Microstructure and magnetic properties of cobalt-(cobalt oxide, cobalt nickel dioxide, nickel oxide) and cobalt nickel-cobalt oxide nanocomposite thin films

Exchange anisotropy was investigated in ferromagnetic (FM)-antiferromagnetic (AFM) nanocomposite thin films for possible applications in computer hard disk drives. Co-(CoO, CoNiO2, NiO) and CoxNi1-x -CoO (x = 0.75, 0.50, 0.25) nanocomposite thin films were fabricated by co-sputtering technique using separate metal and oxide targets. Sputtering conditions were carefully controlled to obtain the desired film compositions. (111) textured (CoO, CoNiO2, NiO) oxide phases were observed in x-ray diffraction (XRD) patterns. For the metal phases, hcp Co phase was observed in the Co-(CoO, CoNiO2, NiO) specimens. No conclusive evidence for fcc Co phase was found in the Co-CoO specimens. The crystallite sizes of the metal and oxide phases calculated from XRD peak broadening were 30∼60 A and 70∼100 A, respectively, and proportional to the volume fraction. A broken columnar structure was observed in cross-section transmission electron microscopy images. Large exchange anisotropy field (H e) as compared to bilayer structures and linear type temperature dependence of He were observed in all specimens after field cooling. A percolation effect was suggested to explain the crystal size effect on the exchange anisotropy in the FM-AFM nanocomposite films. Neel's finite size effect of fine AFM crystallites, and thermal relaxation were proposed to explain the H e. Compared to a bilayer structure, the He was more strongly dependent on the AFM matrix than the FM phase. Very large coercivities were observed at low temperatures due to the exchange coupling. The origin of enhanced coercivity at room temperature was believed to be due to shape anisotropy as well as possible local exchange couplings. A slow decrease with temperature and large thermoremanent moment (TRM) at room temperature indicated that the exchange coupling significantly modified the anisotropy energy barrier of the Co crystallites in the AFM matrices. An upswing of the TRM curve near room temperature indicated the presence of induced anisotropy produced by rotational film deposition process. A large positive peak in the DeltaM curve of the Co-NiO specimen showed that the Co crystallites in the NiO matrix behaved more collectively than in the other systems studied. This was probably due to the higher Neel temperature of the NiO.