Method of manufacturing magnetic recording medium

The method of manufacturing a magnetic recording medium according to the present invention comprises a first step in which a ferromagnetic metal layer of a predetermined thickness is formed on a substrate, and a second step contiguous to said first step and in which another ferromagnetic metal layer is formed by applying a bias voltage to the substrate. Since the ferromagnetic metal layer formed at the second step is excellent in corrosion resistance, even if a protective layer such as a carbon layer incurs a pit defect when the medium is being produced or even when the protective layer is separated, the ferromagnetic metal layer in consideration will not be corroded, thus preventing any error from occurring in data recording or reproduction.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a method of manufacturing a magnetic 
recording medium and, more specifically, to a method of manufacturing a 
magnetic recording medium which is optimally employable for the 
improvement of the corrosion resistance of a magnetic recording medium 
made of a ferromagnetic metal layer. 
(b) Prior Art Statement 
For improving the recording density of conventional magnetic recording 
media, great efforts have been made to improve the properties of the media 
themselves and to reduce the thickness thereof. Similarly for the improved 
recording density of the magnetic recording media, it has been proposed to 
reduce the spacing between the magnetic recording medium and 
recording/reproduction head. Among others, a magnetic recording medium is 
known which uses a ferromagnetic metal layer formed by sputtering and 
evaporation and which has excellent properties and reduced thickness. For 
maintaining the performance as a recording medium, however, these magnetic 
recording media need a protective layer, lubricating layer or both to 
improve the corrosion resistance and sliding resistance. 
The aforementioned magnetic recording media having protective and 
lubricating layers for improved reliability are disclosed in, for exapple, 
Japanese Unexamined Patent Publication Nos. 57-18028, 56-114131 and 
57-138054. 
As described in the foregoing, the conventional magnetic recording media 
require a protective layer to improve the corrosion resistance of the 
ferromagnetic metal layer; in the conventional techniques of manufacturing 
the magneiic recording media, however, no consideration has been given to 
the adhesion between the ferromagnetic metal layer and protective layer 
and the consistency of the manufacturing process. More particularly, since 
the protective layer is generally formed with quite a different material 
from that of the ferromagnetic metal layer, the adhesion between them is 
insufficient so that the protective layer is apt to be separated as the 
case may be. Thus, the ferromagnetic metal layer is corroded so that the 
performance of the magnetic recording medium cannot be maintained and also 
its reliability is low. 
Furthermore, since the ferromagnetic metal layer and protective layer are 
formed in different processes, respectively, no consistency can be ensured 
between the forming process and equipment. Thus, the forming process and 
equipment are complicated, resulting in a reduced manufacturing 
efficiency. 
SUMMARY OF THE INVENTION 
The present invention seeks to overcome the above-mentioned drawbacks of 
the conventional techniques by providing a method of manufacturing a 
highly corrosion-resistant magnetic recording medium by means of a 
simplified forming process and equipment. 
According to another aspect of the present invention, a method of 
manufacturing a magnetic recording medium, whereby a part of the 
ferromagnetic metal layer as an information carrying surface can be formed 
as a protective layer superior in corrosion reiistance. 
The above objects can be attained by providing a method of manufacturing a 
magnetic recording medium, comprising a step of forming a ferromagnetic 
metal layer on a substrate by sputtering, evaporation or the like, more 
particularly, forming a ferromagnetic metal layer to a predetermined 
thickness on the substrate and then forming another ferromagnetic metal 
layer by applying a bias voltage to the substrate. 
According to the present invention, in case a ferromagnetic metal layer is 
formed by sptttering, evaporation or the like, it is made to a necessary 
thickness for a recording medium and then another ferromagnetic metal film 
is formed following the surfacial layer by applying a bias voltage to the 
substrate. 
Since during the formation of a ferromagnetic metal layer by applying the 
bias voltage to the substrate, various impacts are applied to the layer 
surface, the layer thus formed is a fine one denatured to some extent as 
compared with a ferromagnetic metal layer formed without applying any bias 
voltage. The experiments proved that the layer formed by applying a bias 
voltage is very difficult to be corroded. Namely, this film provides an 
effective corrosion-resistant protective layer. 
In the recording medium formed by the method according to the present 
invention, the ferromagnetic recording medium itself also functions as a 
protective layer, and since both the ferromagnetic metal layers are 
continuously formed without any time interval from one to another in the 
same equipment, there occurs no reduction of the adhesion between the 
layers due to any staining on the layers or to creation of any boundary 
between the layers. 
These and other objects and advantages of the present invention will be 
better understood from the ensuing description made by way of example of 
the embodiment according to the present invention with reference to the 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a sectional view showing an example of a magnetic disk 
manufactured by the method of manufacturing a magnetic recording medium 
according to the present invention. In this Figure, the reference numeral 
1 indicates an aluminum-alloy substrate, 2 a NiP-plated primary layer, 3 a 
Cr-sputtered primary layer, 4 and 5 ferromagnetic metal layers, 
respectively, formed by the method according to the present invention. The 
ferromagnetic metal layer 5 is superior in corrosion resistance as will be 
described later. The numeral 6 indicates a carbon-sputtered layer which 
improves the resistance and lubrication of the magnetic disk. 
