Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-09-28
2002-10-08
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S900000, C428S065100
Reexamination Certificate
active
06461750
ABSTRACT:
TECHNICAL FIELD
The present invention relates to magnetic recording media, such as thin film magnetic recording disks, to a method of manufacturing the media and to an apparatus employed to manufacture the media. The present invention has particular applicability to high areal density longitudinal magnetic recording media exhibiting low media noise and enhanced magnetic performance.
BACKGROUND ART
Magnetic recording media are extensively employed in the computer industry and can be locally magnetized by a write transducer or write head to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based upon bits of the information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, grains of the recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response to a read sensor, allowing the stored information to be read.
There is an ever increasing demand for magnetic recording media with higher storage capacity, lower noise and lower costs. Efforts, therefore, have been made to reduce the size required to magnetically record bits of information, while maintaining the integrity of the information as size is decreased. The space necessary to record information in magnetic recording media depends upon the size of transitions between oppositely magnetized areas. It is, therefore, desirable to produce magnetic recording media that will support the smallest transition size possible. However, the output from small transition sizes must avoid excessive noise to reliably maintain the integrity of the stored information. Media noise is generally expressed in signal-to-noise ratio (SMNR).
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., SMNR, and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium. This objective must be accompanied by a decrease in the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high density magnetic hard disk drives, and is attributed primarily to big and inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Longitudinal magnetic recording media containing cobalt (Co) or Co-based alloy magnetic films with a chromium (Cr) or Cr alloy underlayer deposited on a non-magnetic substrate have become the industry standard. For thin film longitudinal magnetic recording media, the desired crystallized structure of the Co and Co alloys is hexagonal close packed (hcp) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable is the Co alloy thin film for use in longitudinal recording to achieve high remanance. For very small grain sizes coercivity increases with increased grain size. The large grains, however, result in greater noise. Accordingly, there is a need to achieve high coercivities without the increase in noise associated with large grains. In order to achieve low noise magnetic recording media, the Co alloy thin film should have uniform small grains with grain boundaries capable of magnetically isolating neighboring grains. This type of microstructural and crystallographic control is typically attempted by manipulating the deposition process, and proper use of underlayers and seedlayers.
Underlayers can strongly influence the crystallographic orientation, grain size and chemical segregation of the Co alloy grain boundaries. Conventional underlayers include Cr and alloys of Cr with elements such as titanium (Ti), tungsten (W), molybdenum (Mo) and vanadium (V).
There are other basic characteristics of magnetic recording media, aside from SMNR, that are indicative of recording performance, such as half-amplitude pulse width (PW50), overwrite (OW), and modulation level. A wide PW50 indicates that readback pulse from adjacent bits are crowded together resulting in interference which limits the linear packing density of bits in a given track and, hence, reduces packing density in a given area thereby limiting the recording capacity of the magnetic recording medium. Accordingly, a narrow PW50 is desirable for high areal recording density.
OW is a measure of the ability of the magnetic recording medium to accommodate overwriting of existing data. In other words, OW is a measure of what remains of a first signal after a second signal, e.g., at a different frequency, has been written over it on the medium. OW is considered low or poor when a significant amount of the first signal remains.
It is extremely difficult to obtain optimum performance from a magnetic recording medium by optimizing each of the PW50, OW, SMNR and modulation level, as these performance criteria are interrelated and tend to be offsetting. For example, if coercivity is increased to obtain a narrower PW50, OW is typically adversely impacted, as increasing coercivity typically degrades OW. A thinner medium having a lower Mr×thickness (Mrt) yields a narrower PW50 and better OW; however, SMNR decreases since the medium signal is typically reduced if the electronic noise of the system is high. Increasing the squareness of the hysteresis loop contributes to a narrower PW50 and better OW; however, noise may increase due to intergranular exchange coupling and magnetostatic interaction. Thus, a formidable challenge is present in optimizing magnetic performance in terms of PW50, OR, SMNR and modulation level.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of hcp Co alloys deposited thereon.
The majority of current conventional longitudinal magnetic recording media exhibit a bi-crystal cluster structure comprising a Co alloy with a (11.0) texture or crystallographic orientation epitaxially grown on a Cr-containing underlayer exhibiting a (200) texture. A bi-crystal cluster structure is characterized by two Co subgrains with an easy axis perpendicular to each other.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films, and can be employed to produce magnetic recording media with a Co-alloy having a (10.0) texture grown on the underlayer with a (112) texture forming the so called “uni-crystal”structure. Li-Lien Lee et al., “NiAl Underlayers For CoCrTa Magnetic Thin Films”, IEEE Transactions on Magnetics, Vol. 30, No. 6, November, 1994, pp. 3951-3953, and U.S. Pat. No. 5,693,426 issued to Li-Lien Lee et al. The “uni-crystal” structure is characterized by Co grains having an easy axis randomly distributed in the film plane. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oerste
Chen Qixu (David)
Huang Lin
Leu Charles
Ranjan Rajiv Yadav
McDermott & Will & Emery
Rickman Holly C.
Seagate Technology Inc.
Thibodeau Paul
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