FePt and CoPt nanoparticles co-deposited on silicon dioxide-a comparative study

We compare CoPt and FePt nanoparticles grown under identical conditions on oxidized Si?substrates by electron beam co-evaporation. Growth was performed under high vacuum conditions at substrate temperatures of 1023?K and was immediately followed by an annealing step. This process forms CoPt and FePt nanoparticles with mean diameters between ～17 and ～22?nm. In particular, the annealing step results in grain size enlargement for all samples and in a progressive magnetic hardening of the nanoparticles which reach maximum perpendicular coercivities of ～6.6?kOe (for the CoPt) and ～10.2?kOe (for the FePt nanoparticles). We show that, during this annealing step, a progressive transition towards the hard magnetic L1(0) ordered phase takes place in both materials. In contrast to FePt, CoPt nanoparticles must be annealed in order to crystallize in this phase.