Control of magnetic film grain structure by modifying Ni-P...

Details Control of magnetic film grain structure by modifying Ni-P... Control of magnetic film grain structure by modifying Ni-P...

2001-02-09

2003-02-25

Rickman, Holly (Department: 1773)

Stock material or miscellaneous articles

All metal or with adjacent metals

Having magnetic properties, or preformed fiber orientation...

C428S336000, C428S667000, C428S690000, C428S900000, C427S128000, C427S129000, C427S131000

Reexamination Certificate

active

06524724

ABSTRACT:

TECHNICAL FIELD
This invention relates to a magnetic medium, such as a thin film magnetic recording medium, and the method of manufacturing the medium. The invention has particular applicability to a magnetic recording medium exhibiting low noise, high coercivity and suitable for high-density longitudinal and perpendicular recording.
BACKGROUND ART
The requirements for high areal density impose increasingly greater requirements on magnetic recording media in terms of coercivity, remanent squareness, low medium noise and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high-density magnetic rigid disk medium for longitudinal and perpendicular recording. The magnetic anisotropy of longitudinal and perpendicular recording media makes the easily magnetized direction (the easy axis of magnetization) of the media located in the film plane and perpendicular to the film plane, respectively. The remanent magnetic moment of the magnetic media after magnetic recording or writing of longitudinal and perpendicular media is located in the film plane and perpendicular to the film plane, respectively.
To accommodate for increased areal density, design of magnetic media is one of the key factors. The main limitation in media is the so-called “superparamagnetic” effect, which can be interpreted simply as follows: to achieve high areal density media, the grain size of the magnetic film needs to be reduced. However, when grain size approaches 100 Å, the energy needed to switch the easy axis of magnetization of one grain to that of the other becomes smaller than the thermal energy if the grains are weakly coupled. That is, the thermal energy destroys the magnetism by randomizing the magnetization of the small grains, and the grains can not hold permanent magnetization any more. Therefore, it has become extremely important to make “thermally stable” magnetic films with grains smaller than 100 Å for high areal density applications.
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al—Mg) alloy. Such Al—Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Other substrate materials have been employed, such as glass, e.g., an amorphous glass, glass-ceramic material which comprise a mixture of amorphous and crystalline materials, and ceramic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks.
A conventional longitudinal recording disk medium is depicted in FIG.
1
and typically comprises a non-magnetic substrate
10
having sequentially deposited on each side thereof an underlayer
11
,
11
′, such as chromium (Cr) or Cr-alloy, a magnetic layer
12
,
12
′, typically comprising a cobalt (Co)-base alloy, and a protective overcoat
13
,
13
′, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer
11
,
11
′, magnetic layer
12
,
12
′, and protective overcoat
13
,
13
′, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer. A conventional perpendicular recording disk medium is similar to the longitudinal recording medium depicted in
FIG. 1
, but does not comprise Cr-containing underlayers.
Conventional methods for manufacturing longitudinal magnetic recording medium with a glass or glass-ceramic substrate comprise applying a seed layer between the substrate and underlayer. A conventional seed layer seeds the nucleation of a particular crystallographic texture of the underlayer.
Conventional Cr-alloy underlayers comprise vanadium (V), titanium (Ti), tungsten (W) or molybdenum (Mo). Other conventional magnetic layers are CoCrTa, CoCrPtB, CoCrPt, CoCrPtTaNb and CoNiCr.
A conventional longitudinal recording disk medium is prepared by depositing multiple layers of metal films to make a composite film. In sequential order, the multiple layer typically comprise a non-magnetic substrate, one or more underlayers, a magnetic layer, and a protective carbon layer. Generally, a polycrystalline epitaxially grown cobalt-chromium (CoCr) magnetic layer is deposited on a chromium or chromium-alloy underlayer.
The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 Å thick.
The linear recording density could be increased by increasing the coercivity of the magnetic recording medium. However, this objective could only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. As the recording areal density increases, conventional magnetoresistive (MR) disks have smaller grain size, which induces superparamagnetic limit and causes the collapse of medium coercivity and magnetic remanance. Also, conventional sputtered media rely on the magnetic alloy composition to increase volume anisotropy.
There exists a need for technology enabling the use of a structure that make “thermally stable” magnetic films with grains smaller than 100 Å for high areal density applications.
SUMMARY OF THE INVENTION
During the course of the present invention, it was found that modifying the substrate plating composition so that during sputtering, special film, such as oxide film, with uniform and extremely fine grains (<100 Å) can form on the top of the substrate surface. The subsequent growth of magnetic films will be pre-defined by the special oxide film. Uniform grains (<100 Å) of magnetic film could be formed on top of the oxide grains underneath, with narrow grain size distribution. Because of the narrow grain size distribution, one could eliminate magnetic grains which are too small to be thermally stable while maintaining the small mean grain size for good signal-to-noise performance. As a result, the “superparamagnetic” effect will be pushed further down to higher areal densities.
An embodiment of this invention is a magnetic recording medium, comprising a substrate, a Ni—P—X containing layer on the substrate and a magnetic layer with segregated Co-containing grains on the Ni—P—X containing layer, wherein X has a higher oxidation potential than that of Ni and X is not W. In other embodiments, X is selected from the group consisting of Al, Co, Cr, Fe, Ti, V, Cd, Zr, Mn and Mo. The magnetic recording medium could further comprise an underlayer comprising at least one layer of Cr or Cr-based alloy on the Ni—P—X containing layer and an optional intermediate layer comprising a CoCr-based alloy on the underlayer. The segregated Co-containing grains have a mean grain diameter of about 100 Å or less. The mean grain diameter could be measured by performing a grain size analysis of transmission electron microphotographs of the magnetic layer. The magnetic recording medium could comprise an oxide layer in between the Ni—P—X containing layer and the magnetic layer. The oxide layer could comprise additive-rich-oxide grains and Ni-rich oxide grains. The additive-rich-oxide grains could have a spacing between adjacent additive-rich-oxide grains of about 100 Å or less. The oxide layer could have a thickness of about 5 to 100 Å. The magnetic layer could have a thickness of about 100 to 300 Å. The substrate could be a glass substrate or an aluminum substrate, the underlayer could be CrW, CrV, CrTi or CrTa and the i

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