Magnetic properties of laser-induced ferromagnetic cobalt quantum dots passivated by ultrathin Ta film

The ferromagnetic quantum dots (FMQDs) are fabricated by exposing a pulsed Nd:YAG laser (wavelength=355 nm) on ultrathin cobalt (Co) films under an external magnetic field of 1000 Oe. Various dot diameters and densities of more than 3.5×1010/cm2 are ultimately realized by changing laser power, scan condition and initial film thickness. In addition, both AlN and Ta thin films as capping layers are used to protect the FMQDs from natural oxidation effect. The observed coercivity values of uncapped and capped FMQDs are changed from 600 up to 200 Oe with the initial Co thickness of 2 nm, respectively. The observation of magnetic force microscopy and hysteresis loops clearly confirms in-plane single domain state for the Ta-capped FMQDs.