The hard disc drive (HDD) which is one kind of a magnetic recording and reproducing device at present has the recording density thereof increase at an annual rate of 60%. It is said that this inclination will continue in the future. In consequence of this inclination, the development of a magnetic recording head and the development of a magnetic recording medium suitable for the high recording density have been being promoted.
The magnetic recording medium which is used in the hard disc drive has been required to follow this inclination toward higher recording density and consequently enhance the coercive force and enhance the signal/noise ratio (SNR) as well.
Concerning the magnetic recording medium to be used in the hard disc drive, the structure having metal films deposited by the sputtering technique on a substrate adapted for a magnetic recording medium is now prevailing. As the substrates suitable for the magnetic recording medium, the aluminum substrate and the glass substrate are being widely used. The aluminum substrate is what is obtained by forming a Ni—P-based alloy layer in a thickness of about 10 μm by electroless plating on a substrate of an Al—Mg alloy ground to specular finish and further grinding the surface of this layer to specular finish. The glass substrate is known in different kinds, such as an amorphous glass and a crystallized glass. The glass substrates of both kinds are ground to specular finish prior to use.
The magnetic recording media which are generally used at present for hard disc drives are in a structure having a nonmagnetic under layer (made of Cr, a Cr-based alloy, a Ni—Al-based alloy, for example), a nonmagnetic intermediate layer (made of a Co—Cr— or Co—Cr—Ta-based alloy, for example), a magnetic layer (made of a Co—Cr—Pt—Ta— or Co—Cr—Pt—B-based alloy, for example), and a protective film (made of carbon, for example) sequentially stacked on a nonmagnetic substrate and having a lubricating film made of a liquid lubricant formed thereon.
Several methods are available for the purpose of enhancing the coercive force of a magnetic recording medium. In the case of the magnetic recording medium of an alloy base having Co as a main component for the magnetic layer, for example, the method resorting to the addition of Pt proves effective. Numerous reports have been already published concerning this method. Also, the use of a CrMn-based alloy as a nonmagnetic under layer has been proposed (refer to U.S. Pat. No. 5,993,956).
As regards the magnetostatic property of the magnetic recording medium, the method of imparting the magnetic anisotropy having an axis of easy magnetization in the circumferential direction and heightening this property is effective as well as the method of enhancing the coercive force in enhancing recording properties and homogenizing the characteristic properties of the medium. At present, therefore, it is generally known that the magnetic recording medium using a substrate resulting from plating an aluminum alloy with a Ni—P-based alloy layer (otherwise called an aluminum substrate) is enabled, by undergoing the texture processing for mechanically inscribing a fine groove in the circumferential direction on the Ni—P-based alloy surface, to manifest the magnetic anisotropy having an axis of easy magnetization in the circumferential direction (refer to IEEE Trans on Mag. Vol. Mag-22, No. 5 (1986), 379 and Journal of Japan Applied Magnetism Society, Vol. 17, No. 5 (1993) 784).
As a nonmagnetic substrate, the glass substrate, for example, possesses rigidity excelling in impact resistance and possesses excellent flatness as well and, therefore, may well be rated as a nonmagnetic substrate appropriate for high recording density. When the magnetic anisotropy in the circumferential direction can be imparted to the magnetic recording medium using glass for the nonmagnetic substrate, it can be expected to acquire outstanding recording and reproducing properties.
Several methods which form texture streaks on a glass substrate by subjecting this substrate to the texture processing have been known. For the purpose of forming fine and uniform texture streaks, the idea of using a woven fabric tape comprising an abrasive grain suspension containing a solution possessing a hydroxyl group and plastic fibers has been proposed (refer to Japanese Patent No. 3117438).
Further, for the purpose of forming fine and uniform texture streaks, the idea of using diamond abrasive grains and CeO2 abrasive grains together has been proposed.
It is, however, difficult to impart a fully satisfactory magnetic anisotropy in the circumferential direction to the glass substrate by merely forming the texture streaks therein. Thus, for the purpose of imparting the magnetic anisotropy in the circumferential direction to the glass substrate having a linear texture formed on the first surface thereof, the idea of forming a pre-coated layer by the sputtering technique (refer to JP-A HEI 4-205916), the idea of forming an amorphous layer containing at least Ni and P (refer to JP-A 2001-209927), and the idea of forming an orientation adjusting layer of a Co—W-based alloy or a Co—Mo-based alloy (refer to JP-A 2004-86936) have been proposed.
As described above, several methods for imparting the magnetic anisotropy to a glass substrate having texture streaks formed thereon have been already proposed. Generally, however, the degree of anisotropy is small as compared with the case of using an aluminum substrate. Not only when the glass substrate is used but also when the aluminum substrate is used, the anisotropy is required to be further enhanced.
This invention has been proposed in view of the state of affairs mentioned above and has for an object thereof the provision of a magnetic recording medium and a magnetic recording and reproducing device which abound in magnetic anisotropy suitable for high density recording and excel in recording properties.