Long-range atomic ordering and magnetic properties of Co1-xPtx/SiO2 granular films

The structure and magnetic properties of Co1-xPtx/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 fcc to the ideal ordered L1(0) structure after annealing at 680 degrees 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 similar to 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 similar to 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. (C) 2000 American Institute of Physics. [S0021- 8979(00)84708-8].