Magnetic and electronic structure of high-coercivity cobalt-carbide nanoparticles for permanent magnet applications

Permanent magnets are important in numerous technological applications. However, those with the largest energy product (\textit{BH}$_{max})$ contain rare earth elements, which increase costs and introduce volatility into the supply chain. Recently, rare-earth free Co$_{\mathrm{2}}$C and Co$_{\mathrm{3}}$C nanoparticles (NPs) with large magnetic coercivity and \textit{BH}$_{max}$ have been synthesized using a polyol process [1]. Optimal \textit{BH}$_{max} $is found in a mixture of the two phases. In this system, the nature of the magnetic interparticle interactions and the origins of intrinsic magnetic properties of the Co-carbide phases are not fully understood. We have investigated the origins of the magnetic properties of Co$_{\mathrm{2}}$C and Co$_{\mathrm{3}}$C NPs using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Co $L$-edge and C $K$-edge. From differences in the electronic structures of the two Co-carbide phases, as determined by XAS, the nature of their unique magnetic properties can be deduced. Furthermore, the role of the spin and orbital moments in determining magnetic anisotropy and \textit{BH}$_{max}$ in these materials is examined with XMCD. [1] V. G. Harris et al. J. Phys. D: Appl. Phys. \textbf{43}, 165003 (2010).