Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-04-03
2002-08-13
Rickman, Holly C. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S900000
Reexamination Certificate
active
06432563
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed generally to magnetic recording media and devices incorporating the media and, more particularly, to zinc (Zn) containing layers for use with cobalt or cobalt alloy based magnetic layers in the formation of magnetic recording media and the recording devices used in data storage.
As the home, office, transportation vehicle, business place and factory becomes more automated and electronically connected, and as electronic devices and appliances such as computers, communication devices, electronic games, entertainment systems, personal data assistants transportation systems, vehicles, manufacturing tools, shop tools, and home appliances become more sophisticated there is, and will be, an ever-increasing demand for low cost magnetic recording media with greater storage capacity. In order to keep the storage devices unobtrusive and inexpensive each product generation must store more information in a smaller space. Hence there are ever increasing demands for technical improvement of the media and storage systems.
In general this means that not only must media attributes be improved, but that the transducer used to record or retrieve the data must be capable of resolving extremely small distances and changes in the media. With the exception of a few optical storage systems, such as those based upon holographic processes or multi-photon processes, this means that for all future storage systems the transducer must be in extremely close proximity to the media. This is certainly the case for all modern magnetic recording systems, where the ability to resolve the recorded data falls off exponentially with distance between the transducer and the media. This is the case in perpendicular, isotropic or longitudinal magnetic recording systems such as used in hard disk, magnetic tape, and floppy disk systems. This is even the case in the proposed near field optical recording systems as well as for the envisioned future x-y addressable systems such as those that might be base upon micro-machined silicon structures. What this means for the magnetic media is that in order to place the transducer close to the magnetic media, so that better resolution is possible, it is best if the magnetic media layer is very thin. Furthermore, there must not be much physical space allocated to be between the surface of the magnetic layer and the transducer, such as might be used for a physical wear layer or lubricant. However, some of these structures are required to actually have a working media. Clearly the entire structure must also be extremely smooth to allow the transducer to approach the media. Hence, since the early 1980s, as shown in the market place, there has been a movement toward thin film technology for the recording media. Thin media has enabled the rapid advances toward higher linear and areal recording densities.
Most commercially available thin film magnetic media is based upon hexagonal closed packed (HCP) cobalt alloys. This is because the HCP Co crystalline phase possesses both a relative large saturation magnetization, Ms, that can be adjusted by alloying, and a large uniaxial magnetocrystalline anisotropy energy density, Ku, which is necessary to achieve high coercivity, H
c
. Certainly for hard disk drive storage, the linear storage density is directly related to the coercivity and so to the anisotropy energy density. Due to the statistical nature of the playback signal a minimum signal to noise ratio (SNR) is required from the recording system in order to guarantee accuracy in reading back the recorded data. For a number of years the SNR in data storage systems has been limited by the statistical nature of the media, as opposed to the other sources of noise such as Johnson noise of the electronics or transducer. Since the media is granular in nature and the data bit cell size is inversely related to the areal recording density the media magnetic switching unit size determines the maximum possible SNR and so areal recording density that can be supported. Even in what is referred to as continuously exchange coupled media the bit cell wall location is determined by localized fluctuations in the media properties and so its storage capability is controlled by the granularity of these fluctuation locations. It is also true in optical recording systems such as the Compact Disk ReWriteable (CD-RW) whether they be based upon magneto-optic, MO, or phase change, PC, media. Basically, the power SNR is proportional to the number of switching units contained in a given data cell. For example for typical, modern hard disk drives approximately 100 to 1000 switching units are required per data bit cell to achieve a sufficient SNR. With the SNR fixed for a given required system data retrieval reliability, this implies that the size of the switching units must be decreased to increase the areal recording density. Unfortunately, the magnetic switching unit size is not always as small as might be thought by measuring the magnetic crystalline grain size. For example, if two magnetic grains, or particles, directly touch then they commonly become magnetically exchange coupled via the material's electronic wave functions. This means that the two grains tend to switch as a single larger unit. This reduces the number of particles or switching units in a data bit cell and causes the media noise to be worse. Because the two grains are usually not oriented in the same direction it also tends to lower the overall anisotropy energy density for the switching unit to a smaller value than that predicted for a single crystal grain. Also, if the crystalline grains are not perfect in structure, including crystalline defects or defects in the surface quality, the Ku value will be less than predicted from bulk crystalline measurements. Lowering Ms can lower Hc, but if Ms does not decrease as fast as Hc increases then it must be due to other effects, such as improved effective Ku. Both of these effects may compromise the coercivity of the media and limit the recording density. Hence, it is a major objective in the construction of magnetic media to have small, isolated magnetic switching units with sufficient anisotropy energy density to provide a coercivity to record short wavelength data bit patterns.
One measurement method developed to determine whether or not magnetic grains are exchange coupled is referred to as the delta M, dM, method. This bulk magnetic measurement compares the difference between the initial magnetization process, from the demagnetized state, to the reversal process from the saturated magnetic state. If there is little fundamental difference between these two magnetization processes then the magnetic particles are said to be non-interacting and the dM values will approach zero. If the dM values are positive then for Co based longitudinal media magnetic grains are said to be exchange coupled. Hence, a near zero or negative dM value is a reasonable measure indicating the media grains are de-coupled.
For longitudinal disk magnetic recording, the recording performance must look the same at all locations around the concentrically recorded tracks on the disk. This usually implies that the disk media has a set of magnetic particles that are randomly oriented with respect to the circumferencial location on the disk. It is the function of the media materials and media construction and process to achieve these attributes and more. Historically, a mechanical grooving in a circumferencial direction of the disk surface, sometimes called mechanical texture, has been used to create a slight orientation of the magnetic direction of the magnetic particles. However, the maximum that has been achieved is quite small. This orientation is measured by determining the ratio of magnetic properties when measure along the circumference direction versus the radial direction on the disk. The ratio of the coercivity, H
c
, or the remanent magnetization, M
r
, values when measured along these two directions, is referred to as the orientation ratio, O.R. Typical ratios ob
Lambeth David N.
Laughlin David E.
Zou Jie
Carnegie Mellon University
Kirkpatrick & Lockhart LLP
Rickman Holly C.
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