Recorded Magnetic Tape

A practical application of electron holography, which required the characterization of magnetic microstructure within the sample as well as the leakage fields outside, involved the examination of magnetic fields in high-density magnetic recording media. Figure 13a shows a schematic diagram of the method used to record bits on a magnetic film. Figure 13b shows results obtained from an in-plane-recorded high-coercivity evaporated Co film that had a thickness of 45 nm, a coercive field of 340 Oe, and a bit length of 5 /m [174]. The sample was prepared by evaporating the Co film obliquely onto a glass substrate that had been coated with photoresist. The Co film was partly oxidized to control its coercive field and remanent magnetization, and the recording was performed using a ring-type magnetic head with a track width of 300 /m and gap lengths of 200 nm and 1 /m. The interference micrograph in Figure 13b shows the projected magnetic lines of force both inside and outside the film. The recorded track runs parallel to the film edge, the film is observed in the upper half of Figure 13b, and the lower half shows empty space. The magnetization

Figure 13. (a) Schematic diagram showing the method used to record bits on a magnetic film. (b) Interference micrograph formed from a holographic phase image of a 45-nm-thick recorded Co film with a bit length of 5 /m. The magnetic stray field in vacuum at the side of the thin film is visible at the bottom of the image. (c) Interference micrograph of a 30-nm-thick recorded Co film with a bit length of 0.15 /m, showing periodic stray fields in vacuum outside the edge of the sample. Reprinted with permission from [174], N. Osakabe et al., Appl. Phys. Lett. 42, 746 (1983). © 1983, American Institute of Physics. (d) Schematic diagram and (e) interference micrograph of a perpendicularly recorded 80-nm-thick Co-Cr film with a bit length of 0.25 /m. Reprinted with permission from [176], K. Yoshida et al., IEEE Trans. Magn. 23, 2073 (1987). © 1987, IEEE.

Figure 13. (a) Schematic diagram showing the method used to record bits on a magnetic film. (b) Interference micrograph formed from a holographic phase image of a 45-nm-thick recorded Co film with a bit length of 5 /m. The magnetic stray field in vacuum at the side of the thin film is visible at the bottom of the image. (c) Interference micrograph of a 30-nm-thick recorded Co film with a bit length of 0.15 /m, showing periodic stray fields in vacuum outside the edge of the sample. Reprinted with permission from [174], N. Osakabe et al., Appl. Phys. Lett. 42, 746 (1983). © 1983, American Institute of Physics. (d) Schematic diagram and (e) interference micrograph of a perpendicularly recorded 80-nm-thick Co-Cr film with a bit length of 0.25 /m. Reprinted with permission from [176], K. Yoshida et al., IEEE Trans. Magn. 23, 2073 (1987). © 1987, IEEE.

directions within the film are indicated by arrows. The maximum packing density for longitudinal recording in such films is limited by the widths of the transition regions between two oppositely magnetized regions. Oppositely magnetized regions in adjacent domains can be seen to collide with each other head-on to produce vortexlike patterns. The magnetic field then meanders to the edge of the film edge and leaks out into the vacuum. (Although stray fields above the film are read by the magnetic head, the contour lines visible at the bottom of Fig. 13b are at the side of the film.)

Figure 13c shows an interference micrograph obtained from an electron hologram of a 30-nm-thick Co film that had been recorded with a bit length of 0.15 /m [174]. Periodic stray fields are again visible in the vacuum outside the sample edge. The narrowest transition region that has been characterized using this approach is 100 nm. Corresponding results have been obtained from recorded CoCr films [175]. Figure 13d and e shows a schematic diagram and an interference micrograph, respectively, of the magnetic field emerging from both the top and the bottom surfaces of a perpendicularly recorded 80-nm-thick Co-Cr film that has a bit length of 0.25 /m [176].

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