Post-synthesis annealing proved to be an efficient way of inducing crystallite growth to a certain limit, thereby improving the magnetic properties. In the annealing experiments crystallite growth was observed to be extremely dependent on the annealing atmosphere, with the size increasing from dynamic vacuum, to air, argon and helium. As prepared crystallite sizes were found to be between 4.74(1) nm and 5.90(1) nm. Heat treatment caused the growth to increase to anywhere between 7.9 nm and 21.7 nm. The largest crystallite sizes, 35.2(1) nm to 44.9(1) nm, were reached by compaction of the powders prior to heating. The largest magnetizations were generally observed in the largest samples containing the least amount of zinc. The highest observed saturation magnetization was 80.49(1) emu g(-1) measured for a sample with 35.2(1) nm sized crystallites of the composition Zn0.35Co0.66Fe1.99O4.