Granular CoPt/C films, consisting of nanoparticles of the face-centered-tet
ragonal (fct) CoPt phase embedded in a carbon matrix, have been made by cos
puttering from CoPt and C targets using a tandem deposition mode. The as-ma
de films have a disordered face-centered-cubic structure and low coercivity
(below 100 Oe). The granular structure is obtained after an aging heat tre
atment, which is followed by a drastic increase in coercivity to values up
to 9 kOe due to the formation of the highly anisotropic fct phase. Transmis
sion electron microscope studies showed the size of the CoPt particles to i
ncrease from 5 nm in the as-made state to 7-12 nm in the optimally annealed
state. The measured coercivities are much lower than the value expected fo
r noninteracting, randomly distributed single-domain particles. The large d
iscrepancy is attributed to interparticle interactions, particle-size effec
ts, and mainly to a lower anisotropy of the fct phase because of incomplete
ordering. (C) 2000 American Institute of Physics. [S0021- 8979(00)00909-9]
.