Observation of Micromagnetic Structure in Computer Hard Disks by Lorentz Transmission Electron Microscopy

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The disks studied have a smooth substrate (supersmooth) with the structure of l5nm C/29nm Co 84 Crj0Ta 6 /50nm Cr/6f.tm NiP/Al(substrate). The metallic films were sputterdeposited using a dc magnetron system with a substrate bias of -200 V and a substrate temperature of 225 °C without breaking vacuum. Magnetic parameters of these films, measured using vibrating sample magnetometry (VSM), are as follows: H, = 1590 Oe and Mrt = 1.34 memu/cm 2 . The radial-to-circumferential orientation ratio of coercivity is 0.98. Magnetic bits were written in alternating direction of magnetization along tracks (in the circumferential direction of the hard disk) in a standard writing procedure for computer hard disks. A magnetoresistive head with an inductive writing element of the size of P1 W/P2W = 7.5/6 gm was used at a flying height of 0.076-0.089 ýtm (3.0-3.5 1"). The recording density was 585 bits/mm (15 Kfci) at the inner diameter. (The bits were written with a constant frequency at different radii, so the recording density decreased with increase of radius.) Track pitch was 10. 14m. The disk was not dc-erased before writing, so the regions between tracks were in the as-deposited magnetic state. The ac-demagnetized state was achieved by rapidly spinning the sample in the VSM machine in a magnetic field which decreased slowly from a saturation value to zero (The sample surface is parallel to this external field). The saturation remanent state was obtained by applying a saturation magnetic field to the samples along the sample surface and then removing the field. Disks on mechanically textured substrates, having the structure of C/48nm Co 86 Cr 8 Ta 6/75nm Cr/NiP/Al(substrate), were also investigated. The metallic films were 21 Mat. Res. Soc. Symp. Proc. Vol. 384 ©1995 Materials Research Society

deposited using conditions similar to those in the smooth substrate case. Magnetic parameters are as follows: Hc = 1900 Oe (in the circumferential direction of the hard disk) and Mrt = 2.5 memu/cm 2 . This disk was dc-erased in the circumferential direction before bits were written. A chemical etching method was used to produce LTEM specimens directly from the C/Co alloy/Cr/NiP/Al(substrate) computer hard disk structures. The resulting specimens typically have 2000 ý.tm2 or larger electron transparent areas of Co alloy/Cr films with uniform thickness. The computer hard disks were first ground from one side to remove most of the Al substrate. The thinned pieces were then mechanically cut into 3mm disks which were subsequently dimpled from the Al side into the NiP layer, which was then etched away with concentrated nitric acid at room temperature. Since both Cr and Al are insoluble in this acid, the Cr underlayer acts as a protective layer, isolating the Co alloy magnetic layer from the etchant while the Al substrate remains as mechanical support for the resulting thin metallic film. The etched samples were suitable for LTEM observation. If necessary, further removal of the Cr underlayer and C overcoat can be accomplished using low-angle