Magnetic properties of Co nanoparticles: role of the coexistence of different geometrical phases.

Following the experimental results of Respaud et al. [Phys. Rev. B 57, 2925 (1998)] we report self-consistent electronic structure calculations in order to analyze the magnetic properties of Co nanoparticles in which a coexistence of bcc and compact (fcc) phases are present within the particles. In all cases, the local spin moments S(i) are found to be saturated (approximately 1.7 microB) while, in contrast, the local orbital moments L(i) and the magnetic anisotropy energy (MAE) are found to be very sensitive to the size and structure of the systems. Interestingly, we obtain considerably enhanced values for L(i) at the internal bcc/fcc interfaces which can be even larger than at surfaces sites and, in addition, we found that by varying the fraction of bcc and fcc phases within the particles, several reorientations of the magnetization can be induced, a result that could open new possibilities to tune the MAE of magnetic nanoparticles.