The storage capacity of hard disk drives has been steadily increasing, as well as the recording density of magnetic recording media mounted therein. In response to these increases, the flying height of a magnetic recording head is becoming smaller over the years, raising the need to reduce the distance between the head and the magnetic recording layer (magnetic spacing) more and more. As one of the methods for reducing the magnetic space, there is a method for thinning a protective layer formed on the magnetic recording layer.
Because the protective layer literally serves to protect the magnetic recording layer, the protective layer needs to have a high corrosion resistance in order to prevent the magnetic recording layer, made of metal or the like, from becoming corroded by reaction with atmospheric moisture or corrosive gas. Furthermore, the head moves on the magnetic recording medium at relatively high speed and flies thereon keeping small magnetic spacing therebetween. The protective layer is also required to be durable to the flying travel motion of the head in order to protect the magnetic recording layer from damage even in a case where the head comes into contact with the magnetic recording medium due to some problem. Currently the protective layers are made of DLC (diamond-like carbon) and as thin as 3.0 nm.
A heat-assisted recording system has been designed as a new recording system for coping with higher recording density, the advent of which is in 1 to 2 years. The heat-assisted recording system is a technique that enables magnetic recording in higher recording density by radiating a laser beam or the like onto the surface of the magnetic recording medium, heating the magnetic recording medium with light absorbed therefrom, reducing the coercivity of the magnetic recording layer using the resultant increased temperature, and assisting the head in writing signals. In this heat-assisted recording system, the protective layer and lubricating layer are expected to be heated up to approximately 300° C. or higher due to the heat resulting from the increased temperature of the magnetic recording layer. Therefore, the protective layer and the lubricating layer that are employed in such a heat-assisted recording system are required to be corrosion-resistant and durable to the heat from the laser radiation (referred to as “heating with laser beam”), as described above.
Unfortunately, employing such a heat-assisted recording system in a conventional magnetic recording medium with a DLC protective layer to write signals, leads to quality degradation in the protective layer and lubricating layer due to the heat from the laser beam. This consequently causes an increase in the number of errors due to deterioration of the corrosion resistance of the protective layer and the lubricating layer, a shortening of the life of the magnetic recording medium due to degradation of the durability of the protective layer and the lubricating layer, and an increase in the risk of damage to the magnetic recording medium. As a measure to prevent these phenomena, Patent Document 1 discloses a method for forming a protective layer on a heat-insulating layer provided on a magnetic recording layer. Patent Document 1 describes that such materials of low thermal conductivity as SiO2, TiO2, and ZrO2 are preferred as the heat-insulating layer and that the heat-insulating layer is made by reactive sputtering method using O2 gas for forming a film with a target of each material.
However, after executing this method, the inventors of the present application discovered that the O2 gas for forming the heat-insulating layer causes oxidation and hence deterioration of the magnetic recording layer and that the method is therefore not for practical use.
Patent Document 1: Japanese Patent Application Publication No. 2010-153012