Magnetic properties of cobalt nano dots fabricated by a new laser irradiation method: enhanced anisotropy and superparamagnetism

In this report, a phenomenon of superparamagnetism through Co nanodots (NDs) prepared by laser irradiation method is studied. These NDs are simply prepared on Si substrates by applying an external laser irradiation on ultra thin Co films at room temperature. The laser irradiation on these ferromagnetic thin films locally melt the film at an extremely shot time so that the melted films get together to form the NDs by the strain effect. Structural properties of these NDs are investigated by scanning electron microscope, atomic force microscope (AFM), and transmission electron microscope (TEM). The magnetization as a function of temperature with differential Co NDs is measured at field cooling (FC) and zero field cooling (ZFC) methods. Results show that at 0.3 T field, remanence magnetization M_r is saturated. The diameter of Co NDs is increased with an increase temperature of zero remanence value. The zero remanent value at each temperature demonstrates that below this temperature, the magnetic moments are at least partially blocked. In addition, Co NDs have relatively high anisotropy energy because of a field growth method which is the additional source of the anisotropy.