Synthesis of Co_100-xPt_x alloy fine particles with high coercivity

In the present study, we have prepared and investigated Co_100-xPt_x (0 ≤ x ≤ 40) fine particles with the average diameter of D = 3 ∼ 40nm. At a fixed D ∼ 10 nm, the room temperature coercivity H_c increases with the Pt content x and takes a maximum value of 1000 Oe at x ∼ 25 at.% for 3D random assemblies of the particles. Such remarkable enhancement of the coercivity due to Pt is attributed to very strong magnetic anisotropy, as evaluated by the law of approach to saturation. At x ∼ 25 at.%, H_c increases with D due to suppression of thermal agitation and takes a maximum of 4000 Oe around at D ∼ 15 nm. Micromagnetic calculations taking into account dipolar interactions between particles have verified that the magnetic behaviours of Co₇₅Pt₂₅ fine particles can be explained by the coherent rotation model.