1. Field of the Invention
The present invention relates to magnetic recording media on which a large amount of information can be recorded, and in particular to magnetic recording media suitable for high density magnetic recording.
2. Description of the Related Art
As the information-oriented society advances, information content handled every day increases, and consequently a magnetic storage device having a larger capacity is in great demand. In order to cope with this, development of a magnetic head having high sensitivity and magnetic recording media with low noise is being conducted vigorously. In the longitudinal magnetic recording now in practical use, adjacent magnetizations recorded on the medium are opposed to each other. For increasing the linear recording density, therefore, it is necessary to increase the coercive force of the recording layer and decrease the film thickness. If the coercive force of the recording layer becomes large, however, a problem of insufficient writing capability of the recording head occurs. If the thickness of the recording layer becomes thin, a problem that recorded information is lost by thermal demagnetization occurs. Because of these problems, it has become difficult to improve the recording density by using the current longitudinal magnetic recording.
As means for overcoming these problems, the perpendicular magnetic recording is now being studied. In the perpendicular magnetic recording, adjacent magnetizations are not opposed to each other, and consequently the high density recording state is stable. Therefore, the perpendicular magnetic recording is considered to be a technique that is essentially suitable for high density recording. Furthermore, the recording efficiency can be increased by combining a single-pole-type magnetic recording head with a double-layer perpendicular magnetic recording medium having a soft magnetic underlayer. Thus it is considered that it is possible to cope with an increase in coercive force of the recording film. For implementing high density recording by using the perpendicular magnetic recording, it is necessary to develop a perpendicular magnetic recording medium having low noise and resistance to thermal demagnetization.
As for the recording layer of the perpendicular magnetic recording medium, a Co—Cr—Pt alloy film put into practical use in longitudinal magnetic recording media, and a super-lattice film formed by laminating a Co layer and a Pd layer having high magnetic anisotropy are being studied. For reducing noise of media using these recording layers and improving the thermal demagnetization characteristic, it is important to make crystal grains forming the recording layer fine and uniform and implement magnetic isolation of the crystal grains.
In the case of a longitudinal magnetic recording medium using a Co—Cr—Pt alloy film as the recording layer, a low noise characteristic is obtained by forming fine and magnetically isolated crystal grains by means of segregation to the grain boundary. In the case of a perpendicular magnetic recording medium, however, crystal grains of the recording layer have a structure in which c-axes of hexagonal closed packed structures are aligned vertical to the film plane, and adjacent crystal grains differs only slightly in crystal orientation in the in-plane direction. Therefore, crystal grains are united to each other and enlarged in the recording film forming process, or magnetic isolation of crystal grains becomes insufficient because of insufficient Cr segregation to the grain boundaries. It is thus difficult to obtain the low noise characteristic.
It is considered that it is effective in solving such problems to form an intermediate layer between a recording layer and a soft magnetic underlayer and control the growth of crystal grains in the recording layer by using the intermediate layer. As for studies concerning the intermediate layer of perpendicular magnetic recording media, a soft magnetic intermediate film of an island structure having a crystal plane of [111] is disclosed in, for example, JP-A-6-295431. Its object is to improve the orientation property of the recording layer by using the intermediate layer, and the magnetic isolation of crystal grains and the effect of noise reduction are not described therein. Use of an oxide layer having superfluous oxygen and poor crystallinity is disclosed in JP-A-7-73429. Its object is to effectively disconnect magnetic and crystal-structural relations between a soft magnetic unerlayer and a recording layer and prevent disappearance of magnetization of the recording layer and reduction of the coercive force. In addition, it is disclosed in JP-A-2001-23140 that a MgO film is suitable for an intermediate layer of a Co—Cr alloy recording layer. Its object is to increase perpendicular magnetic anisotropy energy of a recording magnetic film by controlling the orientation property of magnetic crystal grains.
In these studies mentioned above, magnetic isolation of crystal grains and the effect of noise reduction are not described therein. For obtaining a perpendicular magnetic recording medium on which high density recording can be conducted, a technique of an intermediate layer for controlling crystal grains of the recording layer and reducing noise is required.
On the other hand, in the case of a perpendicular magnetic recording medium using a super-lattice film formed by stacking a Co layer and a Pd layer as the recording layer, magnetic interaction between crystal grains of the recording layer is very strong, and consequently the transition noise is large and high density recording is impossible. In order to solve this problem, a technique of magnetically isolating crystal grains of the super-lattice film is being studied. It is disclosed in, for example, JP-A-2002-25032, that crystal grains of the recording layer are crystallographically isolated and magnetically isolated by making the recording film contain B and O. For controlling the orientation property and size of crystal grains, however, a new technique concerning the intermediate layer is required. As for the study concerning an intermediate layer or an underlayer, it is disclosed in, for example, JP-A-2001-155329, to use an underlayer composed of composite materials including metal, such as Pd or Pt, having a face-centered cubic structure and an oxide such as Al2O3 or MgO. Its object is to make crystal grains of the recording layer small, make magnetic interaction between grains weak, and reduce transition noise remarkably. For preventing the increase of crystal grain diameter of the recording layer, however, it is necessary to divide the recording layer by using division layers. Thus, it is considered that the control of size and orientation property of crystal grains is insufficient.
For obtaining a perpendicular magnetic recording medium on which high density recording can be conducted, a new technique concerning an intermediate layer for controlling crystal grains of the recording layer is required. As for a condition required of the intermediate layer of the double-layer perpendicular magnetic recording medium on which high density recording can be conducted, it is necessary not only to conduct structure control of the recording layer but also to make the film thickness small in order to prevent the degradation of the recording efficiency. In the case where the intermediate layer is thick, the distance between the magnetic pole of the recording head and the soft magnetic underlayer becomes long and it is expected that the recording efficiency falls and the recording resolution falls. Therefore, a technique of a thin intermediate layer capable of conducting structure control of the recording layer is required.
Judging the conventional technique concerning the intermediate layer of the perpendicular magnetic recording medium from the viewpoint of the grain control of the recording layer, making grains fine and uniform and magnetic isolation are not taken into consideration in the case where orientation property has been improved, and orientation property improvement and making grains fine and uniform are not taken into consideration in the case where magnetic isolation has been promoted. From the viewpoint of the thickness of the intermediate layer, making the film thickness thin is not sufficiently taken into consideration. For these reasons, there are problems of high medium noise, weakness for thermal fluctuation, low recording efficiency and low recording resolution. Because of these problems, it is impossible to fabricate a perpendicular magnetic recording medium suitable for high density recording.