Hysteresis-loop measurements using an extraordinary Hall effect are carried out at different temperatures (4.2–300 K) in Pt=Co=AlO x trilayers. The AlO x layer is prepared by plasma oxidation of the Al capping layer during various oxidation times. The samples are subsequently annealed at different temperatures ð300 °C–450 °CÞ. Most of the samples exhibit perpendicular anisotropy of increasingly high amplitude as the measurement temperature decreases. In addition, some samples exhibit unusual phenomena such as a temperature-induced reorientation of anisotropy from in plane to out of plane or a large increase of anisotropy at low temperature associated with the onset of exchange bias. The perpendicular magnetic anisotropy, coercivity, thermally induced anisotropy reorientation, and exchange-bias effects are explained by the influence of different chemical bonds, namely, Co─Al, Co─O, and Co─Pt, which appear and/or evolve as a function of oxidation time and annealing temperature. They are linked to the formation of new phases such as CoO or CoPt alloy and to the evolution of the Co=AlO x , Co=Pt, and Co=CoO interfaces modified by the oxidation time and the interdiffusion between species taking place during the anneals.