Magnetic properties of polycrystalline cobalt nanoparticles

The energy diagram of stationary magnetization states existing in polycrystalline cobalt nanoparticles in the range of diameters 20 ≤ D ≤ 60 nm has been calculated by means of numerical simulation. It is shown that in polycrystalline cobalt nanoparticles in the range of diameters D ≥ 32 nm only vortex states with low average magnetization are present, whereas mostly quasi-uniform states are realized in nanoparticles with diameter D ≤ 24 nm. Thus, the effective single-domain diameter of polycrystalline cobalt nanoparticles is estimated to be Dc = 24 nm. It is approximately two times smaller than the actual single-domain diameter of monocrystalline cobalt nanoparticle, Dc0 = 45 nm. The hysteresis loops of a dilute assembly of polycrystalline cobalt nanoparticles in the range of diameters D ≤ Dc are characterized by a coercive force that is approximately 2.5 times less compared with that of the randomly oriented assembly of monocrystalline cobalt nanoparticles.