1. Field of the Invention
The present invention relates to a magnetic recording/reading apparatus used as a hard disk drive and, more particularly, to a perpendicular magnetic recording/reading apparatus using perpendicular magnetization, and a perpendicular magnetic recording medium for use in the apparatus.
2. Description of the Related Art
Presently commercially available magnetic recording/reading apparatuses use a longitudinal magnetic recording system in which the magnetization direction of a magnetic recording layer is longitudinal. To increase the recording density in this longitudinal magnetic recording system, it is necessary to decrease the size of magnetic particles in the magnetic recording layer in order to increase the medium signal-to-noise ratio. Unfortunately, this allows easy occurrence of thermal decay, with the result that information disappears. Therefore, a method of increasing the magnetic anisotropy of the magnetic recording layer is conventionally used. However, from the viewpoint of the ease with which a recording head records data, further increasing the magnetic anisotropy is inadequate. As described above, it is difficult to increase the medium signal-to-noise ratio by reducing medium noise and increase the thermal decay resistance at the same time.
By contrast, in a perpendicular magnetic recording system in which the magnetization direction of the magnetic recording layer is perpendicular, magnetic fields which stabilize magnetization interact in a magnetization transition region. This forms a steep transition region to realize high density. In addition, compared to a longitudinal recording medium, the thickness of the recording layer can be increased to obtain the same recording resolution. This is also advantageous to thermal decay, since the volume of magnetic particles in the magnetic recording layer can be increased. If the recording density is very low, a demagnetizing field in the center of a recording bit is large, so the influence of thermal decay is found in this case. However, if the recording density is high, the perpendicular magnetic recording medium is stable unlike the longitudinal magnetic recording medium. Furthermore, in a double-layered perpendicular media in which a soft magnetic film is formed below the magnetic recording layer, the head magnetic field can be made larger than in the longitudinal recording medium. Therefore, a material having large anisotropy can be used as this magnetic recording layer. From these points, the perpendicular magnetic recording system has currently attracted attention.
In a perpendicular magnetic recording medium suitable for a magnetic recording system, it is necessary to reduce a decrease in output when data is recorded in a low-recording-density bit or in an adjacent track. For this purpose, the recording medium must have a perpendicular recording layer having magnetic characteristics by which magnetization does not reverse with respect to a certain magnetic field in the opposite direction of the magnetization direction, having a film thickness for obtaining an appropriate output, and having a fine structure with a high medium signal-to-noise ratio and high recording resolution.
An example of such a magnetic recording medium suited to the perpendicular magnetic recording system is described in Jpn. Pat. Appln. KOKAI Publication No. 2001-6158. In this magnetic recording medium, a CoPt alloy having appropriately large magnetic anisotropy is used as a magnetic recording layer. Below this magnetic recording layer, a nonmagnetic Co alloy to which Cr, oxygen, oxide which causes magnetic separation of crystal is added is formed. Below this nonmagnetic Co alloy, an underlayer, e.g., a nonmagnetic CoCr alloy or Ru alloy film, which perpendicularly aligns the magnetic recording layer is formed. Below this underlayer, a seed layer such as a Ti alloy, NiNb alloy, or NiTa alloy layer is formed to decrease a crystal size by decreasing the thickness of the underlayer. However, it is being required to further increase the recording density, so a perpendicular magnetic recording medium having a higher signal-to-noise ratio and better characteristics is being demanded.