Atomic structure of high-coercivity cobalt-carbide nanoparticles ensembles

Permanent magnets are increasingly important in numerous applications, including the quickly expanding area of green technologies ($e.g.$ high efficiency electric car motors and wind turbine power systems). We present studies of novel permanent magnet materials based on cobalt carbide nanoparticles (NPs), where the energy product ($BH_{max}$) exceeds 20 kJ / m$^3$ [1]. The NPs are synthesized via a polyol process, which offers a flexible approach to modify the Co-carbide phase (Co$_2$C and Co$_3$C), and NP morphology, size and size dispersion. The Co$_2$C and Co$_3$C phases have unique magnetic properties, and the combination exhibits the high $BH_{max}$. We present a detailed assessment of the structure of mixtures of Co$_2$C and Co$_3$ NPs, measured by high-resolution, synchrotron based powder x-ray diffraction (p-XRD). Both the Co$_2$C and Co$_3$ phases exhibit an orthorhombic structure (Pnnm and Pnma space groups, respectively). The high-resolution p-XRD facilitates identification of mixed phase samples, enabling detailed comparisons of the atomic structure with the magnetic properties, measured by both lab-based magnetometry and x-ray spectroscopy (soft x-ray XAS \& XMCD). \newline [1] V. G. Harris et al., J. Phys. D: Appl. Phys. 43, 165003 (2010).