Enhancement of the polar coercive force for annealed Co/Ir(111) ultrathin films

The alloy formation and the magnetic properties of Co/Ir(111) ultrathin films have been investigated. As the temperature is increased above 400 K, interdiffusion of Co and the Ir substrate occurs. Due to a compositional change in the surface layers, the polar coercive force is greatly enhanced. At temperatures above 600 K, magnetic hysteresis appears only in the polar configuration. This shows that the easy axis of the magnetization of Co/Ir(111) may be stabilized in the direction of the surface normal by thermal-annealing treatments. From systematic investigations of Co/Ir(111) ultrathin films thinner than 4 monolayers, a magnetic phase diagram has been established. According to the compositional changes and related magnetic properties, the phase diagram can be separated into three regions. In region I at temperatures below 400 K, Co films are ferromagnetic. In region II where atomic interdiffusion occurs in the surface layers, an enhanced polar coercive force is observed. The phase transition from phase I to II is related to the interdiffusion between the Co overlayer and the iridium substrate. In region III for low coverage or at high temperatures, a nonferromagnetic behavior is observed. The phase transition from phase II to III is mainly due to the reduced atomic percent of cobalt in the Co-Ir alloy and to the lowered Curie temperature caused by a reduction in the thickness of the magnetic layers.