Thin Film Recording Media
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MARCH1990
inside the saturation hystérésis loop can be reached by an infinité number of différent minor loops. The H field required to reduce the component of M in the direction of H to zéro is called the coercivity Hc. The reciprocal of the sldpe of the M-H loop at the coercivity Hc may be set equal to (1 - S*) Hc/Mr. The term S* is called the coercivity squareness and should be near unity for recording sharp bit transitions. The values of Ms (emu/cm3) and 4w Ms (gauss) at 20°C for pure Fe, Co, and Ni are 1,714 (21,580), 1,422 (17,900), and 484 (6,084), respectively. Typical thin film recording média (100-1,000 Â thick) hâve Ms values ranging 500-1,000 emu/cm3, Hc ranging 5001,500 Oe, and S or S' ranging 0.5-1. Demagnetizating Fields The magnetic pôle density on the surface of the uniformly magnetized sheet, cylinder, and sphère shown in Figure 3 produces an internai uniform demagnetizing field Hdemag proportional and directed opposite to M(emu/cm3) as defined by Hdemag = - N d M
[Oersteds]
where Nd is the linear demagnetizing factor. The sum of three orthogonal linear demagnetizing factors is iw. The magnitude of the internai magnetizing field Hmt is less than the magnitude of the applied field Happ by the amount of the Hdemag, or l"int|
=
l"app|
—
l"demagl
which means that the applied field must overcome the demagnetizing field to saturate the material. The external shape as well as internai microstructure can severely limit the magnitude of the internai magnetizing field. For example, a thin film média composed of grains magnetically decoupled at their boundaries will require a larger applied field to rotate the dipole moments than that
which is needed in a continuous film. The operating point in a magnetic body is shown in Figure 3 as the intersection of the linear demagnetizing field Une with the second quadrant of a hystérésis loop. It is easy to see that the demagnetized remanence approaches Hc/N and the demagnetizing field approaches Hc as t h e d e m a g n e t i z i n g factor N approaches 47r. The demagnetizing fields produced inside magnetized materials also limit the recording performance of thin film média at high bit densities. The rectangular bar shown in Figure 3 represents approximately an isolated bit. As the bit length b approaches zéro at high bit densities, the demagnetizing factor Nj of a longitudinal recorded bit approaches 47r whereas the demagnetizing factor N x for a perpendicular recorded bit approaches zéro. At very high bit densities, longitudinal média must be very thin (100-500 Â) whereas perpendicular recording média can be much thicker (500-2,500 Â).
Magnetic Cristalline Anisotropy Magnetic crystalline anisotropy energy density is a measure of the change in magnetic energy as the magnetization M is rotated with respect to the crystal lattice by a magnetizing H field. As an applied magnetic field orients the spin moments, the électron orbits also tend to be oriented by spin-orbit coupling. The crystal field acts to resist this reorientation of the électron orbit. Thiis, the
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