Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-10-03
2002-12-31
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S332000, C428S336000, C428S690000, C428S690000, C427S128000, C427S129000, C427S130000, C427S131000, C204S192200
Reexamination Certificate
active
06500567
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, the 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 60%.
In order for the media to be able to support the high KFCI (e.g., over 200 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 PW
50
and hence increase resolution, the magnetic recording media must have high coercivity, Hc. For today's typical recording density of 2 Gb/in
2
, the value of Hc needs to be on the order of 2500 Oe, and in future it needs to be 3000 Oe or more 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).
For increased TPI, the media should also have high H
c
to support a high off track compressibility (OTC). OTC is the measure of how much the individual tracks can be squeezed together before they start to interfere with each other and degrade the error rate as the track being squeezed. For example, in order to support 10 KTPI, the H
c
of the media should be equal to or higher than 2500 Oe. In summary, in order for the media to be able to support high areal recording density, H
c
of the media must be as high as possible.
Other parameters of importance for recording performance are overwrite (OW), signal to noise ratio (SNR), and total non-linear distortion (TNLD). Overwrite is a measure of the ability of the medium to accommodate overwriting of existing data. In another words, OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over the original data. OW will be low or poor when a significant amount of the first signal still remains after over-writing. OW is generally affected by the coercivity, the squareness, and the SFD of the medium. For future high density recording, higher Hc media will be preferred for narrower PW
50
and high resolution. However, an increase in Hc is generally accompanied by reduction in OW. Thus, there is a need in the art to improve the S* and the SFD to obtain improvements in OW for a given Hc. High hysteresis loop squareness and narrow switching field distribution can be obtained by having uniform distribution of the grain size of the media.
Another factor which is important for increased KFCI and KTPI is that the signal to noise ratio 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. As the recording density approaches 2 Gb/in
2
and beyond, the bit size along the track will be in the order of 0.1 &mgr;m or less. 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 approximately 0.01 &mgr;m (10 nm) or less for media in the 2.0-2.5 Gb/in
2
media range. The grain size should be approximately 7.5 nm or less for 5 Gb/in
2
media, and should be approximately 5 nm or less for 10 Gb/in
2
media. 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.
There is another intergranular interaction, called magnetostatic interaction, which acts over a much greater distance between particles as compared to the exchange interaction. Reducing the magnetostatic interaction does reduce intrinsic media noise slightly. However, the effects of magnetostatic interaction actually improve hysteresis loop squareness and narrow the switching field distribution (but to a lesser extent than the exchange interaction), and hence improve PW
50
and OW. Therefore, magnetostatic interaction is generally desirable and hence tolerated.
Total non-linear distortion (TNLD) is another parameter that needs to be reduced for high density recording, and it comes about from intersymbol interference between adjacent bits, and partial erasure of a bit at the transition during writing. TNLD can be reduced by increasing the coercivity, reducing the remanent magnetization Mr, and reducing the film thickness T, for a reduction in remanence thickness product MrT. TNLD is also improved by orienting the easy magnetization direction of the magnetic particles in the plane of the media. Increasing the Hc is desirable not only for TNLD but also for PW
50
as mentioned previously. Since TNLD increases as the recording density increases, it is becoming an important parameter for higher recording density.
In order to obtain the best performance from the magnetic media, each of the above criteria such as PW
50
, resolution, OW, SNR, and TNLD must be optimized. This is a formidable task, as each of these performance criteria are interrelated. For example, obtaining a narrower PW
50
and reducing TNLD by increasing the Hc will adversely affect OW. A thinner medium also provides narrower PW
50
, better OW, and lower TNLD, however the SNR decreases because the media signal itself is reduced. Increasing squareness of the hysteresis loop contributes to narrower PW50, better OW, and lower TNLD, but may increase noise due to intergranular exchange coupling and magnetostatic interaction. The SNR of the media can be reduced by decreasing the grain of the media, however smaller grain size may reduce Hc due to onset of super-paramagnetic effect which comes about due to inability of the grain to support the magnetization when it competes with the thermal fluctuation. In general, the effect of onset of super-paramagnetic can be delayed by increasing the K
u
of the magnetic grain through addition of platinum which has high orbital moment, and also 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 sacrifici
Bertero Gerardo
Chen Charles Changqing
Chen Tu
Imakawa Makoto
Suekane Michinobu
Bernatz Kevin M
Komag, Inc.
Thibodeau Paul
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