Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-06-05
2004-03-23
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S336000, C428S690000, C428S900000, C427S129000, C427S131000, C427S599000, C204S192150, C204S192200, C360S097010
Reexamination Certificate
active
06709773
ABSTRACT:
FIELD OF INVENTION
This invention relates to perpendicular recording media, such as thin film magnetic recording disks having perpendicular recording, and to a method of manufacturing the media. The invention has particular applicability to high areal density magnetic recording media exhibiting low noise.
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.
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.
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 filed 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.
The ease of magnetization or demagnetization of a magnetic material depends on the crystal structure, grain orientation, the state of strain, and the direction and strength of the magnetic field. The magnetization is most easily obtained along the easy axis of magnetization but most difficult along the hard axis of magnetization. A magnetic material is said to possess a magnetic anisotropy when easy and hard axes exist. On the other hand, a magnetic material is said to be isotropic when there are no easy or hard axes.
“Anisotropy energy” is the difference in energy of magnetization for these two extreme directions, namely, the easy axis of magnetization and the hard axis of magnetization. For example, a single crystal of iron, which is made up of a cubic array of iron atoms, tends to magnetize in the directions of the cube edges along which lie the easy axes of magnetization. A single crystal of iron requires about 1.4×10
5
ergs/cm
3
(at room temperature) to move magnetization into the hard axis of magnetization, which is along a cubic body diagonal.
The anisotropy energy U
A
could be expressed in an ascending power series of the direction cosines between the magnetization and the crystal axes. For cubic crystals, the lowest-order terms take the form of Equation (1),
U
A
=K
1
(&agr;
1
2
&agr;
2
2
+&agr;
2
2
&agr;
3
2
+&agr;
3
2
&agr;
1
2
)+
K
2
(&agr;
1
2&agr;
2
2
&agr;
3
2
) (1)
where &agr;
1
, &agr;
2
and &agr;
3
are direction cosines with respect to the cube, and K
1
, and K
2
are temperature-dependent parameters characteristic of the material, called anisotropy constants.
Anisotropy constants can be determined from (1) analysis of magnetization curves, (2) the torque on single crystals in a large applied field, and (3) single crystal magnetic resonance.
The total energy of a magnetic substance depends upon the state of strain in the magnetic material and the direction of magnetization through three contributions. The first two consist of the crystalline anisotropy energy of the unstrained lattice plus a correction that takes into account the dependence of the anisotropy energy on the state of strain. The third contribution is that of the elastic energy, which is independent of magnetization direction and is a minimum in the unstrained state. The state of strain of the crystal will be that which makes the sum of the three contributions of the energy a minimum. The result is that, when magnetized, the lattice is always distorted from the unstrained state, unless there is no anisotropy.
“Magnetostriction” refers to the changes in dimension of a magnetic material when it is placed in magnetic field. It is caused by the rotation of domains of a magnetic material under the action of magnetic field. The rotation of domains gives rise to internal strains in the material, causing its contraction or expansion.
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 causes the easily magnetized direction of the media to be 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.
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 comprises 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.
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
Chang Chung-Hee
Ranjan Rajiv Y.
Morrison & Foerster / LLP
Rickman Holly
Seagate Technology Inc.
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