Synthesis and characterization of magnetic nanocomposite films

Nanocomposite films were made consisting of Co nanoparticles, with coatings of amorphous Al2O3. Nanoparticles were deposited on room-temperature substrates by sputtering in 0.10-0.20 Torr Ar. Transmission electron microscope images showed average Co particle sizes between 70 and 223 Angstrom, for different sputtering rates and pressures. Selected-area electron diffraction and x-ray diffraction showed that the particles had a fcc structure for sizes less than or equal to 122 Angstrom and a mixture of fcc and hcp phases for larger sizes. Magnetization measurements on a sample with 122 Angstrom mean-particle size showed significant hysteresis at room temperature, with a coercivity of 390 Oe. At 10 K, the coercivity increased to 3145 Oe, and a loop shift of 2020 Oe was measured when the sample was field cooled. This indicates a fraction of the Co was oxidized by reaction with the Al2O3 coating, resulting in a surface layer of antiferromagnetic Co oxide. (C) 1999 American Institute of Physics. [S0021-8979(99)50608-7].