Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

1999-12-14

2003-04-29

Rickman, Holly (Department: 1773)

Stock material or miscellaneous articles

All metal or with adjacent metals

Having magnetic properties, or preformed fiber orientation...

C428S667000, C428S690000, C428S900000

Reexamination Certificate

active

06555248

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium and a production method thereof, and particularly, to a magnetic recording medium whose ferromagnetic metal layer has a high coercive force Hc, a high anisotropic magnetic field Hk
grain
, and a high normalized coercive force (Hc/Hk
grain
). 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 production method, the following is known.
FIG. 17
is a schematic view illustrating a hard disk as an example of a magnetic recording medium. In
FIG. 17
, FIG.
17
(
a
) is a perspective view of the whole magnetic recording medium, and FIG.
17
(
b
) is a cross section along the A-A′ line of FIG.
17
(
a
).
A substrate body
1
consists of an Al substrate
2
and a non-magnetic (Ni—P) layer
3
provided on a surface of the Al substrate
2
. On this substrate body, are laminated a Cr underlying layer
4
, a ferromagnetic metal layer
5
, and a protective layer
6
.
The non-magnetic (Ni—P) layer
3
is formed by plating or sputtering, on the surface of 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 is in the radial direction 5 nm to 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 to 15 &mgr;m for the non-magnetic (Ni—P) layer
3
, 50 nm to 150 nm for the Cr underlying layer
4
, 30 nm to 100 nm for the ferromagnetic metal layer
5
, and 20 nm to 50 nm 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 at 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, 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 a 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, 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 layer contains Ta element, a normalized coercive force (expressed as Hc/Hk
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 coercive force (Hc/Hk
grain
) of the ferromagnetic metal layer is a value obtained by dividing a coercive force Hc by an anisotropic magnetic field Hk
grain
of a crystal grain, and expresses degree of increase of magnetic isolation of the crystal grain. Namely, when normalized coercive force of a ferromagnetic metal layer is high, it means that magnetic interaction between respective crystal grains constituting the ferromagnetic metal layer decreases, and high coercive force can be realized.
Further, an international patent application PCT/JP94/01184 discloses a technique relating to a cheap high-density recording medium whose coercive force 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 underlying layer and utilizes magnetic reversal, Ar gas 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 wtppm or less. Further, it is also reported that, the coercive force 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 to 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 coercive force of a magnetic recording medium and its medium noise, and, in order to obtain a low noise medium, its normalized coercive force 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 recording medium and its manufacturing method, in which, when oxygen concentration of a ferromagnetic metal layer consisting of CoNiCr or CoCrPt is 100 wtppm or less, a grain boundary layer of amorphous structure can be formed between crystal grains constituting the ferromagnetic metal layer, and, as a result, a signal-to-noise ratio of electromagnetic transduction characteristics is high, and a stable coercive force can be obtained in mass production.
However, it is still obscure how various magnetic characteristics (coercive force: Hc, anisotropic magnetic field: Hk
grain
, and normalized coercive force: Hc/Hk
grain
) of a ferromagnetic metal layer or to composition distribution in a grain boundary layer of amorphous structure existing between the crystal grains constituting the ferromagnetic metal layer as high values in all the coercive force, anisotropic magnetic field and normalized coercive force, and which is adaptable to promotion of high recording density.
An object of the present invention is to provide a magnetic recording medium whose ferromagnetic metal layer has high coercive force, high anisotropic magnetic field and/or high normalized coercive force, so that it is adaptable to promotion of high recording density.
SUMMARY OF THE INVENTION
A magnetic recording medium of the present invention comprises a ferromagnetic metal layer that contains at least Co and Cr, and is formed on a base body via a metal underlying layer having Cr as its main component, and is characterized in that, the surface roughness of the base body is less than 1 nm when measured as a center line average height Ra, and, between crystal grains constituting the ferromagnetic metal layer, the magnetic recording medium has a first region in which Cr segregates, the first region penetrating the ferromagnetic metal layer, and that, in the first region, Cr concentration is lower in the neighborhood of the middle in the thicknesswise direction of the ferromagnetic metal layer than in the neighborhood of the surface and in the neighborhood of the metal underlying layer.
Film formation is performed under extra clean atmosphere to form a magnetic recording medium comprising a ferromagnetic metal layer which contains at least Co and Cr and is formed on a base body via a metal underlying layer containing Cr as its main component. Surfac

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