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Magnetic Recording Medium with Improved Magnetic Performance

IP.com Disclosure Number: IPCOM000124905D
Publication Date: 2005-May-11
Document File: 2 page(s) / 34K

Publishing Venue

The IP.com Prior Art Database

Abstract

Magnetic anisotropy with the easy axis preferentially aligned in the circumferential direction is necessary for good orientation ratio (OR) and signal to noise ratio (SNR) of longitudinal magnetic recording media. The magnetic anisotropy derives from crystalline anisotropy of the magnetic grains as they are epitaxially grown on the disk substrate during sputtering of the magnetic layers. Grain anisotropy is induced by a circumferential texture that is polished into the substrate. The quality of the anisotropy is determined by nanometer scale features of the substrate texture. The substrates most widely used throughout the recording industry are NiP plated on AlMg disks. We discovered that atmospheric corrosion by moisture and carbon dioxide forms a layer of Ni carbonates on the textured surface during substrate shipping and storage. The carbonates are removed by rewashing the substrates before sputtering. Formation of carbonate and its removal by washing sufficiently alters the texture such that the high magnetic anisotropy of the initially textured substrates can not be recovered. Here we disclose that the carbonate formation, and consequent loss in magnetic signal to noise ratio, is mitigated by packaging the substrates within impermeable foil bags in which the carbon dioxide concentration has been sufficiently lowered by pumpdown to subambient pressure and backfilling with an inert gas.

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Magnetic Recording Medium with Improved Magnetic Performance

  Carbonate formation on the textured substrates requires both moisture and carbon dioxide. When ambient levels of carbon dioxide are present, the relative humidity must be less than 5% in order to substantially reduce the carbonate formation on a time scale that is useful for shiipping. Elimination of moisture can, in principle, be accomplished by incorporating a desiccant along with the substrates and sealed inside a foil bag. In practice, the moisture cannot be sufficiently reduced inside the sealed foil bag because too much water is released by the polycarbonate cassette. We have discovered that reducing the carbon dioxide below a critical level by pumping out the air from the disk containers and backfilling them with an inert gas hinders, or prevents, the formation of carbonates, Such packaging improves OR and SNR when the substrates are employed in disk manufacturing.

The total amount of carbon dioxide inside the cassette package is adequately reduced by pumping the air out to at least 29.5 in Hg below atmospheric pressure in a vacuum chamber bag sealer. At this pressure, the number of carbon dioxide molecules inside the packaging is low enough so that significanly less than a monolayer of carbonate can form on the surface. It is critical to ensure that the amount of residual carbon dioxide does not exceed this value. Therefore, the pressure that the packaged disks need to be pumped down to, will depend on the number of disks and the volume of the shipping cassette. The vacuum chamber of the sealer is then backfilled with dry nitrogen, or other inert gas, to about 8 inHg below atmospheric pressure to avo...