Abstract: Structures and magnetic properties of anodic-oxide coatings on Al containing electrodeposited Co, Ni, and Co–Ni
Authors: Ueda R. , Kawai S. , Ishiguro I. 
Published: July 2002

When the surface of aluminum is anodized in some electrolytes, a film of γ-Al2O3, having micropores approximately 100–300 Å diam, is formed. In this work, cobalt, nickel, and Co–Ni alloys were then electrodeposited into these micropores. The electrochemical process of this deposition has not yet been fully explained. The metal deposits were extracted and examined by electron microscopy. The extracted crystals showed needle-like structures which were about 100 Å diam when aluminum was anodized in sulfuric acid baths.

By using an x-ray microanalyzer, a microprobe x-ray fluorescence analysis was performed. On cross sections of the γ-Al2O3 films, the metal deposits were found in each micropore and dispersed uniformly parallel to the surface. However, the amount of metal in the micropores decreased roughly linearly as the analyzer moved perpendicularly from the surface into the film.

By x-ray diffractometer analysis, the crystal structure of Co deposits was found to be hexagonal-close-packed α-Co (a=2.507 Å, C=4.070 Å). However, in the case of Co–Ni alloy depositions, a material with a face-centered-cubic crystal geometry gradually appeared as the Co content decreased, until the hexagonal-close-packed structure disappeared at about 50% Co content.

The magnetic properties of the films were very interesting. Depending upon deposition parameters, the Co-deposited γ-Al2O3 films had properties ranging from strong magnetic anisotropies along directions perpendicular to the film with good squareness ratios but rather weak along the parallel direction, to almost opposite directional effects. Coercive forces also varied with deposition parameters, with values ranging from 500 to 1000 Oe. The strength of residual magnetization increased in proportion to the amount of metal in the micropores and attained values as high as 1000 G.

For one set of deposition parameters, the Co–Ni alloy deposits give very characteristic magnetic anisotropies, depending upon alloy content. The coercive force along the perpendicular direction varied from about 750 Oe at 100% Ni content to about 1100 Oe at 100% Co content. On the other hand, the coercivities along the parallel direction rapidly increased from a low value of 300 Oe at 100% Ni content to a maximum of 1100 Oe at 50% Co content, followed by a slow decrease to about 600 Oe at 100% Co content. The remanence strength along perpendicular and parallel directions showed a similar trend, indicating a maximum value along the parallel direction at about 50% Co content. The magnetic anisotropies are apparently caused by a preferred crystal orientation of the metal needles in the micropores. Both deposition parameters and metal composition probably affect the crystal orientation during deposition. It is considered that the anodic-oxide coatings deposited with ferromagnetic metals have potential applications for high-bit-density memories and recordings. Other details on this work can be found in S. Kawai and R. Ueda, J. Electrochem. Soc. 122, 32 (1975).