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Magnetism of films with controlled nanostructure
Abstract
NdFeB permanent-magnet films, CoCrPt magnetic recording media, Co:C nanocomposite films, and CoPt:C nanocomposite films were prepared by sputtering with different sputtering parameters, underlayers, and annealing conditions. Rapid thermal-processing techniques were used on amorphous as-deposited NdFeB films to produce isotropically oriented Nd2Fe14B nanocrystallites with grain sizes of about 50 nm and strong intergrain exchange interactions. The small size of the Nd2Fe14B nanocrystallites is the key to develop coercivities up to 20 kOe and the strong intergrain exchange interaction leads to remanence ratios as high as 0.8. The (magnetic) thermal stability at ambient temperatures of current CoCrPt recording media was investigated. A CoCrPt recording media inherits the columnar structure of a Cr or CrV underlayer and has lateral grain size of about 15 nm. The grain volume is found to decrease with CoCrPt thickness. When the CoCrPt layer is less than 10 nm, the magnetization of the film becomes thermally unstable due to the moderate uniaxial anisotropy constant of the CoCrPt alloy. This suggests that new media material with high uniaxial anisotropy constants are necessary for extremely high-density recording. Co:C nanocomposite films were investigated to find the correlation among film preparation conditions, nanostructures, and magnetic properties. After the as-deposited Co:C films were annealed appropriately, hcp Co grains of about 20 nm were formed in an amorphous C matrix. Higher annealing temperatures and lower C concentrations generally result in larger Co grain sizes. The maximum coercivities are usually obtained in films near the percolation threshold, which typically occurs at a C concentration of about 50 at% in thick films. Due to reduced dimension, this threshold shifts to lower C concentration in thin films with thicknesses comparable to the Co grain size. The highest coercivity obtained in Co:C nanocomposite films is about 1 kOe. In order to improve coercivity as well as thermal stability, CoPt:C nanocomposite films were chosen for further investigation as another candidate for EHDR media. The main motivation is that the uniaxial anisotropy constant of the fct CoPt phase is about 10 times higher than that of Co-based alloys. fct CoPt grains of about 10 nm with large coercivity can be formed in amorphous C matrix. CoPt grain size and coercivity increase with increasing annealing temperature and decreasing C concentration. The magnetic activation volumes of CoPt:C nanocomposite films change little with annealing temperature and C concentration. When the fct CoPt grain sizes are larger than 10 nm, the activation volumes are found to be much smaller than the physical volumes. And when the sizes are smaller than 10 nm, the activation volumes are found to be close to the physical volumes. Coercivity and activation volume are insensitive to the CoPt:C layer thickness over a large range (10–100 nm). These results indicate that CoPt:C nanocomposite films have great potential as EHDR media. (Abstract shortened by UMI.)
Subject Area
Condensed matter physics|Materials science|Electrical engineering
Recommended Citation
Yu, Mingjun, "Magnetism of films with controlled nanostructure" (1999). ETD collection for University of Nebraska-Lincoln. AAI9953894.
https://digitalcommons.unl.edu/dissertations/AAI9953894