permanent magnet properties of Fe₅C₂ from first principles

Iron carbides, such as Fe₃C, Fe₂C and the Hagg carbide Fe5C₂ are commonly produced in steel-making and in fact often play an important role in strengthening such steel. Here we study the permanent magnet properties of Fe₅C₂ from first principles, focusing on the magnetization and magnetic anisotropy – two indispensable components of a permanent magnet material. We find a substantial magnetization exceeding 1.5 T, along with a significant magnetic anisotropy exceeding 1 MJ/m³ (here defined as the energetic difference between the magnetically easiest and hardest directions). While the anisotropy for an intermediate direction is much smaller, likely precluding the use of Fe₅C₂ for permanent magnet applications, the 1 MJ/m³ anisotropy value is rather significant for a material containing only relatively light elements such as Fe and C. It is in fact more than half again as large as the value for hcp Co. We discuss the implications of these results for permanent magnet applications.