Unexpected size and temperature dependence of the magnetic switching field of individual Co nanoislands

We present results on the magnetic switching field of individual, nm small Co islands on Cu(111), which we obtained by low temperature spin-polarized STM in magnetic fields. We find that the magnetic switching field of the islands depends on both island size and temperature. A transition from a superparamagnetic to a blocked magnetization state is found with increasing islands size for islands with appr. 800 atoms at 8 K. The maximum switching field reaches 2.2 T for islands with 5000 atoms, and it decreases for larger islands. The switching field of an island decreases with increasing temperature. A quantitative analysis reveals the failure of the N\'eel-Brown model of thermally assisted magnetization reversal by coherent rotation, which has been successfully applied to similar systems. Our calculations find a spatial variation of the magnetic anisotropy and of the magnetization within a Co island. The implication of the inhomogeneous magnetic properties for the field-induced magnetization reversal is discussed.