Multi-layered anti-ferromagnetically coupled magnetic media

Details Multi-layered anti-ferromagnetically coupled magnetic media Multi-layered anti-ferromagnetically coupled magnetic media

2001-12-07

2004-05-18

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

All metal or with adjacent metals

Having magnetic properties, or preformed fiber orientation...

C428S637000, C428S668000, C428S678000, C428S686000, C428S215000, C428S336000, C428S690000, C428S216000

Reexamination Certificate

active

06737172

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to very high areal density magnetic recording media exhibiting improved thermal stability, such as hard disks. More particularly, the present invention relates to improved longitudinal magnetic recording media including a plurality of spacer layers providing enhanced anti-ferromagnetic coupling (“AFC”) between a plurality of vertically spaced-apart ferromagnetic layers.
BACKGROUND OF THE INVENTION
Magnetic recording (“MR”) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A portion of a conventional longitudinal recording, thin-film, hard disk-type magnetic recording medium I commonly employed in computer-related applications is schematically illustrated in
FIG. 1
in simplified cross-sectional view, and comprises a substantially rigid, non-magnetic metal substrate
10
, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited or otherwise formed on a surface
10
A thereof a plating layer
11
, such as of amorphous nickel-phosphorus (Ni—P); a seed layer
12
A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Ni—Al) or chromium-titanium (Cr—Ti) alloy; a polycrystalline underlayer
12
B, typically of Cr or a Cr-based alloy; a magnetic recording layer
13
, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer
14
, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer
15
, e.g., of a perfluoropolyether. Each of layers
11
-
14
may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and layer
15
is typically deposited by dipping or spraying.
In operation of medium
1
, the magnetic layer
13
is locally magnetized by a write transducer, or write “head”, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer
13
, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto 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 layer
13
can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (“SMNR”) 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.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, including that presented by the superparamagnetic limit, is to increase the crystalline anisotropy, thus the squareness of the magnetic bits, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (“AFC”) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability.
The strength of coupling can be described in terms of the total exchange energy. For a pair of ferromagnetic layers separated by a non-magnetic spacer layer, the total exchange energy generally results from RKKY-type interaction (i.e., oscillation from anti-ferromagnetic to ferromagnetic with increasing spacer film thickness), dipole-dipole interactions between grains of the ferromagnetic layers across the spacer layer (which favors anti-ferromagnetic alignment of adjacent grains across the spacer layer), and exchange interaction (which favors ferromagnetic alignment of the ferromagnetic layers). In AFC media the thickness of the spacer layer is chosen to maximize anti-ferromagnetic coupling between the ferromagnetic layers, i.e., to maximize the RKKY-type anti-ferromagnetic coupling and the dipole-dipole interactions. According to this approach, the total exchange energy between the ferromagnetic layer pairs is a key parameter in determining the increase in stability.
In general, the ferromagnetic recording layer in longitudinal recording media is comprised of weakly coupled single-domain grains. The magnetic energy of each single-domain grain in the absence of an applied field (i.e., zero applied field) is identical to KV, where K is the anisotropy constant and V is the volume of the grain. Due to the interactions between grains in the recording layer (i.e., primarily direct exchange and dipole-dipole interactions), the magnetic energy of the grains in the recording layer is modified, i.e., changed to an effective magnetic energy, E
Meff
.
In the simplest case, AFC media consist of two recording layers, i.e., a “main” layer (hereinafter layer “
1
”), and a “stabilization” layer (hereinafter layer “
2
”), which layers are anti-ferromagnetically coupled (AFC) across a thin, non-magnetic spacer layer. In general, the grains of the main and stabilization layers grow one above the other. Thus, an AFC grain may be defined as two adjacent grains grown one above the other, one grain being from the stabilization layer and one grain being from the stabilization layer, the two grains being anti-ferromagnetically coupled (AFC) across a non-magnetic spacer layer. During information storage in the media, the so-called “remanent state” of the media (wherein the external magnetic field is zero) is such that the magnetizations of the main (
1
) and stabilization (
2
) layers oppose each other, i.e., the magnetic moments of the pair of grains comprising each AFC grain (i.e., one grain being from the main layer and one grain being from the stabilization layer) are anti-parallel. If the external magnetic field is sufficiently large, the media is in a so-called “saturation state” and the magnetizations of the main (
1
) and stabilization (
2
) layers are parallel. In AFC media, in the absence of an external magnetic field, the total exchange energy favors the anti-ferromagnetic coupling (AFC) and is large enough such that the stabilization layer (
2
) can overcome the effective energy barrier and “flip” from the “saturation state” to a state in which its magnetic moment is anti-parallel to the magnetic moment of the main layer

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