The traditional arc discharge process for producing metallic nanoparticles was modified to be applicable for the direct preparation of oxide nanoparticles by using both conductive and insulative precursors. The cobalt oxide nanoparticles were synthesized by feeding Co and Co 2 O 3 into the air plasma. The as-prepared nanoparticles are spherical in shape with size in the range of 10 to 40 nm. The phases of CoO and Co 3 O 4 were detected in the samples prepared from the precursors of Co and Co 2 O 3 , respectively. Weak ferromagnetism and an exchange bias up to 17.1 mT were observed in these antiferromagnetic nanoparticles, owing to the presence of a large fraction of the uncompensated surface spins and possibly the local disordered regions.

Exchange coupling has been investigated in the [Cr/Gd] 5 multilayers deposited at 25, 200, and 400 • C, where the Néel temperature (T N ) of antiferromagnetic Cr is slightly higher than the Curie temperature (T C ) of ferromagnetic Gd. It was found that the exchange coupling existed not only at T C < T < T N , but also above the temperature (T N ) of antiferromagnetic orderings with incommensurate spin-density wave structures transiting to paramagnetic state. These results can be discussed in terms of the crucial role played by the antiferromagnetic spins of Cr with commensurate spin-density wave structures in the vicinity of the Cr/Gd interfaces. Moreover, the exchange coupling of the multilayers grown at different temperatures exhibited different dependencies on the measuring temperature and the cooling field, respectively. Positive exchange bias was observed in the multilayers grown at 200 and 400 • C. The interfacial roughness, grain size, and the antiferromagnetic orderings of Cr may be responsible for the anomalous exchange coupling of the multilayers. In addition, the competition between the exchange coupling at Cr/Gd interfaces and the external field-Cr surface magnetic coupling can explain the appearance of negative or positive exchange bias.