The thin films of Pt/Co/AlO x , showing perpendicular magnetic anisotropy, were grown by magnetron sputtering with AlO x formed by the oxidation of thin Al layers using an oxygen atom source. Films were studied as a function of Pt thickness and Al oxidation, and films that showed full remanence and sharp switching coercivity were achieved. In order to prevent further oxidation of the interface in ambient conditions, we use a double Al growth and oxidation process. The magnetooptical Kerr effect and vibrating sample magnetometry were used to analyze these films. We find an effective perpendicular anisotropy of $2 \times 10^{6}$ erg/cm3, with the majority of the perpendicular anisotropy coming from the Pt/Co interface. From the sweep rate dependence on the coercivity, we are able to extract an activation volume of 4.3 $\pm 0.5 \times 10^{-18}$ cm3, similar to other Co-based perpendicular systems.