Storage media with non-uniform properties

Details Storage media with non-uniform properties Storage media with non-uniform properties

2002-01-17

2004-10-26

Bernatz, Kevin M. (Department: 1773)

Stock material or miscellaneous articles

Circular sheet or circular blank

Recording medium or carrier

C428S065100, C428S156000, C428S212000, C428S215000, C428S336000, C428S409000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06808783

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to information storage media and specifically to magnetic recording media, such as thin-film magnetic disks.
BACKGROUND OF THE INVENTION
Magnetic hard-disk drives can store and retrieve large amounts of information. The information is commonly stored as a series of bits on a stack of thin-film magnetic disk platters, each of which is an aluminum alloy or glass substrate coated on each side with a thin-film magnetic material and one or more protective layers. A bit is identified as a flux transition, and bit density is measured as the number of flux transitions per unit length. Typically, the higher the bit density, the lower the signal-to-noise ratio. Read-write heads, typically located on both sides of each platter, record and retrieve bits from circumferential tracks on the magnetic disks.
Although great strides have been made over the past decade in increasing the bit density of hard drives, information storage requirements have increased dramatically. An ongoing challenge of disk drive manufacturers is to provide even higher areal (bit) densities for thin-film magnetic disks.
The annular disk shape has complicated the ability to obtain further significant increases in bit density because of the existence of differing operating conditions in different parts of the disk. Due to the annular shape of disks, the lengths of the inner tracks (in the inner diameter (“ID”) disk region) are significantly less than the lengths of the outer tracks (in the outer diameter (“OD”) disk region), and therefore the track velocity in the ID region is less than the track velocity in the OD region.
The disparate track velocities in the ID and OD disk regions together with the substantial uniformity in disk properties across the face of the disk cause the User Bit Density or UBD in the ID and OD regions to be subject to different limiting factors. In the ID and OD regions, permissible bit densities are typically limited by one or more of the percolation limit (which is a measure of how close together magnetic field transitions in adjacent bits can be positioned before the adjacent magnetic fields are subject to mutual interference or cancellation), the signal strength (or signal-to-noise ratio or SNR), pulse width, the performance characteristics of the data detection channel, and/or the grain size of the recording medium. Presently in the ID region, the UBD is limited by data detection channel performance. Beyond a certain UBD value, the Bit Error Rate (BER) of the channel is unacceptable. In the OD region by contrast, the bit density is limited not by data detection channel performance such as UBD but by noise and signal strength (or by the minimum acceptable SNR). The data rate in the OD region is generally much higher than in the ID region due to the higher track velocities in the OD region. Higher data rates introduce more system noise during recording and therefore provide a lower SNR (compared to the same signal strength in the ID region) when information is later accessed by the head. To maintain noise at acceptable levels, the UBD in the OD region is typically reduced; that is, the performance degradation from high data rates in the OD region is offset by the low UBD in the same region. Thus, unused channel capacity exists in the OD region.
There is thus a need for providing a thin-film magnetic disk having a high areal (bit) density, such as by utilizing a larger percentage of the unused channel capacity in the OD region.
SUMMARY OF THE INVENTION
The present invention is directed to a storage media with at least substantially nonuniform properties to enhance performance of the media and/or provide a high areal bit density of the media. By using nonuniform properties, for example, the ID and OD regions can be configured differently to provide optimum or near optimum disk properties for the differing operating conditions of the two regions.
In one embodiment of the present invention, a disk for information storage is provided that includes:
(a) a substrate and
(b) an information layer.
An underlayer can be located between the substrate and the information layer to provide a consistent surface structure for deposition of the information layer and/or to control the properties such as coercivity of the information layer. The disk has one or more (or all) of the following properties: (i) two or more recording parameters that vary radially, (ii) a writing property (e.g., the coercivity of magnetic materials) that varies radially, (iii) an underlayer having a thickness that varies radially which, in one configuration, causes a recording parameter (e.g., the coercivity) of the disk to vary radially, and (iv) the information layer has a thickness that increases from an inner disk diameter to an outer disk diameter. As will be appreciated, the “coercivity” of a magnetic material refers to the value of the magnetic field required to reduce the remanence magnetic flux to zero, i.e., the field required to erase a stored bit of information. A higher coercivity allows adjacent recorded bits to be placed more closely together without mutual interference or cancellation.
Many disk variations are possible according to the concepts of the present invention. For example in one configuration, the two or more magnetic parameters of condition (i) include a magnetization-thickness product (Mrt) and a coercivity. The Mrt or magnetization thickness product (or magnetization product or magnetic moment) is the product of the remnant or remanence magnetization Mr and the thickness of the magnetic material. The remanence moment or magnetic moment is a measure of the signal amplitude that can be read from pulses stored in the medium—the greater the remanence moment, the greater the signal amplitude that can be detected in a reading operation. In order to cause the MRT to vary radially, the thickness of the information layer is varied radially, such as by increasing the thickness of the Mrt from the inner disk diameter to the outer disk diameter and/or by otherwise increasing the magnetic remanence from the inner disk diameter to the outer disk diameter, such as by a altering radially the chemical composition of the information layer.
In a preferred configuration, the magnetic remanence and/or the magnetic moment (Mrt) of the disk increases from the inner disk diameter to the outer disk diameter, and the coercivity decreases from the inner disk diameter to the outer disk diameter. These trends reflect the unique operating conditions in each of the two regions. That is, the higher coercivity and lower magnetic moment in the ID region support a higher linear density due to reduced UBD, and the lower coercivity and higher magnetic moment in the OD region improves writing properties and signal-to-noise ratio. In one configuration, the underlayer has a thickness that decreases from an inner diameter of the disk to an outer diameter of the disk to decrease the coercivity from the inner to outer disk diameters. In one configuration, the increased Mrt or magnetic remanence in the OD region provides a higher signal strength (or SNR), thereby permitting more noise to be tolerated and a higher linear bit density (or UBD) to be utilized. The decrease in the coercivity towards the OD region further provides better writing properties in the OD region (in which recording heads typically encounter more resistance to recording or writing bits), thereby providing reduced demands (relative to existing storage media) on the write head, the data detection channel, and the pre-amplifier, and permitting the head to write to the disk at a higher data rate. As a result of the foregoing, the Bit Per Inch (BPI) can be high enough in the OD region to be limited by data detection channel performance, as in the case of the ID region. The use, in the OD region, of a higher BPI than has been previously possible provides a significant increase in the areal density of the disk. For example, in a conventional disk the BPI reduction from the ID to the OD regions typically varies between abo

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