Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2000-08-29
2004-06-01
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S690000, C428S690000, C428S690000, C428S900000, C204S192100, C204S192200
Reexamination Certificate
active
06743503
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved perpendicular magnetic recording medium comprising a sputtered multilayer magnetic superlattice exhibiting very high values of perpendicular magnetic coercivity and areal storage density, and a method for manufacturing same. The invention finds particular utility in the fabrication of very high areal recording density magnetic recording (“MR”) media and devices such as hard disks.
BACKGROUND OF THE INVENTION
Magnetic recording media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval purposes, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the grains of magnetic material.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in
2
in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits. In this regard, the perpendicular recording media have been found superior to the more common longitudinal media in achieving very high bit densities.
As indicated above, much effort has been directed toward enhancing the density of data storage by both types of magnetic media, as well as toward increasing the stability of the stored data and the ease with which the stored data can be read. For example, it is desirable to develop magnetic media having large magnetic coercivities, H
c
, since the magnetic moments of such materials require large magnetic fields for reorientation, i.e., switching between digital 1 and 0. Thus, when the magnetic medium has a large coercivity H
c
, exposure of the magnetic medium to stray magnetic fields, such as are generated during writing operations, is less likely to corrupt data stored at adjacent locations.
The density with which data can be stored within a magnetic thin-film medium for perpendicular recording is related to the perpendicular anisotropy (“Ku” or “K
⊥
”) of the material, which reflects the tendency for the magnetic moments to align in the out-of-plane direction. Thin-film magnetic media having high perpendicular anisotropies have their magnetic moments aligned preferentially perpendicular to the plane of the thin film. This reduces the transition length, thereby allowing a larger number of magnetic bits (domains) to be packed into a unit area of the film and increasing the areal density with which data can be stored.
A large perpendicular anisotropy is also reflected in a larger value of the magnetic coercivity H
c
, since the preferential out-of-plane alignment of the magnetic moments raises the energy barrier for the nucleation of a reverse magnetization domain, and similarly, makes it harder to reverse the orientation of the magnetic domains by 180° rotation. Further, the magnetic remanence of a medium, which measures the tendency of the magnetic moments of the medium to remain aligned once the magnetic field is shut off following saturation, also increases with increasing K
⊥
.
A promising new class of materials for use as the active recording layer of perpendicular MR media includes multilayer magnetic “superlattice” structures comprised of a stacked plurality of very thin magnetic
on-magnetic layer pairs, for example cobalt/platinum (Co/Pt)
n
and cobalt/palladium (Co/Pd)
n
multilayer stacks. As schematically illustrated in the cross-sectional view of
FIG. 1
, such multilayer stacks or superlattice structures
10
comprise n pairs of alternating discrete layers of Co (designated by the letter A in the drawing) and Pt or Pd (designated by the letter B in the drawing), where n is an integer between about 5 and about 50. Superlattice
10
is typically formed by a suitable thin film deposition technique, e.g., sputtering, and can exhibit perpendicular magnetic isotropy arising from metastable chemical modulation in the direction normal to the underlying substrate S on which superlattice
10
is formed. Compared to conventional cobalt-chromium (Co-Cr) magnetic alloys utilized in magnetic data storage/retrieval disk applications, such Co/Pt)
n
and (Co/Pd)
n
, multilayer magnetic superlattice structures offer a number of performance advantages. For example, a sputtered (Co/Pt)
n
multilayer stack or superlattice
10
suitable for use as a magnetic recording layer of a perpendicular MR medium can comprise n Co/Pt or Co/Pd layer pairs, where n=about 5 to about 50, e.g., 20, and wherein each Co/Pt layer pair consists of an about 3 Å thick Co layer adjacent to an about 10 Å thick Pt or Pd layer, for a total of 40 separate (or discrete) layers. Such multilayer magnetic superlattice structures are characterized by having a large perpendicular anisotropy, high coercivity H
c
, and a high squareness ratio of a magnetic hysteresis (M-H) loop measured in the perpendicular direction. By way of illustration, (Co/Pt)
n
and (Co/Pd)
n
multilayer magnetic superlattices, wherein n=about 10 to about 30 Co/Pt or Co/Pd layer pairs having thicknesses as indicated supra and fabricated, e.g., by means of techniques disclosed in U.S. Pat. No. 5,750,270, the entire disclosure of which is incorporated herein by reference, exhibit perpendicular anisotropies exceeding about 2×10
6
erg/cm
3
; coercivities as high as about 5,000 Oe; squareness ratios (S) of a M-H loop, measured in the perpendicular direction, of from about 0.85 to about 1.0, and carrier-to-noise ratios (“CNR”) of from about 30 dB to about 60 dB.
A key advance in magnetic recording (MR) technology which has brought about very significant increases in the data storage densities of magnetic disks has been the development of extremely sensitive magnetic read/write devices which utilize separate magnetoresistive read heads and inductive write heads. The magnetoresistive effect, wherein a change in electrical resistance is exhibited in the presence of a magnetic field, has long been known; however, utilization of the effect in practical MR devices was limited by the very small magnetoresistive response of the available materials. The development in recent years of materials and techniques (e.g., sputtering) for producing materials which exhibit much larger magnetoresistive responses, such as Fe—Cr multilayer thin films, has resulted in the formation of practical read heads based upon what is termed the giant magnetoresistive effect, or “GMR”. Further developments in GMR-based technology have resulted in the formation of GMR-based head structures, known as GMR “spin valve” heads, which advantageously do not require a strong external magnet or magnetic field to produce large resistance changes, and can detect weak signals from tiny magnetic bits.
The use of such GMR-based spin valve heads can significantly increase the areal density of MR media and systems by increasing the track density, as expressed by the number of tracks per inch (“TPI”) and the linear density, as expressed by the number of bits per inch (“BPI”), where areal density=TPI×BPI. Currently, GMR-based spin valve heads are utilized for obtaining areal recording densities of more than about 10 Gb/in
2
; however, even greater recording densities are desired. A difficulty encountered with further increase in the BPI of conventional MR media is that the smaller grain sizes necessary for increase in the BPI results in thermal instability of the media due to exceeding the supe
McDermott & Will & Emery
Rickman Holly
Seagate Technology LLC
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