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Perpendicular recording media consisting of isolated CoPt magnetic grains separated with a nonmagnetic oxide grain boundary can be prepared by sputtering a Ru interlayer and then cosputtering CoPt with an oxide material, with low adatom mobility. The oxide material moves into the grain boundaries and isolates the magnetic grains. An increase in the oxide volume fraction in the magnetic layer does not affect the saturation magnetization but significantly reduces the magnetocrystalline anisotropy of magnetic grains. This may impose a limit on the areal density capability of this media design since media with smaller grains will require a larger oxide volume fraction that will reduce magnetocrystalline anisotropy of the CoPt based magnetic grains.

For this analytical TEM study, nonmagnetic oxygen-rich boundaries were introduced into Co-Pt-alloy perpendicular recording media by cosputtering Co and Pt with TiO2. Increasing the TiO2 content resulted in changes to the microstructure and elemental distribution within grains and boundaries in these films. EFTEM imaging was used to generate composition maps spanning many tens of grains, thereby giving an overall depiction of the changes in elemental distribution occurring with increasing TiO2 content. Comparing EFTEM with spectrum-imaging maps created by high-resolution STEM with EDXS and EELS enabled both corroboration of EFTEM results and quantification of the chemical composition within individual grain boundary areas. The difficulty of interpreting data from EDXS for these extremely thin films is discussed. Increasing the TiO2 content of the media was found to create more uniformly wide Ti- and O-rich grain boundaries as well as Ti- and O-rich regions within grains.

The nanoscale grain size of current hard disk media demands high-resolution methods for characterization. Therefore, high-resolution transmission electron microscopy (TEM), energy-filtered TEM (EFTEM) imaging, and nanoprobe energy-dispersive spectroscopy were chosen for the examination of Co84−x−yCr16PtxTay longitudinal hard disk media samples. Experiments were conducted to elucidate the relationship between nanoscale variations in grain-boundary composition and the atomic structure of the grain boundaries. Results show that Cr enriches the grain boundaries of the magnetic layer in all examined cases, but the addition of Ta greatly enhances this enrichment at some boundaries. Nanoprobe and EFTEM results show that enrichment varies according to the grain-boundary orientation of the hcp cobalt alloy crystals. Grains with tilt-boundary orientations near 0°, 30°, and 60°, found by measuring the angle between adjacent c-axis directions, show the least amount of segregation. Grains with tilt-boundary orientations near 15°, 45° and 75° show maximum amounts of Cr segregation. This trend exists in both CoCrTa and CoCrTaPt samples, and efforts to interpret the results within the context of hcp coincident site lattice theory are described.

The remarkable increase in the magnetic recording density of computer hard disk systems (currently about 60% per annum) is achieved by microstructural control. Of particular importance are the grain size and orientation of the active cobalt alloy layer, the alloy content, and grain boundary segregation or separation effects. Since typical grain sizes are now close to 10 nm or so, it is clear that only transmission electron microscopy (TEM) has the capacity to study these effects in the necessary detail. This paper reviews some recent progress in this field. TEM samples are prepared in both through-foil and cross-section orientations, although the former often contain most of the information for the magnetic technologists. High resolution (HREM) imaging is generally required for the grain size analysis [1,2] because of the lack of clarity in bright field images in identifying each individual grain. Segregation effects are studied with nano-probe Xray energy dispersive spectroscopy (EDS), combined with HREM for direct grain misorientation analysis, and collaborative work using electron energy filtered imaging has also been successful [3,4]. The basic alloys are cobalt-chromium based with additions of tantalum, platinum and/or boron. Tantalum promotes the grain boundary segregation of chromium, platinum improves the magnetic coercivity and boron refines the grain structure significantly. Fig. 2a shows its grain size analysis in the form of a cumulative percentage plot. Such curves allow immediate derivation of the median volume, and can easily distinguish grain size distributions of materials with apparently similar averages [1,2]. Because of the uniformity of the microstructure, and indeed the magnetic properties across the hard disk, we have found that sampling only 100-200 grains yields a satisfactory analysis, a result obtained elsewhere on a study of sputtered aluminum thin film grains [5]. Data such as these are related to the experimental deposition conditions and the thin film magnetic properties. Fig. 2b shows EDS analysis of grain boundaries in a Co 80 Cr 16 Pt 4 alloy. As found previously [6], the degree of chromium segregation is significantly lower for platinum-bearing alloys compared to tantalum bearing. Indeed for the latter, up to twice the nominal chromium composition can be accommodated at the boundaries [3,4,7], which noticeably improves the magnetic recording signalto-noise performance. We are currently correlating segregation effects to grain misorientation. An earlier report [8] indicates that an important structural relationship exists. Fig. 1 shows low and high magnification views of a CoCrPtB alloy andIn summary, TEM is essential for microstructural studies of such fine-grain, high performance magnetic thin films.