Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

1999-03-25

2003-03-18

Rickman, Holly C. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S690000, C428S336000, C428S900000

Reexamination Certificate

active

06534203

ABSTRACT:

RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P10-090243 filed Apr. 2, 1998 which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic recording medium having a ferromagnetic recording layer on its substrate. More particularly, it relates to a magnetic recording medium, comprised of a ferromagnetic recording layer and an anti-ferromagnetic recording layer, layered together, to enable recording to a higher recording density.
2. Description of the Related Art
As a recording medium for e.g., a computer, a random-accessible magnetic disc is extensively used. Of these magnetic discs, a magnetic disc employing a substrate of a hard material, such as a glass plate, plastic plate, Ni—P plated Al alloy plate or an almite-processed Al alloy plate, or a so-called hard disc, is now in widespread use because it is superior in response characteristics and suited for high-density recording. In this magnetic disc, at least a recording layer of a ferromagnetic material is formed on a major surface of the substrate.
Meanwhile, for the magnetic recording medium, such as the above-mentioned hard disc, a higher recording density is required, such that there is raised a demand for a reduced thickness of the recording layer and for finer magnetically independent particles of the ferromagnetic material of the recording layer.
That is, from the viewpoint of reducing the counter magnetic field from the recording medium, summed to the magnetic field of the magnetic head during recording, a thinner thickness of the recording layer is desirable. Also, in longitudinal recording in which the direction of magnetization is along the length in the medium plane, a thinner thickness of the recording layer is similarly desirable for alleviating the phenomenon of demagnetization due to the same N or S poles of the recording pits facing each other. Also, in order for a transition area between recording areas of different directions of magnetization not to serve as a magnetic wall presenting moderate change in the direction of magnetization, it is desirable for the particles of the ferromagnetic material to be fine and free from magnetic coupling therebetween.
However, if the grain size of the ferromagnetic material is finer, it becomes impossible for a magnetic recording medium to maintain the recording in stability for prolonged time. The reason is that, if the ferromagnetic grains are comminuted, the energy of magnetic anisotropy for the entire volume of a sole ferromagnetic particle is reduced so that the ‘potential mountain’ which stabilizes the direction of magnetization can be easily traversed by the magnetization vector due to thermal fluctuations. The phenomenon in which comminuted magnetic particles cannot maintain the constant direction of magnetization due to outstanding thermal fluctuation is termed super-paramagnetism, while the limit of the magnetic recording density thus imposed is termed the limit of the super-paramagnetism.
For achieving high recording density, it is necessary to thrust this limit of the super-paramagnetism towards the high recording density side. To this end, the energy of magnetic anisotropy needs to be increased. Thus, such a material having a high energy of magnetic anisotropy, specifically, such a material presenting various sorts of high anisotropic ions, crystal structure, crystal orientation, crystal grain boundary or precipitates, either alone or in combination, are being developed.
However, if the usefulness of the magnetic material constituting the recording layer of the magnetic recording medium is taken into consideration, not only the energy of magnetic anisotropy but also various other properties, such as corrosion resistance, smoothness, abrasion resistance or ease of manufacture, are required of the magnetic material. As for ease of manufacture, it is required that the magnetic material can be manufactured at a lower process temperature. It is extremely difficult to satisfy the energy of magnetic anisotropy and the above-mentioned various other properties simultaneously and satisfactorily.
An instance of a magnetization curve of the magnetic recording medium is shown in
FIG. 1
, in which the abscissa and the ordinate denote an external magnetic field and the intensity of magnetization, respectively. In such magnetic recording medium, the coercivity is not larger as a principle than one-half the anisotropic magnetic field. In the instance of
FIG. 1
, in which the anisotropic magnetic field is 5 kOe, the coercivity cannot exceed 2.5 kOe.
Since the upper limit of the coercivity is regulated by the magnitude of the anisotropic magnetic field, it is necessary to increase the energy of magnetic anisotropy of the magnetic material and the anisotropic magnetic field, if it is desired to increase the coercivity to assure recording stability of the magnetic recording medium.
As a method for increasing the energy of magnetic anisotropy of the particles of the magnetic material to increase the coercivity, there may be recited a method known as ‘pinning by a different sort of a magnetic material’. Here, a structure comprised of two magnetic layers of different sorts is taken as an example for illustration. That is, an illustrative structure is now explained, in which a pinning layer
102
, as a thin film of a first ferromagnetic material, is formed on a major surface
101
a
of a substrate
101
, and a recording layer
103
, as a second thin film of a ferromagnetic material, is formed thereon, as shown in FIG.
2
.
It is assumed that the pinning layer
102
meets two requirements, that is, it is high in magnetic anisotropy and low in saturation magnetic flux density in comparison with the recording layer
103
.
When two sorts of the magnetic material are in contact with each other, the magnetic moments of the two tend to be parallel or anti-parallel. Thus, the magnetization of these two layers tend to be rotated in unison on both sides of the interface between the pinning layer
102
and the recording layer
103
.
If it is presupposed that the magnetization of the two materials is turned in unison parallel to each other, the coercivity is derived in the following manner:
It is assumed that the recording layer
103
has a thickness t
1
[m], saturation magnetization ms
1
[T] and uniaxial magnetic anisotropy ku
1
[J/m
3
], the pinning layer
102
has a thickness t
2
[m],saturation magnetization Ms
2
[T] and a uniaxial magnetic anisotropy ku
2
[J/m
3
] and that the easy axes of the two materials run parallel to each other. At this time, the average saturation magnetization ms [T] and the uniaxial magnetic anisotropy ku
1
J/m
3
of the compound system may be found from the following equations 1 and 2:
ms=(
t
1
·ms
1
+t
2
·ms
2
)/(
t
1
+t
2
)  (1)
and
ku=(
t
1
·ku
1
+t
2
·ku
2
)/(
t
1
+t
2
)  (2).
Since the coercivity hc is expressed as approximately one-half of the anisotropic magnetic field hk=2 ku/ms, it is expressed in this compound system by the following equation (3):
hc=(
t
1
·ku
1
+t
2
·ku
2
)/(
t
1
·ms
1
+t
2
·ms
2
)  (3)
If now the pinning layer
102
meets the aforementioned two conditions, that it is high in magnetic anisotropy and low in saturation magnetization flux density in comparison with the recording layer
103
, the magnetic anisotropy ku of the present compound system is higher than the magnetic anisotropy ku
1
of a system comprised only of the ferromagnetic layer, while its coercivity is higher than ku
1
/ms
1
of a system comprised only of the recording layer
103
. Therefore, the compound system has a stable direction of magnetization of the recording than the system comprised only of the recording layer
103
.
As a method for increasing coercivity by increasing the energy of magnetic anisotropy of the particles of the magnetic mate

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