Ferromagnetic behavior of carbon nanospheres encapsulating silver nanoparticles

We report on the structure and magnetic properties of a silver and carbon nanocomposite. The as-synthesized nanocomposite consists of a matte-black powder composed of Ag nanoparticles encapsulated in carbon nanospheres (similar to 10 nm diameter) that are interconnected in necklace-like structures. Magnetic measurements of the Ag and C nanocomposite, in its powder form, showed weak ferromagnetic behavior up to at least room temperature with a coercive field of 389 Oe at 2 K and 103 Oe at 300 K, from which we estimate magnetic ordering up to 425 K. However, pressing the Ag-C powder samples into tablets suppressed the ferromagnetism; the pressed samples instead exhibited diamagnetic behavior. Chemical analysis with EDS and trace metal analysis with ICP-MS indicated that there are no magnetic contaminants in the sample. Therefore, we attribute the ferromagnetism to the carbon nanospheres and propose a model for the observed magnetism. We also measured a pronounced peak in the magnetization between 50 and 90 K that was completely suppressed when measurements were made upon cooling; we attribute this peak to a first-order spin reorientation.