Metastable Co3C nanocrystalline powder produced via reactive ball milling: Synthesis and magnetic properties

• The magnetic properties of metastable Co3C synthesized via reactive ball milling in nitrogen and argon are reported.
• Dissolved nitrogen is effective in increasing transformation rate, coercivity, and decomposition temperature of Co3C.
• A maximum coercivity value of 106 kA/m (1.34 kOe) is obtained in a 473 K annealed Co3C sample.

This study investigates reactive ball milling synthesis of a single phase metastable Co3C compound from starting mixtures of cobalt and graphite powders under argon and nitrogen environments. It reports their phase formations, magnetic, and structural properties. Regardless of the starting powder composition (25.0 or 33.3 at.% C) and milling atmosphere, reactive ball milling leads to the formation of the Co3C compound with an orthorhombic structure. Coercivities of the starting mixtures first increases with milling time as the Co3C compound starts to form and subsequently decreases due to structural defects induced by further milling. Higher graphite containing starting powder mixtures yield higher peak coercivity values and higher transformation rates. The formed Co3C compound exhibits a Curie temperature of 563 K and a decomposition temperature around 710 K. Compared to argon milled powders, reactive milling under a nitrogen atmosphere increases the coercivity and decomposition temperature of the Co3C compound.