Observation of the effect of grain orientation on chromium segregation in longitudinal magnetic media

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.