Magnetic tapes have found various applications in audio tapes, videotapes, magnetic tapes for data-backup, etc. In particular, in the field of magnetic tapes for data-backup (or backup tapes), tapes having memory capacities of several ten GB or more per reel are commercialized in association with the increased capacities of hard discs for back-up. Therefore, it is indispensable to increase the capacity of this type of tape for data-backup, so as to meet the demand for a hard disc having a far larger memory capacity. It is also necessary to increase the feeding speed of a tape and a relative speed between the tape and heads in order to quicken an access speed and a transfer speed.
To increase the recording capacity of a magnetic recording medium, it is tried to shorten a recording wavelength. With the shortening of the recording wavelength, the particle size of magnetic powder used is decreased. As a result, the durability of a magnetic layer decreases so that it becomes difficult to maintain sufficient reliability against damages caused by sliding of the medium on a head.
When the thickness of a magnetic coating layer is decreased and a track width is narrowed in response to the increase of a recording capacity, leakage magnetic fluxes from the magnetic recording medium becomes smaller. Therefore, MR heads are increasingly used as reproducing heads, since they comprise magnetoresistance elements capable of obtaining high outputs even if magnetic fluxes are very small. The MR heads are made of soft materials and in the form of a very thin layer so that they are less abrasion resistant, while conventional magnetic induction type heads are made of relatively hard materials.
Examples of the magnetic recording media which can correspond to MR heads are disclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In these magnetic recording media, skewness of outputs from the MR heads is prevented by controlling the magnetic fluxes from the magnetic recording medium (a product of a residual magnetic flux density and the thickness of the a magnetic layer) to a specific value or less, or the thermal asperity of the MR heads is reduced by lessening the dents on the surface of the magnetic layers to a specific value or less.
To improve the durability of the magnetic layer, inorganic powder is usually added to the magnetic layer. In particular, inorganic powder having high Mohs hardness such as alumina, etc. is added to the magnetic layer. The addition of alumina powder is disclosed in JP-A-11-126325, JP 3240672B, JP-A-2001-155324 and JP 3046580B.
However, when the inorganic powder having high Mohs hardness such as alumina is added to the magnetic powder, the head is heavily abraded with the inorganic powder although the durability of the magnetic layer increases. In JP-A-11-126325, JP 3240672B, JP-A-2001-155324 and JP 3046580B, the relationship of the pH and particle size of alumina with an amount of a lubricant, the relationship of the particle size of alumina with the surface roughness of the magnetic layer and the like are studied to achieve the low abrasion of the head and the increased durability of the magnetic layer at the same time. However, the results are not satisfactory.