Synthesis, composition and spin-dynamics of FCC and HCP phases of pyrolysis derived Co-nanoparticles embedded in amorphous carbon matrix

We report the structure, magnetic domain state and spin dynamics of biphasic Co nanoparticles embedded in amorphous carbon matrix. Two types of samples were synthesized by heating the precursors (Co-acetylacetonate and toluene) from RT to the pyrolysis reaction temperature (800 degrees C) at two different heating rates, viz. similar to 3 degrees C/min and similar to 20 degrees C/min. We observe that the sample prepared at higher heating rate (similar to 20 degrees C/min) predominantly contains fcc-phase of Co in the Co-nanoparticles, whereas the sample synthesized at lower heating rate exhibits higher amount of the hcp-phase-content in the Co-nanoparticles. The Co-nanoparticles are surrounded by graphitic carbon layers forming core-shell type morphology. The hcp-phase of Co is characterized by a higher saturation magnetization and coercivity (higher magnetic hardness) than the fcc-phase. The nature of the measured hysteresis loop in combination with the estimated critical size for the particles to be in a single magnetic domain state suggest that both hcp and fcc-phases of Co coexist in the same particle and most of the particles are multidomain-particles. Furthermore, our results reveal that the dynamics of the spins present at the grain boundaries are the slowest followed by those in the fcc-phase and hcp-phase of Co. The dynamics of the spins present at the domain walls are slower than those inside the magnetic domains. The responsive nature of the spins on the domain walls in comparison to those inside the domains leads to the so called domain wall enhancement effect.