MICROSTRUCTURE AND MORPHOLOGY OF Cu-Co GRANULAR THIN FILMS EXHIBITING ANISOTROPIC GIANT MAGNETORESISTANCE

We have investigated the magnetoresistive properties of Cu rich-Co thin films. Samples with nominal composition Cu₉₅Co₅ have been deposited on glass substrates, at room temperature, by pulsed laser ablation of the pure elements from rotating circular targets. Post isothermal annealing optimizes the magnetoresistive effect to give a non saturated value of 10% in 1 Tesla at 77 K. X-ray diffraction analysis of the films show broad diffraction peaks, slightly shifted from pure Cu (111) reflection peak, indicating that both constituents form nanometer sized grains, and some Co is dissolved in the Cu rich grains. Scanning tunnelling microscopy investigations, in the topographic mode, have revealed pyramidal growth with basal dimensions about 100 nm. The atomic force microscopic analyses show the existence of nanoparticles with an average height value of about ≈ 9-10 nm. Furthermore, we observe a highly anisotropic magnetoresistive behaviour for samples obtained at low rotation velocities, 10-15 rpm. A reversible magnetoresistance is observed when the magnetic field is applied perpendicular to the film plane. Coercive fields as large as ≤1.7 kOe are found when the magnetic field is applied at angles to the film plane in the range 70 - 80 deg. On the other hand values of 0.4 kOe are observed for the case when the magnetic field is in the plane of the film. These properties are discussed in terms of the shape anisotropy of magnetic granules.