Thin films of Pt/Co/AlO x , showing perpendicular magnetic anisotropy, were grown by magnetron sputtering with the AlO x formed by oxidation of thin Al layers using an oxygen atom source. Films were studied as a function of Pt thickness and Al oxidation and films which showed full remanence and sharp switching coercive were achieved. In order to prevent further oxidation of the interface in ambient conditions we use a double Al growth and oxidation process. Magneto optical Kerr effect and vibrating sample magnetometry were used to analyse these films. We find an effective perpendicular anisotropy of 2.0 x 10 6 erg/cm 3 , with the majority of the perpendicular anisotropy coming from the Pt/Co interface. From the sweep rate dependence of the coercivity we are able to extract an activation volume of 4.3 ± 0.5 x 10-18 cm 3 , similar to other Co-based perpendicular systems.