1. Field of the Invention
The present invention relates to magnetic recording and assisted magnetic recording, and particularly to a magnetic recording medium capable of realizing an areal recording density of at least 150 gigabits per square centimeter and a method of manufacturing the same.
2. Background Art
Hard disk drives (HDDs) are indispensable devices for usage requiring large-capacity information recording in computers and consumer-electronics products. In the future too, needs for large-capacity recording will be high. It is required to increase the areal recording densities of recording media in order to realize large capacity while serving the needs for savings in space and energy. Presently, approaches to high density by improvement in perpendicular magnetic recording have been attempted. However, according to conventional perpendicular magnetic recording, it is estimated that a feasible maximum areal recording density is 150 Gbit/cm2 (1 Tbit/inch2). The reason why the areal recording density has the limit is interpreted to be due to a fundamental principle of recording according to which a medium suitable for high density recording deteriorates in thermal stability. High density magnetic recording requires magnetic grains forming a magnetic recording medium to be finer to form highly accurate recording bit boundaries (magnetic transition region). However, in a case of making the magnetic grains fine, the magnetic energy KuV that stabilizes magnetization directions of respective grains cannot retain a magnitude sufficient against thermal energy kBT as a disturbance. Accordingly, a phenomenon occurs that recorded magnetization information deteriorates (thermal decay of magnetization) immediately after recording. Here, Ku, V, kB, and T are a uniaxial magnetic anisotropy energy, a magnetic grain volume, the Boltzmann constant, and the absolute temperature, respectively.
Improvement in areal recording density while maintaining thermal stability requires use of a magnetic recording layer having a high magnetic anisotropy energy Ku. As described in IEEE Trans. Magn., vol. 36, p. 10 (2000) and the like, an L10 FePt ordered alloy is a material having perpendicular magnetic anisotropy energy Ku higher than that of existing CoCrPt alloys, and receives attention as a next-generation magnetic recording layer. Use of the L10 FePt ordered alloy as a magnetic recording layer absolutely necessitates reduction in exchange interaction between crystalline grains. Accordingly, in recent years, many attempts of adding a non-magnetic material, such as MgO, SiO2 or C, to an L10 FePt ordered alloy to form granular structure have been reported. Here, the granular structure represents a structure including magnetic crystalline grains made of an FePt alloy and grain boundaries made of surrounding non-magnetic material.
However, recording cannot be made on the magnetic recording layer material having such a high Ku, using an existing magnetic head. This is because a soft magnetic material used for a magnetic writer pole has the maximum value of saturated magnetic flux density B of approximately 2.5 T, and thus the magnitude of the magnetic field generated by the magnetic writer pole is limited. Thus, assisted magnetic recording, or a new concept of magnetic recording, has been proposed. Presently, two assisting schemes, laser heating and microwave irradiation schemes have mainly been proposed, and referred to as thermally assisted magnetic recording (IEEE Trans. Magn., vol. 37, p. 1234 (2001)) and microwave assisted magnetic recording (IEEE Trans. Magn., vol. 44, p. 125 (2008)), respectively. These assisted magnetic recording schemes irradiate a magnetic recording layer with assist energy to facilitate magnetization reversal and then form a recording bit using a magnetic field generated by the magnetic writer pole.
Since FePt has a disordered fcc structure as a metastable phase in addition to the L10 ordered structure, this requires to be subjected to an ordering process by heat treatment. It has been known that, the higher the degree of ordering (degree of ordering S), the higher the magnetic anisotropy energy is obtained. Improvement in degree of ordering requires heat treatment. The methods therefor are broadly divided into a method of heating after forming a film of an FePt alloy (post annealing method), and a method of forming a film of an FePt alloy on a preheated substrate (substrate heating method). In a case of granulation by adding a nonmetal element to an FePt alloy thin film, a fabrication method is required to be determined on the basis of any of heating methods as a premise.
An example of a fabrication method using the post annealing method is disclosed in Appl. Phys. Lett., vol. 91, p. 072502 (2007). According to this document, a post annealing process is applied to a multilayer film structure in which a periodic structure including an Fe layer, Pt layer, and a SiO2 layer as a grain boundary material is repeatedly stacked n times, thereby obtaining L10 FePt alloy magnetic thin film having a granular structure. The diameters of the FePt magnetic grains at this time are approximately 6 nm. Accordingly, the grains can be applied to high density magnetic recording. On the other hand, an example of the fabrication method using the substrate heating method is disclosed in Appl. Phys. Lett., vol. 91, p. 132506 (2007) and J. Appl. Phys., vol. 103, p. 023910 (2008). These documents have reported that a granular structure can be obtained without using the periodically laminated structure such as in Appl. Phys. Lett., vol. 91, p. 072502 (2007), and the diameters of the grains can relatively easily be controlled according to a heating temperature of a substrate and an amount of addition of non-magnetic material. Various oxides and carbon have been discussed as a grain boundary material. It has been understood that C is a specific grain boundary material which can realize an excellent granular structure among these materials. J. Magn. Magn. Mater., vol. 322, p. 2658 (2010) discloses an example of fabricating an L10 FePt alloy magnetic thin film which realizes both a favorable granular structure with the diameters of magnetic grains of about 6 nm and a high coercivity Hc of at least 3 T (30 kOe).