Quantitative Measurements


Quantitative measurements of segregation in CoCrX/Cr magnetic recording media by EFTEM








SHaRE Program collaborative research by J. Bentley (ORNL), J.E. Wittig (Vanderbilt University), and T.P. Nolan (Komag Inc.)




Development of high-density longitudinal magnetic recording media with good noise performance and high thermal stability requires optimization of both alloy composition and processing methods. In CoCr(PtTa) thin films, intergranular Cr segregation decouples the magnetic exchange between the small ferromagnetic grains. The corresponding Cr depletion within the grains affects the “bulk” magnetic anisotropy. The nanoscale structural and chemical details are not well understood but are needed for modeling and for guiding material development. Reliable core-loss spectroscopic methods were developed for mapping the segregation by energy-filtered transmission electron microscopy (EFTEM) and were used to characterize CoCrTa/Cr, CoCrPt/Cr, and CoCrPtTa/Cr media, to understand their structure-property-processing relationships.






Elemental maps were produced by EFTEM methods that have been refined and optimized in a wide range of applications at the SHaRE User Facility over the last 4 years. Extraction of quantitative compositions at a spatial resolution approaching 1 nm involved sophisticated treatments for diffraction contrast, variations in specimen thickness, and closely spaced ionization edges. The procedures yield elemental “maps” that are quantitative measures of the concentration at 256K pixels. An example for Co84Cr12Ta4 d.c. magnetron sputtered at 250ºC is shown in the figure. The extensive intergranular Cr segregation of <4nm width is readily apparent and typically reaches 25 ± 2 at.% Cr at random-angle grain boundaries, and 15 ± 2 at.% Cr at special 90º boundaries induced by the film texture and orientation relationship with the Cr underlayer. Regions adjacent to boundaries are depleted to less than 5 at.% Cr. Localized intragranular segregation at defects can also reach levels of ~25 at.% Cr. Compositions for hundreds of grain boundaries can be readily extracted from a single concentration map; such a statistically significant sampling is uncommon for analytical electron microscopy. In addition, elemental maps are ideal for grain size measurements, since grains with sufficient intergranular segregation to be considered as isolated magnetic units are clearly defined. The work has also shown that the uniformly distributed Ta is more effective than Pt in promoting intergranular Cr segregation, and that the segregation is not due to Cr diffusion from the underlayer.

The success of this research led to funding at Vanderbilt University from the National Storage Industry Consortium to extend these collaborative SHaRE studies to other related media.





 Oak Ridge National Laboratory