In recent years, magnetic recording apparatuses such as a magnetic disk apparatus, a flexible disk apparatus and a magnetic tape apparatus are widely used with their importance being increasing. Recording density of a magnetic recording medium used in the magnetic recording apparatus is greatly enhanced. Especially, since the development of MR head and PRML technique, the areal recording density is more and more increasing. Recently GMR head and TMR head have been developed, and the rate of increase in the areal recording density is about 100% per year. There is still increasing a demand for further enhancing the recording density, and therefore, a magnetic layer having a higher coercive force, and a higher signal-to-noise ratio (SNR) and a high resolution are eagerly desired.
An attempt of increasing the track density together with an increase of a liner recording density to enhance an areal recording density is also being made.
In a recent magnetic recording apparatus, the track density has reached about 110 kTPI. However, with an increase of the track density, magnetic recording information is liable to interfering with each other between adjacent tracks, and magnetization transition regions in the boundary regions thereof as a noise source tend to impair the SNR. These problems result in lowering in bit error rate and impede the enhancement of the recording density.
To enhance the areal recording density, it is required to render small the size of each recording bit and give the maximum saturated magnetization and magnetic film thickness to each recording bit. However, with a decrease in the bit size, the minimum magnetization volume per bit becomes small, and the recorded data are tend to disappear due to magnetization reversal caused by heat fluctuation.
Further, in view of the reduction in distance between the adjacent tracks, a high-precision track servo system technology is required for the magnetic recording apparatus, and an operation is generally adopted wherein recording is carried out widely but the reproduction is carried out narrowly so that the influence of the adjacent tracks is minimized. This operation is advantageous in that the influence of the adjacent tracks can be minimized, but it is disadvantageous in that the reproduction output is rather low. This also leads to difficulty in enhancement of the SNR to a desired high level.
To reduce the heat fluctuation, maintain the desired SNR and obtain the desired reproduction output, a proposal has been made wherein elevations and depressions are formed, which extend along the tracks on a surface of a magnetic recording medium, so that each of patterned tracks on the elevations is partitioned by the depressions whereby the track density is enhanced. This type of magnetic recording media is hereinafter referred to as a discrete track media, and the technique for providing this type of magnetic recording media is hereinafter referred to as a discrete track method.
An example of the discrete track medium is a magnetic recording medium disclosed in, for example, patent document 1, which is made by providing a non-magnetic substrate having a pattern with elevations and depressions formed on the surface thereof, and forming a magnetic layer having a corresponding surface configuration on the non-magnetic substrate, to give physically discrete magnetic recording tracks and servo signal patterns (see, for example, patent document 1).
The above-mentioned magnetic recording medium has a multilayer structure such that a ferromagnetic layer is formed via a soft magnetic underlayer on the non-magnetic substrate having the pattern with elevations and depressions, which are formed on the surface thereof, and an overcoat is formed on the ferromagnetic layer. The magnetic recording pattered regions form magnetic recording regions on the elevations physically partitioned from the surrounding regions.
In the above-mentioned magnetic recording medium, the occurrence of ferromagnetic domain wall in the soft magnetic underlayer can be prevented or minimized and therefore the influence due to the heat fluctuation is reduced and the interfere between the adjacent signals is minimized with the result of provision of a magnetic recording medium with high recording density exhibiting a large SNR.
The discrete track method includes two type of methods: a first type method wherein tracks are formed after the formation of a multilayer magnetic recording medium comprising several laminated films; and a second type method wherein patterns having elevations and depressions are formed directly on a substrate or formed on a film layer for forming tracks thereon, and then a multilayer magnetic recording medium is formed using the patterned substrate or the patterned film layer (see, for example, patent document 2 and patent document 3). The first type method is often called a magnetic layers-treating type method, and the second type method is often called as an embossing type method.
Another discrete track method has been proposed wherein a previously formed magnetic layer is, for example, subjected to an implantation of nitrogen ion or oxygen ion or irradiated with laser whereby regions partitioning magnetic tracks in a discrete track medium are formed. (see, for example, patent document 4).
Further, a method of producing a magnetic recording medium has been proposed, which comprises a step of subjecting a magnetic layer to an ion milling using a carbon mask (see patent document 5).
Further, another method for forming a magnetic pattern has been proposed in patent document 6, which comprises a step of forming a ferromagnetic layer comprising at least one element selected from Fe, Co and Ni, a step of selectively masking the surface of the ferromagnetic layer, a step of exposing the regions of the surface of ferromagnetic layer, which have been exposed by the selective masking, to a halogen-containing reactive gas whereby the ferromagnetic layer and an under layer are chemically reacted with an active ingredient in the reactive gas in said exposed regions of ferromagnetic layer to be thereby rendered non-ferromagnetic regions.
Patent document 7 teaches that variation in an areal ratio of the magnetic pattern to the non-magnetic regions along the radius directions in the magnetic disk enables to suppress or minimize the reproduction errors.
Patent document 1 JP 2004-164692 A1
Patent document 2 JP 2004-178793 A1
Patent document 3 JP 2004-178794 A1
Patent document 4 JP H5-205257 A1
Patent document 5 JP 2006-31849 A1
Patent document 6 JP 2002-359138 A1
Patent document 7 JP 2006-48751 A1