The present invention relates to a magnetic recording medium.  More particularly, the present invention relates to a magnetic recording medium which is particularly used in a magnetic disc apparatus equipped with a flying type magnetic head, which is excellent in a characteristic of friction sliding with a magnetic head, which can improve a reproducing output and a signal-to-noise (S/N) ratio by keeping the low noise of the magnetic recording medium without any change, and which hardly produces bit errors.  The present invention also relates to a magnetic disc apparatus in which such a magnetic recording medium is used and which records and reproduces information. 
High-density assembly of a magnetic disc apparatus used as a computer external memory has come to be highly required with the development of information processing technologies.  In order to meet the requirements, as is well known, a CSS (contact-stop-and-start) system wherein a magnetic head is contacted with a magnetic recording medium when the magnetic recording medium stops, the magnetic head slides on the medium in the initial stage of starting, the magnetic head then departs from the medium and floats, and the head then flies while maintaining a constant flying distance, has widely been adopted in currently used magnetic disc apparatuses. 
Magnetic disc apparatuses based on the CSS system have been variously improved.  Specifically, in a reproducing head part of a magnetic disc apparatus, a magnetoresistance effect type head in which a magnetoresistance element changing its resistance in accordance with the intensity of the magnetic field is used, namely, an MR head has been used, in place of a conventional winding type inductive thin film magnetic head.  The MR head uses the magnetoresistance effect, that is a change in the resistance of the magnetic material caused by application of an external magnetic field, in the reproduction of signals on a recording medium, and is characterized by the following advantages: a reproducing output amplitude several times as much as that of a conventional inductive thin film magnetic head can be obtained; the inductance is small; and a large S/N ratio can be expected.  Moreover, in addition to the MR head, users have begun to use an AMR head utilizing an anisotropic magnetoresistance effect, a GMR head utilizing a giant magnetoresistance effect and a spin bubble type GMR head that is a practical type of GMR head.  Note that these magnetoresistance effect type heads are generically referred to as xe2x80x9cMR heads, etc.xe2x80x9d in the present specification. 
Furthermore, a magnetic recording medium to be used in the magnetic disc apparatus is also required to have improved characteristics corresponding to those of the MR head, etc.  That is, a magnetic recording medium is required to have an excellent CSS resistance, have a high coercive force (Hc) and have a low noise. 
A conventional magnetic recording medium usually has a layer construction as shown by the cross-sectional view in FIG.  1.  That is, a magnetic recording medium 110 comprises a nonmagnetic substrate 101 of aluminum or its alloy, having applied thereon a magnetic recording layer 105 of an alloy that contains cobalt as its major component.  Moreover, a nonmagnetic underlayer 104 of chromium or its alloy is sandwiched between the substrate 101 and the magnetic recording layer 105.  The nonmagnetic underlayer 104 is aimed at making the direction of easy magnetization of the magnetic recording layer locate in a film plane of the magnetic recording layer.  Furthermore, when the substrate 101 is composed of aluminum or its alloy, an electroless Nixe2x80x94P plating layer 103 is applied to improve the adhesion of the Cr-based underlayer 104 to the substrate 101.  A protective layer 106 of carbon or the like is formed on the surface of the magnetic recording layer 105 to protect the medium 110.  A lubricating layer (not shown) is generally applied to the protective layer 106. 
As explained above, an electroless Nixe2x80x94P plating layer is ordinarily formed on the substrate of aluminum or its alloy in a magnetic recording medium.  The Nixe2x80x94P plating layer is formed to lower the friction coefficient between the magnetic head and the magnetic recording medium in the magnetic disc apparatus in which the CSS system is adopted.  Stripe-like grooves composed of protruded portions and recessed portions are formed on the surface of the Nixe2x80x94P plating layer on the substrate in the circumferential direction.  That is, as schematically shown in FIG.  2, grooves formed of fine protruded portions 3a and fine recessed portions 3b having shapes different from each other are formed on the surface of the electroless Nixe2x80x94P plating layer 103 on the substrate 101.  The treatment for forming the recessed and protruded portions is generally referred to as xe2x80x9ctexture processingxe2x80x9d or xe2x80x9ctexturingxe2x80x9d.  Texture processing is conducted by pressing grinding abrasive grains or the like to a surface of the rotating substrate to mechanically produce grooves in the circumferential direction or by similar procedures.  Moreover, as is well known, the treatment is also aimed at further directing the direction of easy magnetization in the magnetic recording layer to the circumferential direction so that the treatment contributes to the improvement of a S/N ratio. 