The method of manufacturing a magnetic disk of which the construction is 
shown in FIG. 1 will be explained with reference to FIG. 2. The process of 
applying the NiP-plated primary layer 2 onto the aluminum-alloy substrate 
1 and further the Cr-sputtered primary layer 3 onto the NiP-plated primary 
layer 2 is similar to that of the conventional techniques. 
The ferromagnetic metal layers 4 and 5 are applied onto the Cr-sputtered 
primary layer 3 in the following process. That is, the ferromagnetic metal 
layers 4 and 5 are formed on the Cr-sputtered primary aayer 3 by 
sputtering basically a Co alloy such as Co-Ni, Co-Ni-Cr or the like. This 
process will be described in more detail below. There are disposed within 
a chamber 10 a Co-alloy target 12 and a disk 14 on the alluminum-alloy 
substrate 1 of which the NiP-plated primary layer 2 and Cr-sputtered 
primary layer 3 are formed, the target 12 and disk 14 being separated 70 
mm from each other. The target 12 is 8 inches in diameter, while the 
diameter of the disk 14 is 5 inches. The sputtering system adopted in this 
embodiment is one using a planar magnetron. In an actual system, a 
sputtering coil (not shown) is disposed below the target 12. The chamber 
10 is supplied with an Ar gas from the direction of arrow A, and exhausted 
in the direction of arrow B. Thus, the interior of the chamber 10 is kept 
at a vacuum of 2.times.10.sup.-2 Torr. The target 12 is connected to a DC 
power supply and applied with a negative DC voltage. The disk 14 is 
arranged by means of a holder (not shown) inside the chamber 10, and the 
aluminum-alloy substrate 1 is connected to a high frequency power supply 
18 of 13.56 kHz in power frequency and thus applied with a high-frequency 
power. 
The sputtering is started first with the DC power supply 16 electrically 
connected to the target 12. In this embodiment, the target 12 is applied 
with a DC power of 1.2 kW, and the sputtering is done for about 30 sec to 
form a ferromagnetic metal layer 4 of approximately 600 .ANG.. 
Furthermore, a high-frequency power of 200W is applied to the 
aluminum-alloy substrate 1 of the disk 14 for about 10 sec with the DC 
power supply 16 electrically connected to the target 12, whereby a 
ferromagnetic metal layer 5 of about 50 .ANG. is formed on the 
above-mentioned ferromagnetic metal layer 4. The experiments showed that 
the ferromagnetic metal layer 5 formed while applying a high-frequency 
bias voltage to the disk 14 is superior in corrosion resistance, and that 
the ferromagnetic metal layer 4 formed with no high-frequency bias voltage 
applied to the disk 14 is dissolved in an HNO.sub.3 solution of 25% in 
volume while the layer 5 cannot be dissolved in the solution after being 
dipped for 10 min. 
A carbon layer indicated at the numeral 6 is formed on the ferromagnetic 
metal layer 5 by sputtering using another carbon target as target 12 after 
stopping the supply of the high-frequency power to the aluminum-alloy 
substrate 1. 
Since the surface of the ferromagnetic metal layer as the recording medium 
of the magnetic disk formed by the method according to the present 
invention is excellent in corrosion resistance, even if a pit defect is 
created when the carbon-sputtered layer 6 is formed, the ferromagnetic 
metal layer 5 will not be corroded when in contact with H.sub.2 O or 
Cl.sub.2 in the atmosphere. Therefore, no error will be caused in 
recording/reproduction of data by the corrosion of the recording medium. 
Even if the carbon-sputtered layer 6 is ground or separated due to the 
contact of the magnetic head with the carbon-sputtered layer 6, the 
ferromagnetic metal layer 5 will act as a corrosion-resistant protective 
layer, so that no error will occur in recording or reproduction of any 
data. In addition, since the ferromagnetic metal layers 4 and 5 are formed 
continuously with the same material, there exists no inter-layer boundary 
between the layers (in the illustration, the boundary is shown for the 
convenience of explanation), and they are bonded to each other with a 
strength as in metal bonding. Therefore, these layers will not be 
separated from each other even if they are in contact with the magnetic 
head. 
The magnetic disk manufactured by the method according to the present 
invention is highly reliable in that no error will occur in data recording 
or reproduction even when a corrosion is found in the ferromagnetic metal 
layer. 
In the embodiment described in the foregoing, a high-frequency power is 
applied to the aluminum-alloy substrate 1. However, the present invention 
is not limited only to this embodiment, but it is possible to form a fine, 
highly corrosion-resistant ferromagnetic layer by controlling the metal 
molecules flying from the target through application of a bias voltage 
like a negative voltage or the like.