Long-range atomic ordering and magnetic properties of Co_1-xPt_x/SiO₂ granular films

The structure and magnetic properties of Co_1-xPt_x/SiO₂ granular films with the Pt atomic content x varying from 0.42 to 0.71 have been investigated. It was found that the CoPt grains transform from disordered fee to the ideal ordered L 1₀ structure after annealing at 680°C. In the range of x = 0.42-0.60, the lattice parameter ratio a/c slightly varies from 0.965 to 0.979. For the Pt content x∼0.7, the superstructure exists but the lattice constant ratio is equal to 0.998. The coercivity dramatically increases from 1.8 to 9.1 kOe and then decreases to several hundred Oe while x increases from 0.42 to 0.71 for the grain size d ∼ 10 nm. We also found that the coercivity increases with the grain size. Our results show that the highly ordered fct structure, accompanied by the high crystalline anisotropy, is a prerequisite for the high coercivity, and the coercivity can be tailored by controlling either the composition or the grain size.