Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-06-02
2002-08-27
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S336000, C428S900000, C428S690000, C428S212000, C427S131000, C427S132000
Reexamination Certificate
active
06440589
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic layered structures for use in magnetic recording media, particularly magnetic recording disks.
BACKGROUND
Conventional magnetic recording disks have a magnetic recording layer, typically of a ferromagnetic alloy, such as a cobalt (Co) alloy, that is sputter deposited as a continuous thin film having grains of the crystalline magnetic material. It is well known that to achieve high density recording in magnetic recording disks it is necessary to decrease the grain size, increase the coercivity (H
c
), and reduce the remanent magnetization-thickness product (M
r
t) of the magnetic recording layer. Therefore, most attempts to increase the magnetic recording density of magnetic media have focused on these three parameters by altering the composition and microstructure of the magnetic material in the magnetic layer or by reducing the magnetic layer thickness to achieve low M
r
t. Unfortunately, very thin magnetic recording layers with small grains can become thermally unstable, wherein the magnetic moments of the small grains can spontaneously switch their magnetization direction, resulting in loss of the recorded data. This is of concern, especially at the elevated operating temperatures of the disk drive. Thermally unstable grains, at sufficiently small grain sizes, are generally referred to as superparamagnetic grains.
These superparamagnetic grains represent a serious obstacle to further increases in magnetic recording densities. This limitation of conventional media is evident by the fact that a reduction in grain volume by a mere factor of one half, for instance, may change the thermal stability from being on the order of several years to less than a minute.
Conventional media may show amplitude loss, noise increase, increased data error rate, loss of resolution, etc., in short, a general loss of magnetic recording performance due to thermally driven demagnetization processes even before catastrophic and rapid thermal demagnetization sets in at the superparamagnetic limit. This slower but steady magnetization decay can be characterized by a number of experimental measurement methods, including magnetic “viscosity” (magnetization decay rate) measurements. The conventional magnetic recording media can exhibit significant magnetization decay rates which impose serious limitations on the magnetic recording densities that can be achieved.
The conventional mechanism to stabilize thermally activated magnetization processes is by raising the magnetocrystalline anisotropy and coercivity of the media. This route is however subject to the constraint that the magnetic write head can produce only a maximum field magnitude which is limited by the magnetic moment density of the material of the pole pieces of the write head. The writeability of high-performance media is critical in order to increase the magnetic recording densities.
J. Chen et al. in
IEEE Trans. Mag.,
Vol. 34, no. 4, pp. 1624-1626, 1998; describes using “keeper layers” for thermally stabilizing the magnetic recording layers in the media. A chromium (Cr) break layer is laminated between the keeper layer and the magnetic layer. For example, a Cr break layer approximately 25 Angstroms thick is deposited on a magnetic layer prior to depositing a relatively thick (50 to 150 nm) keeper layer. The Cr break layer is used to control the growth of the keeper layer and to prevent exchange coupling between grains within the magnetic layer. Depositing such a thick magnetically soft keeper layer directly on a magnetic recording layer will cause strong magnetic inter-granular coupling in the magnetic layer and hinder its use for high density recording. The function of the keeper-layer/break layer construction is to reduce the demagnetization fields from adjacent magnetic transitions. Keeper layers are known to be a large source of recording noise. Furthermore, the keeper layer construction reported are 50-150 nm thick. It is preferred to have the total thickness of the layers and any overlays and protective overcoats making up the medium to be very thin to optimize the recording performance.
What is needed is a magnetic recording medium with a thin magnetic layer that has high coercivity, large coercivity squareness (S*), low remanent product thickness (M
r
t), and that is thermally stable and writeable to be able to supporting very high recording densities. It is preferable that such a medium can be fabricated using conventional film deposition methods.
OBJECTS AND ADVANTAGES
Accordingly, it is a primary object of the present invention to provide magnetic recording media exhibiting enhanced thermal stability. Because the magnetic media of this invention exhibits improved thermal stability, thinner magnetic recording layers containing smaller grains can be used, resulting in higher magnetic data recording densities.
It is a second object of the invention to improve the recording characteristics of very thin magnetic layers by providing ferromagnetic overlays or capping layers deposited on these magnetic layers. The ferromagnetic overlays increase the effective volume of the small thermally unstable grains in the magnetic layers and thus increase thermal stability.
It is a third object of the present invention to provide magnetic recording media with improved writeability.
Lastly, it is an object of the present invention to provide magnetic media with sharp magnetic transitions, high S* and low M
r
t.
SUMMARY OF THE INVENTION
The objects and advantages of the present invention are obtained by providing a magnetic media with a magnetic recording layer that includes a ferromagnetic “host” layer and a thin overlay or capping layer of ferromagnetic material deposited on the host layer. The ferromagnetic host layer is a granular layer with grains that are weakly coupled or uncoupled, wherein the grains are capable of independently changing magnetization directions in the presence of local magnetic fields generated by the magnetic write head. The ferromagnetic overlay enhances the thermal stability and writeability of the ferromagnetic material in the host layer, is substantially thinner than the host layer, and is exchange coupled to the host layer. Ferromagnetic “overlay” is meant to refer to a ferromagnetic material that is either a continuous layer of ferromagnetic material, a discontinuous layer in the form of a dispersion of islands or grains of ferromagnetic material, or a multiple phase layer containing islands or grains of a first ferromagnetic material separated by a second non-ferromagnetic material. Because the ferromagnetic overlay can be a discontinuous film or dispersion of material, it is convenient to refer to an “effective thickness” which is the thickness that would be attained with the same particle flux from the deposition apparatus for a film exhibiting continuous coverage. In addition, because multiple phase materials are also considered as ferromagnetic overlays, it is convenient to refer to effective concentrations of materials, which describe an average concentration of a material in the overlay. When referring to either a ferromagnetic overlay or a ferromagnetic host layer as a continuous layer the intent is to distinguish a continuous layer from a discontinuous layer or a dispersion of material, and is not intended to imply that the layer is a single phase material.
A magnetic medium of the present invention has at least one magnetic Co-based layer that is preferably 1 to 30 nm thick and that is deposited on a prepared substrate. The substrate is any suitable disk substrate, such as an aluminum disk blank coated with nickel phosphorus, glass coated with NiAl, silicon, ceramic, quartz, MgO and silicon-carbide. The substrates are covered with an underlayer in order to achieve the desired crystalline orientation of the subsequently deposited ferromagnetic host layer. Pure Cr is a typical underlayer, but underlayers may also include Cr alloys containing an element of Co, V, Ti and O. The choice of substrates and underlayers is dependent on the ferromagn
Fullerton Eric Edward
Margulies David Thomas
Marinero Ernesto Esteban
Schabes Manfred Ernst
Lumen Intellectual Property Services Inc.
Rickman Holly C.
Thibodeau Paul
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