As of now, the most commonly employed method as the texturing processing is the mechanical texture processing wherein, as explained above, grinding abrasives or the like are pressed to an aluminum substrate plated with Nixe2x80x94P or others to form grooves.  However, since the aluminum used as a substrate and the Nixe2x80x94P plating layer formed thereon to improve the CSS resistance are relatively soft materials, deep grooves of about 15 to 20 nm depth may be partially formed when grinding abrasive grains or the like are pressed in the processing method.  Since a signal output is decreased in such deep groove portions, bit errors are produced. 
Recently, since magnetic disc apparatuses have been used for portable applications, glass or ceramic substrates have been used in place of aluminum substrates or the like in order to increase the impact resistance.  However, the conventional mechanical texture processing causes the following problems: when a material having no brittleness and having a high hardness such as glass or ceramics is used, accurate and controlled formation of grooves in the circumferential direction cannot be performed; moreover, the processing deteriorates the anisotropy of the axis of easy magnetization in the circumferential direction to lower the S/N ratio.  Methods of forming recesses and protrusions without using the mechanical texture processing include a method of chemically etching the surface of a glass substrate, as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos.  60-136035, 63-225919, etc.  However, even this chemical processing method cannot make the magnetic material have anisotropic properties of the axis of easy magnetization in the circumferential direction, and thus there still remains the problem that the S/N ratio is lowered. 
An object of the present invention is to solve the above-described prior art problems, thus providing a magnetic recording medium capable of conducting high density recording, in which various nonmagnetic materials including glass and ceramics which are particularly excellent in impact resistance, can be used as a substrate, on which fine recessed and protruded grooves having a high friction sliding characteristic can be formed while the anisotropy of an axis of easy magnetization in the circumferential direction, leading to a low noise and a high S/N ratio, is maintained, and on which formation of deep grooves reaching the substrate surface can be prevented during the formation of the grooves, thereby enabling removal of bit errors. 
Another object of the present invention is to provide a magnetic recording medium which can be particularly advantageously used in a magnetic disc apparatus equipped with a flying-type magnetic head. 
Furthermore, another object of the present invention is to provide a magnetic disc apparatus in which the magnetic recording medium according to the present invention is used. 
The above objects and other objects of the present invention will be easily understood from the following detailed description of the present invention. 
As a result of intensively carrying out investigations to achieve the above objects, the present inventors have found that when fine recessed and protruded grooves are to be formed on a nonmagnetic substrate having a smooth surface, if a relatively hard reinforcing coat layer (also referred to as xe2x80x9ca precoat layerxe2x80x9d in the present specification) is inserted between the substrate and a soft, recesses-and protrusions-forming layer on the substrate, it becomes possible to provide a magnetic recording medium on which fine recessed and protruded grooves for improving the friction sliding characteristic between a magnetic head and the medium can be formed while the anisotropic properties of the axis of easy magnetization in the circumferential direction are being kept; moreover, formation of deep grooves is suppressed so that generation of bit errors can be prevented. 
The present invention provides, in one aspect thereof, a magnetic recording medium comprising a nonmagnetic substrate having applied thereon, through a nonmagnetic underlayer, a magnetic recording layer composed of a magnetic metal material, characterized in that the nonmagnetic substrate comprises a substrate having a smooth surface with an average surface roughness Ra of not more than 20 xc3x85, a recesses-and protrusions-forming layer having stripe-like protrusions and recesses in the circumferential direction, formed on the substrate, and a reinforcing coat layer having a hardness higher than that of the recesses-and protrusions-forming film is sandwiched between the substrate and the recesses-and protrusions-forming layer. 
It is particularly preferred to use the magnetic recording medium of the present invention in a magnetic disc apparatus equipped with a flying-type magnetic head. 
Furthermore, the present invention provides, in another aspect thereof, a magnetic disc apparatus equipped with a recording head part for recording information and a reproducing head part for reproducing information in a magnetic recording medium, characterized in that the magnetic recording medium is the one according to the present invention that is described above and will be explained below in detail, and that the reproducing head part is equipped with a magnetoresistance-effect-type head. 
The magnetoresistance-effect-type head used in the magnetic disc apparatus of the present invention is preferably an MR head, an AMR head or a GMR head (including a spindle type GMR head).