Submicron-scale magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ hollow spheres were prepared by a template method using polystyrene beads. The obtained particles were very uniform in size with a diameter of $650\pm 20\mathrm{nm}$ and a shell thickness of $40\mathrm{nm}$. The temperature dependence of the zero-field cooled magnetizations indicated a broad anomaly at around $110\mathrm{K}$ that was ascribable to the Verwey transition, while the magnetite nanoparticles $\left(25\mathrm{nm}\right)$, obtained by grinding the hollow spheres, exhibited no anomalies in this temperature range. Particularly below the Verwey transition temperature, the coercive fields of the hollow spheres increased much more significantly than did those of nanoscale and bulk magnetite. The Day plots [Phys. Earth Planet. Inter. 13, 260 (1977)] for the submicron hollow spheres indicated that their domain states were similar to the single-domain state at low temperature.

• Received 12 November 2006

DOI:https://doi.org/10.1103/PhysRevB.75.224404