Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-10-31
2004-05-04
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S900000, C428S216000, C428S336000, C427S131000, C427S132000
Reexamination Certificate
active
06730420
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to magnetic recording media used in rigid disc drives commonly used for computer data storage and methods of making the same.
BACKGROUND
In the hard disk drive industry, ever-increasing recording density demands continuous improvement in hard disk recording media so as to support a higher linear recording density (thousands of flux changes per inch-KFCI) and track density (thousands of tracks per inch-KTPI). Recording density is proportional to the product of KFCI and KTPI, and is typically expressed as giga-bits per square inch (Gb/in
2
). Currently the recording density is increasing at compound annual growth rate of approximately 100%.
In order for the media to be able to support high KFCI (e.g., over 250 KFCI), the pulse width (PW
50
-pulse width at 50% of pulse amplitude) needs to be as small as possible to reduce the inter-symbol interference so that high resolution at high recording density can be obtained. The resolution is defined as the pulse amplitude at high frequency divided by the pulse amplitude at low frequency. Based on generally known magnetic recording theory, in order to reduce PWso and hence increase resolution, the magnetic recording media must have high coercivity, Hc. For today's typical recording density of 4 Gb/in
2
, the value of Hc needs to be on the order of 3000 Oe, and in future it needs to be greater for even higher recording density. Other means of reducing PW
50
include increasing the hysteresis loop-squareness, generally defined as “S” which is ratio of remanent to saturation magnetization (Mr/Ms), increasing the coercivity squareness “S*”, increasing the remanent coercivity squareness “S*
rem
”, and narrowing the switching field distribution (“SFD”), as described by William and Comstock in “An Analytical Model of the Write Process in Digital Magnetic Recording,” A.I.P. Conference Proceedings on Magnetic Materials 5, p. 738 (1971).
Another factor which is important for increased KFCI and KTPI is that the signal to noise ratio (SNR) must be maximized. There are contributions to SNR from the electronics and the channel used to process the magnetic signal. But there is also intrinsic noise from the media that must be minimized. The largest contribution to the media noise is generated from the interparticle (or intercrystalline) magnetic exchange interaction. To suppress this exchange interaction, one must isolate the magnetic crystals from each other by one or more nonmagnetic elements (such as Cr atom) or compounds. The amount of separation need be only a few angstroms for there to be a significant reduction in intergranular exchange coupling. Another source of intrinsic media noise is the size or dimension of the magnetic grain. At recording densities of 4 Gb/in
2
and greater, the bit size along the track is less than 0.1 &mgr;m. Therefore to prevent the excessive noise arising from the physical dimensions of the grain, the diameter of each magnetic grain on the average should be less than approximately 0.01 &mgr;m (10 nm) at this density, and even smaller for greater densities. Intrinsic media noise has been theoretically modeled by J. Zhu et al. in “Micromagnetic Studies of Thin Metallic Films” in Journal of Applied Physics, Vol. 63, No. 8, (1988) p. 3248-53 which is incorporated by reference herein. T. Chen et al. also describe the source of intrinsic media noise in “Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media” in IEEE Transactions on Magnetic, Vol. 24, No. 6, (1988) p. 2700-05 which is also incorporated by reference herein.
The noise of the media can be reduced by decreasing the grain of the media, however smaller grain size may reduce Hc due to the onset of the superparamagnetic effect which comes about due to an inability of the grain to support the magnetization when it competes with the thermal fluctuation. In general, the onset of the superparamagnetic effect can be delayed by increasing the K
u
of the magnetic grain through addition of platinum which has a high orbital moment, and also by improving the crystalline perfection of the hexagonal close packed (HCP) cobalt grains.
Therefore, an optimal thin film magnetic recording medium for high density recording applications that can support high bit density will require low noise and high signal without adversely sacrificing PW
50
, overwrite (OW) and total non-linear distortion (TNLD). Cobalt alloys which are currently used for optimization of certain of the above performance criteria typically include the addition of chromium (Cr), tantalum (Ta) and platinum (Pt), due to their ability to provide high Hc and high magnetic moment. Chromium is typically added in an amount greater than 10 atomic % to act as segregant to separate the cobalt alloy grains for noise reduction and for corrosion resistance. Other additives such as Ti, V, W, Mo, B and others are sometimes used. In all cases, the cobalt crystal structure must be hexagonal close-packed (HCP), and it is preferable to have the c-axis of the grains oriented in the film plane. This is accomplished by depositing a chromium film below the cobalt layer and arranging for the epitaxial growth of cobalt alloy grain above the chromium layer.
In order to describe the crystallography of the cobalt alloy and chromium, planes and directions in the crystal are denoted by generally accepted conventions, such as described in “Elements of X-ray Diffraction” by B. D. Cullity published by Addison-Wesley Publishing Co. Inc., herein incorporated by reference. It is typical to describe the crystallographic planes and directions in hexagonal crystals such as cobalt by a 4 indices notation called Miller-Bravais indices, while cubic structure crystals such as chromium are denoted by 3 indices notation called Miller indices.
Brackets, “< >” are used to describe crystallographic directions, while parenthesis “( )” are used to denote specific lanes. “{ }” are used to denote a class of planes which are crystallographically equivalent. For example with chromium with body-centered cubic (BCC) crystallographic structure, <001> direction is normal to a (001) plane. For a hexagonal crystal structure such as cobalt, the crystal surface with the most dense atomic packing is the (0001) plane and the direction normal to that plane is <0001> direction. The <0001> direction is often referred to as the c-axis as described earlier. The crystallographic directions and the surfaces for cobalt are shown in FIG.
1
and those for chromium are shown in FIG.
2
.
The crystallographic orientation relationship that occurs between hexagonal cobalt film and BCC chromium film was originally reported by J. Daval & D. Randet in “Electron microscopy on High-coercive-force Co-Cr Composite Films” in IEEE Transaction on Magnetics, MAG-6, No. 4, (1970) p. 768-73. This work was preceded by work by J. P. Lazzari, I. Melnick and D. Randet in “Experimental studies using in-contact recording on chromium-cobalt films” in IEEE Transactions on Magnetics, Vol. MAG-5, No. 4, (1969) p. 955-59 where they reported that Hc of the cobalt film is increased by its deposition on top of a chromium underlayer.
The crystallographic orientation of chromium which promotes the cobalt c-axis to lie in the plane of the film is to arrange for chromium film to grow with <001> preferred growth, which means that {001} type planes of chromium lie in the plane of the film. It has been found that the atomic spacing of cobalt {11{overscore (2)}0} type planes matches reasonably well with the atomic spacing of a {001}
cr
plane as shown in
FIG. 3
hereof. Lattice spacings for pure Cr (a
o
=2.885 Å) and pure Co (c=4.069 Å, a
o
=2.507 Å) are illustrated in FIG.
3
. As seen in the Figure, the <0001> direction of cobalt is aligned with the <110> direction of the chromium lattice in the plane of epitaxy. In this direction, the Cr and Co lattices are closely matched and the mismatch is around 0.3%.
Bertero Gerardo
Cao Wei
Chen Charles
Chen Tu
Komag, Inc.
Rickman Holly
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