Analysis of grain boundaries in CoCrTa and CoPtCrB HDD media by analytical transmission electron microscopy

Crystallographic structures and elemental distributions of grain boundaries in cobalt (Co)-based hard disk drive (HDD) media of CoCrTa and CoPtCrB were analysed by analytical transmission electron microscopy (TEM). CoCrTa medium had crystalline grain boundaries with extremely distorted (0002) planes of hexagonal close-packed structure. The width of grain boundaries of 1.5 nm obtained by high-resolution TEM (HRTEM) imaging agreed with that of 1.5 nm obtained by energy-dispersive spectroscopy (EDS) mapping. The EDS analysis revealed that Cr was segregated to grain boundaries while Ta was not segregated to either grain boundaries or grains. Cobalt was retained within grains. CoPtCrB medium had amorphous grain boundaries and spots in grains. The EDS and electron energyloss spectroscopy analyses revealed that both Cr and B were segregated to grain boundaries and spots. Both Co and Pt were retained within grains. The width of the boundaries of 0.4 nm obtained by HRTEM differed from that of 1.5 nm obtained by high-angle angular dark-field scanning TEM; the former indicates the crystallographic boundary and the later corresponds to the magnetic boundary, which is significantly correlated to the magnetic properties of HDD media.