Analytical Electron Microscopy


Characterizing Intergranular Segregation in Co-based Magnetic Recording Media by AEM








SHaRE collaborative research by J. Bentley and N.D. Evans (ORNL) with J.E. Wittig and J.F. Al-Sharab (Vanderbilt University). Support at Vanderbilt from Komag, Inc., National Storage Industry Consortium (NSIC), and IBM Corp.




Thin-film longitudinal magnetic recording media that provide the technological basis for modern personal computer hard drives are prototypical complex nanostructured materials. A combination of a complex multi-element Co-Cr-based composition with judicious selection of a host of processing variables is used to achieve a nanoscale domain structure, comprised of Co-rich grains a few tens of nanometers in size interspersed by Cr-enriched grain boundaries a few nanometers in width, that optimizes key performance criteria such as signal-to-noise and thermal stability. The intergranular segregation decouples the magnetic exchange between the nano-scale ferromagnetic grains and is a key microstructural effect that allows high data-density recording. In order to uncover the basic phenomena governing these complex materials, an intensive multi-year collaboration has been established to characterize these media by analytical electron microscopy (AEM) techniques. Preliminary efforts focussed on the development of quantitative elemental mapping of composition at ~1 nm resolution by energy-filtered transmission electron microscopy (EFTEM). These efforts have met with great success in correlating Cr concentration variation with media composition and processing, but only following the development of sophisticated treatments to mitigate the effects of diffraction contrast, specimen thickness variations, and closely spaced ionization edges. However, the mapping of common alloying additions are poorly suited to EFTEM characterization.






Through a combination of AEM techniques, the roles of grain boundary misorientation and a variety of compositional and processing variables on the grain boundary segregation levels in Co-Cr-based longitudinal magnetic recording media with additions of Pt, Ta, and B have been revealed. Key to the success of this study has been a high degree of confidence in the reliability of the extracted Co and Cr compositions, which has been achieved through refinement of EFTEM data acquisition and processing procedures, and strong statistical sampling (>100) of grain boundaries in each sample, which is uncommon for AEM. Combined lattice imaging and EFTEM mapping have revealed interesting minima in intergranular Cr segregation at ~30° and ~60° grain boundary misorientations, comparable with Cr levels at the special 0° and 90° grain boundaries that arise from multiple nucleation on a single Cr underlayer grain (Fig. a). Quantitative data analysis reveal that in-plane coercivity correlates strongly with these integrated intergranular Cr levels. Scanning transmission electron microscopy (STEM) spectrum imaging methods have been applied in order to characterize the segregation of the Pt, Ta, and B additions. EFTEM mapping of boron has been challenging because of the background shape and low peak/background. STEM spectrum imaging has confirmed that boron strongly co-segregates with Cr (Fig.b,c, where images are 64 x 64 nm). Similar characterization of Pt and Ta additions show that these elements are relatively homogeneously distributed in the media. However, a comprehensive study shows that Ta promotes intergranular Cr segregation, as does increasing substrate temperature in the sequence 150, 200 and 250°C during sputter deposition, whereas Pt additions and deposition bias of the order of hundreds of volts have little influence.





 Oak Ridge National Laboratory