1. Field of the Invention
The present invention relates to a magnetic recording medium known as a discrete track type.
2. Description of the Related Art
An example of recording mediums used for storage devices such as hard disk drives is a magnetic disk (a magnetic recording medium). A magnetic disk has a multilayer structure consisting of a disk substrate and a recording layer having a predetermined magnetic structure. The ongoing increase in amount of information that needs to be processed by a computer system is driving development of the magnetic disk for much higher recording density.
When recording information on the magnetic disk, a magnetic head for recording is disposed close to a recording surface (substantially a recording layer) of the magnetic disk, and the magnetic head applies to the recording layer a recording magnetic field stronger than the coercive force thereof. Sequentially reversing the direction of the recording magnetic field applied by the magnetic head while relatively moving the magnetic head with respect to the magnetic disk leads to formation of a plurality of recording marks (magnetic domains), alternately magnetized in opposite directions, aligned circumferentially of an information track or tracks in the recording layer of the disk. Controlling the timing to reverse the direction of the recording magnetic field during this process enables forming recording marks of predetermined lengths. Thus on the recording layer, signals or information is recorded based on the variation of the magnetizing direction.
In the field of the magnetic disk, a so-called discrete track (DT) type magnetic disk has been developed, for achieving higher recording density. DT type magnetic disks are described in Patent documents 1 to 3 below.                Patent document 1: JP-A-2005-71467        Patent document 2: JP-A-2005-166115        Patent document 3: JP-A-2005-293730        
FIGS. 31 and 32 depict a conventional DT type magnetic disk 90. FIG. 31 is a fragmentary plan view of the magnetic disk 90, and FIG. 32 is a cross-sectional view taken along the line XXXII-XXXII in FIG. 31.
As shown in FIG. 32, the magnetic disk 90 has a multilayer structure including a disk substrate 91, a recording layer 92, and a cover layer 93. The recording layer 92 includes, as shown in FIG. 31, a plurality of information tracks 92A extending circumferentially of the disk D or winding around the center of the disk, and non-magnetic regions 92B located between the information tracks. The information track 92A includes a track servo signal region TS and a user data region YD. The track servo signal region TS includes a plurality of magnetic regions 92a and a plurality of non-magnetic regions 92b, such that the mutually adjacent magnetic regions 92a are isolated by the non-magnetic region 92b. The magnetic regions 92a respectively have what is known as perpendicular magnetic anisotropy, and are magnetized in the same direction. The magnetic regions 92a and the non-magnetic regions 92b thus configured constitute track servo information. The user data region YD includes a magnetic region 92c, where user data is to be written. The magnetic region 92c, which is made of the same material as that of the magnetic region 92a, has perpendicular magnetic anisotropy, and is in so-called an “as-depo” state in which the region is randomly and generally uniformly magnetized in a vertical direction, before the user data is first written thereon.
When recording information on the DT type magnetic disk 90, the magnetic head applies the recording magnetic field, thus to create a plurality of recording marks (magnetic domains), alternately magnetized in opposite directions, serially aligned circumferentially of the disk recording layer 92 on the magnetic region 92c in the user data region YD in one of the information tracks 92A. During this process, since the information track 92A to which the magnetic field is sequentially applied for recording information and the adjacent information track 92A are isolated by the non-magnetic region 92B, a cross-write effect, which erases or degrades the recording mark in the adjacent information track 92A, can be prevented. The capability of preventing the cross-write effect is an advantageous feature of the magnetic disk in achieving a finer pitch of the tracks and higher recording density.
FIGS. 33(a) to 34(c) depict a manufacturing method of the magnetic disk 90. To manufacture the magnetic disk 90, firstly a magnetic film 94 is formed on the disk substrate 91 as shown in FIG. 33(a). Then as shown in FIG. 33(b), a resist pattern 95 is formed on the magnetic film 94. The resist pattern 95 is provided with openings 95a located according to the pattern of the non-magnetic region 92b of the recording layer 92. The resist pattern 95 also includes openings (not shown) located according to the pattern of the non-magnetic region 92B of the recording layer 92. Proceeding to FIG. 33(c), an etching process is performed utilizing the resist pattern 95 as the mask, to thereby delineate the pattern on the magnetic film 94. At this stage, the magnetic regions 92a, 92c in the recording layer 92 are formed. After removal of the resist pattern 95, a non-magnetic material 96 is loaded among the magnetic regions 92a, 92c, as shown in FIG. 33(d). This process forms the non-magnetic regions 92B, 92b in the recording layer 92.
Proceeding to FIG. 34(a), the cover layer 93 is formed on the recording layer 92. Then as shown in FIG. 34(b), a magnetic field is collectively applied to the entire recording layer 92, so as to magnetize the magnetic regions 92a, 92c in the same direction. In this process, the magnetic field Hr applied to the magnetic regions 92a, 92c is stronger than the coercive force thereof. Magnetizing thus the magnetic regions 92a in the same direction permits formation of the track servo information in the track servo signal region TS. Referring now to FIG. 34(c), an AC erasion process is performed to bring the magnetic region 92c in the user data region YD into the as-depo state. In the AC erasion process, a predetermined magnetic head is employed so as to form a high-frequency repeating pattern mark in the magnetic region 92c of the user data region YD, in each information track 92A. Through such steps, the magnetic disk 90 including the track servo information in the recording layer 92 can be obtained.
The manufacturing of the conventional magnetic disk 90, however, involves inefficient steps. The manufacturing process of the conventional magnetic disk 90 requires once magnetizing the magnetic region 92a of the recording layer 92 in the same direction, and then performing the AC erasion process to turn only the magnetic regions 92c into the as-depo state. The AC erasion process has to be performed for each single information track 92A. Accordingly, the AC erasion process takes a long time (often 10 minutes or more per disk) in turning the magnetic region 92c in the information track 92A, hence all the magnetic regions 92c of the recording layer 92, into the as-depo state. Such AC erasion process is undesirable for achieving higher manufacturing efficiency and lower manufacturing cost of the magnetic disk.