Subseedlayers for magnetic recording media

Details Subseedlayers for magnetic recording media Subseedlayers for magnetic recording media

2001-05-03

2004-05-25

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S336000, C428S690000, C428S690000

Reexamination Certificate

active

06740397

ABSTRACT:

FIELD OF INVENTION
This invention relates to magnetic recording media, such as thin film magnetic recording disks, and to a method of manufacturing the media. The invention has particular applicability to high areal density magnetic recording media having thin films for longitudinal magnetic recording media, and more particularly, to B2-structured underlayers for use with a cobalt or cobalt alloy based magnetic layer.
BACKGROUND
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., signal-to-medium noise ratio (SMNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
Magnetic discs and disc drives provide quick access to vast amounts of stored information. Both flexible and rigid discs are available. Data on the discs is stored in circular tracks and divided into segments within the tracks. Disc drives typically employ one or more discs rotated on a central axis. A magnetic head is positioned over the disc surface to either access or add to the stored information. The heads for disc drives are mounted on a movable arm that carries the head in very close proximity to the disc over the various tracks and segments.
FIG. 1
shows the schematic arrangement of a magnetic disk drive
10
using a rotary actuator. A disk or medium
11
is mounted on a spindle
12
and rotated at a predetermined speed. The rotary actuator comprises an arm
15
to which is coupled a suspension
14
. A magnetic head
13
is mounted at the distal end of the suspension
14
. The magnetic head
13
is brought into contact with the recording/reproduction surface of the disk
11
. The rotary actuator could have several suspensions and multiple magnetic heads to allow for simultaneous recording and reproduction on and from both surfaces of each medium.
An electromagnetic converting portion (not shown) for recording/reproducing information is mounted on the magnetic head
13
. The arm
15
has a bobbin portion for holding a driving coil (not shown). A voice coil motor
19
as a kind of linear motor is provided to the other end of the arm
15
. The voice motor
19
has the driving coil wound on the bobbin portion of the arm
15
and a magnetic circuit (not shown). The magnetic circuit comprises a permanent magnet and a counter yoke. The magnetic circuit opposes the driving coil to sandwich it. The arm
15
is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion
17
. The ball bearings provided around the pivot portion
17
are held by a carriage portion (not shown).
A magnetic head support mechanism is controlled by a positioning servo driving system. The positioning servo driving system comprises a feedback control circuit having a head position detection sensor (not shown), a power supply (not shown), and a controller (not shown). When a signal is supplied from the controller to the respective power supplies based on the detection result of the position of the magnetic head
13
, the driving coil of the voice coil motor
19
and the piezoelectric element (not shown) of the head portion are driven.
A cross sectional view of a conventional longitudinal recording disk medium is depicted in
FIG. 2. A
longitudinal recording medium typically comprises a non-magnetic substrate
20
having sequentially deposited on each side thereof an underlayer
21
,
21
′, such as chromium (Cr) or Cr-alloy, a magnetic layer
22
,
22
′, typically comprising a cobalt (Co) -base alloy, and a protective overcoat
23
,
23
′, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer
21
,
21
′, magnetic layer
22
,
22
′, and protective overcoat
23
,
23
′, 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.
According to the domain theory, a magnetic material is composed of a number of submicroscopic regions called domains. Each domain contains parallel atomic moments and is always magnetized to saturation, but the directions of magnetization of different domains are not necessarily parallel. In the absence of an applied magnetic field, adjacent domains may be oriented randomly in any number of several directions, called the directions of easy magnetization, which depend on the geometry of the crystal. The resultant effect of all these various directions of magnetization may be zero, as is the case with an unmagnetized specimen. When a magnetic field is applied, the domains most nearly parallel to the direction of the applied field grow in size at the expense of the others. This is called boundary displacement of the domains or the domain growth. A further increase in magnetic field causes more domains to rotate and align parallel to the applied field. When the material reaches the point of saturation magnetization, no further domain growth would take place on increasing the strength of the magnetic field.
A magnetic material is said to possess a uniaxial anisotropy when all domains are oriented in the same direction in the material. On the other extreme, a magnetic material is said to be isotropic when all domains are oriented randomly.
Important magnetic properties, such as coercivity (Hc), remanent magnetization (Mr) and coercive squareness (S*), which are crucial to the recording performance of the Co alloy thin film for a fixed composition, depend primarily on its microstructure. For thin film longitudinal magnetic recording media, the desired crystalline 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 higher the coercivity of the Co alloy thin film used for longitudinal recording. This is required to achieve a high remanence. For very small grain sizes coercivity increases with increased grain size. Large grains, however, result in greater noise. There is a need to achieve high coercivities without the increase in noise associated with large grains. To achieve a low noise magnetic medium, the Co alloy thin film should have uniform small grains with grain boundaries which can magnetically isolate neighboring grains. This kind of microstructure and crystallographic texture is normally achieved by manipulating the deposition process, by grooving the substrate surface, or most often by the proper use of an underlayer.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium, and by decreasing 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 inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Underlayers can strongly influence the crystallographic orientation, the grain size and chemical segregation at the Co alloy grain boundaries. Underlayers that have been reported in the literature include Cr, Cr with an additional alloy element X (X=C, Mg, Al, Si, Ti, V, Co, Ni, Cu, Zr, Nb, Mo, La, Ce, Nd, Gd, Th, Dy, Er, Ta, and W), Ti, W, Mo, and NiP. While there would appear to be a number of underlayer materials available, in practice, only a very few work well enough to meet the demands of the industry. Among them, the most often used and the most successful underlayer is pure Cr. For high density recording, in-plane orientation has heretofore been achieved by gra

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