Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-11-26
2004-06-15
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
69, 69
Reexamination Certificate
active
06749956
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium, and, in particular, to a magnetic recording medium formed under an ultra clean atmosphere, i.e., a high density magnetic recording medium utilizing magnetization reversal and having a ferromagnetic metal layer whose oxygen concentration is 100 wt ppm or less, wherein normalized coercivity (Hc/H
k
grain
: Hc=coercivity, H
k
grain
=anisotropic magnetic field) is high, and is superior in a medium S/N at the time of recording and reproduction. The magnetic recording medium of the present invention can be suitably applied to a hard disk, a floppy disk, a magnetic tape, and the like.
2. Description of the Related Art
As the conventional magnetic recording medium and its manufacturing method, is known the following technique.
FIG. 8
is a schematic view illustrating a hard disk as an example of a magnetic recording medium. In FIG.
8
(
a
) is a perspective view of the whole magnetic recording medium, and FIG.
8
(
b
) is a cross section along the line A-A′ in FIG.
8
(
a
).
As a substrate body
1
, is used one in which a non-magnetic (Ni—P) layer 3 is provided on a surface of an Al substrate
2
. On this substrate body
1
, are laminated a Cr underlying layer
4
, a ferromagnetic metal layer
4
, a ferromagnetic metal layer
4
, and a protective layer
6
.
The non-magnetic (Ni—P) layer
3
is formed by planting or sputtering, on the disk-shaped Al substrate
2
of 89 mm(3.5 inches) in diameter and 1.27 mm (50 mil) in thickness, to form the substrate body
1
. Further, on the surface of the non-magnetic (Ni—P) layer
3
, are provided concentric scratches (hereinafter, referred to as texture) by a mechanical grinding process. Generally, surface roughness of the non-magnetic (Ni—P) layer
3
, i.e., a center line average height Ra measured in the radial direction is 5 nm-15 nm. Further, the Cr underlying layer
4
and the ferromagnetic metal layer
5
(generally, a magnetic film of Co alloy family) are formed on the surface of the above-mentioned substrate body
1
by sputtering, and, lastly, the protective layer
6
comprising carbon and the like is formed by sputtering to protect the surface of the ferromagnetic metal layer
5
. Typical thicknesses of respective layers are 5 &mgr;m-15 &mgr;m for the non-magnetic (Ni—P) layer
3
, 50 nm-150 nm for the Cr underlying layer
4
, 30 nm-100 nm for the ferromagnetic layer
5
, and 20 nm-50nm for the protective layer
6
.
The conventional magnetic recording medium having the above-described layer structure has been manufactured under the condition that back pressure of the deposition chamber is a the level of 10−7 Torr before the sputter deposition and impurity concentration of Ar gas used for film formation is 1 ppm or more.
In the magnetic recording medium obtained by the above-described manufacturing method, and particularly in the case of a ferromagnetic metal layer
5
containing Ta element (for example, a CoCrTa alloy magnetic film), it is reported by Nakai et al. that, between crystal grains forming the ferromagnetic metal layer, exists a grain boundary layer of amorphous structure, and that this grain boundary layer has non-magnetic alloy composition (J. Nakai, E. Kusumoto, M. Kuwabara, T. Miyamoto, M. R. Visokay, K. Yoshikawa and K. Itayama, “Relation Between Microstructure of Grain Boundary and the Integranular Exchange in CoCrTa Thin Film for Longitudinal Recording Media:, IEEE Trans. Magn., vol. 30, No. 6, pp. 3969, 1994). However, in the case of a ferromagnetic metal layer that does not contain Ta element (for example, a CoNiCr or CoCrPt alloy magnetic film), the above-mentioned grain boundary layer has not been found. Further, the above report describes that, when a ferromagnetic metal layer contains Ta element, a normalized coercivity (expressed as Hc/H
k
grain
) of the magnetic recording medium is as large as 0.3 or more, and when a ferromagnetic metal layer does not contain Ta element, its value is less than 0.3.
The above-mentioned normalized coercivity (Hc/H
k
grain
) of the ferromagnetic metal layer is a value obtained by dividing a coercivity Hc by an anisotropic magnetic field H
k
grain
of a crystal grain, and express a degree of increase of magnetic isolation of the crystal grain. Namely, when normalized coercivity of a ferromagnetic metal layer is high, it means the magnetic interaction between respective crystal grains constituting the ferromagnetic metal layer decreases, and high coercivity can be realized.
Further, an international patent application PCT/JP94/01184 discloses a technique relating to a cheap high-density recording medium whose coercivity is increased without using an expensive ferromagnetic metal layer, and its manufacturing method. According to PCT/JP94/01184, regarding a magnetic recording medium which has a ferromagnetic metal layer formed on a surface of a substrate body via a metal underlaying layer and utilizes magnetization reversal, Ar has whose impurity concentration is 10 ppb or less is used for film formation, so that oxygen concentration of the metal underlying layer and/or ferromagnetic metal layer is made 100 wt ppm or less. Further, it is also reported that, the coercivity is further increased when Ar gas of 10 ppb or less impurity concentration is used in a cleaning process by high frequency sputtering on the surface of the above-mentioned substrate body to remove the surface of the substrate body by 0.2 nm-1 nm, before forming the above-mentioned metal underlying layer. Further, in this report, it is described that there is a correlation between a normalized coercivity of a magnetic recording medium and its medium noise, and, in order to obtain a low noise medium, its normalized coercivity should be more than or equal to 0.3 and less than 0.5.
Further, an international patent application PCT/JP95/00380 discloses a magnetic recoding medium and its manufacturing method, in which, when oxygen concentration of a ferromagnetic metal layer consisting of CoNiCr or CoCrPt is 100 wt ppm or less, a grain boundary layer of amorphous structure can be formed between crystal grains constituting the ferromagnetic metal layer, and, as a result, an S/N of electromagnetic transduction characteristics is high, and a stable coercivity can be obtained in mass production.
However, in the magnetic recording medium formed under the ultra clean atmosphere, i.e., the magnetic recording medium utilizing magnetization reversal and having a ferromagnetic metal layer whose oxygen concentration is 100 wt ppm or less, it is still obscure what are conditions in which a high normalized coercivity (Hc/H
k
grain
) of, for example, 0.3 or more is obtained stably without depending on the crystal grain diameter of a ferromagnetic metal layer, and conditions of grain diameters of the crystal grains constituting the ferromagnetic metal layer for realizing a high medium S/N. It has been desired to elucidate these relations, in order to develop a magnetic recording medium adaptable to promotion of high recording density.
An object of the present invention is to provide a magnetic recording medium adaptable to promotion of high recording density, having a high normalized coercivity of the ferromagnetic metal layer and, at the same time, a superior medium S/N.
SUMMARY OF THE INVENTION
The present invention provides a magnetic recording medium utilizing magnetization reversal, and comprises a Co-based ferromagnetic metal layer formed on a base body via a metal underlying layer with surface roughness of the base body being less than 1 nm in terms of a center line average height Ra, and oxygen concentration of said metal underlying layer and said ferromagnetic layer being 10 wt ppm or less.
When saturation magnetization of the ferromagnetic metal layer is expressed by Ms, and an anisotropic magnetic field of individual crystal grains constituting the ferromagnetic metal layer is expressed by H
k
grain
, then 4ΠMs/H
k
grain
is 1 or less.
In the magnetic recording medium of the present inv
Kikuchi Akira
Takahashi Migaku
Knuth Randall J.
Migaku Takahashi
Rickman Holly
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