1. Field of the Invention
This invention relates to magnetic recording media used in hard disk devices and similar and to a method of manufacture of such media, as well as to a magnetic recording/reproduction device.
2. Description of the Related Art
In recent years there has been a remarkable expansion of the range of application of magnetic disk devices, flexible disk devices, magnetic tape devices, and other magnetic recording devices, and the increasing importance of such devices has been accompanied by progress to markedly increase the recording densities of the magnetic recording media used in such magnetic recording devices. In particular, since the introduction of MR heads and PRML technology, areal recording densities have surged upward, and with the further appearance of GMR heads, TMR heads and other technologies in recent years, areal recording densities have continued to shoot upward at a rate of roughly 100% per year.
Hence the attainment of still higher recording densities for magnetic recording media is being sought, and to this end magnetic recording layers with higher coercive forces, higher signal-to-noise ratios (SNRs), and higher resolution are demanded. Also, in recent years continuing efforts have been made to further increase areal recording densities through increases in track densities together with improvements in linear recording densities.
In the most recent magnetic recording devices, track densities of 110 kTPI have been achieved. However, if track densities are increased, mutual interference of magnetic recorded information in adjacent tracks occurs, and magnetization transition regions in these boundary areas become a source of noise, so that the problem that the SNR is degraded tends to occur. This leads directly to a reduced bit error rate, and so impedes increases in recording density.
In order to further increase areal recording densities, the size of each recorded bit on the magnetic recording media must be made still more minute, and as large a saturation magnetization and magnetic film thickness as possible must be secured for each recorded bit. However, if recorded bits are further reduced in size, the minimum magnetized volume per bit is decreased, and so there is the problem that magnetization inversion due to thermal fluctuation causes loss of recorded data.
Moreover, because the distance between tracks is decreased, extremely high-precision track servo technology is required in magnetic recording devices, and at the same time, a method is generally employed in which recording is executed with a wide width, and reproduction is executed over a width narrower than during recording in order to eliminate insofar as possible the effect of adjacent tracks. In this method, the effects of other tracks can be suppressed to a minimum, but on the other hand it is difficult to obtain an adequate reproduction output, and consequently there is the problem that an adequate SNR cannot easily be secured.
As one method of addressing such problems with thermal fluctuations, and of securing an adequate SNR and sufficient output, attempts have been made to raise the track density by forming depressions and protrusions along tracks on the recording media surface, to physically separate recording tracks from each other. (Below, such technology is called the discrete track method, and magnetic recording media manufactured using this method is called discrete track media.)
As one example of discrete track media, magnetic recording media is known in which the magnetic recording media is formed on a nonmagnetic substrate on the surface of which a pattern of depressions and protrusions has been formed, so that physically separated magnetic recording tracks and servo signal patterns are formed (see Patent Reference 1).
In this magnetic recording media, a ferromagnetic layer is formed on top of the nonmagnetic substrate having a plurality of depressions and protrusions on the surface, with a soft magnetic layer intervening. In this magnetic recording media, magnetic recording areas, physically separated from the surrounding areas, are formed in protrusion areas.
By means of this magnetic recording media, the occurrence of domain walls in the soft magnetic layer can be suppressed, so that the effects of thermal fluctuations tend not to appear, nor is there interference between adjacent signals, and high-density magnetic recording media with low noise can be fabricated.
Discrete track methods include a method in which tracks are formed after magnetic recording media comprising a number of thin film layers is fabricated, and a method in which, after forming a depression/protrusion pattern either directly on the substrate surface in advance or else in a thin film layer for track formation, magnetic recording media thin film layers are formed (see for example Patent Reference 2 and Patent Reference 3).
Of these, the former method, often called the magnetic layer machining method, entails physical machining of the surface after media fabrication, and so has the drawback that contamination tends to occur during media manufacturing processes, and moreover manufacturing processes are extremely complicated. On the other hand, the latter method is often called the embossing method; contamination does not readily occur during manufacturing processes, but because the depression/protrusion shape formed on the substrate is transferred to film deposited thereupon, there are the problems that the flying attitude and flying height of a recording/reproduction head which performs recording or reproduction while flying over the media are unstable.
In addition, a method has also been disclosed in which areas between magnetic tracks in discrete track media are formed by implantation of nitrogen ions and oxygen ions into the magnetic layer in advance, and then performing laser irradiation (see Patent Reference 4). However, the areas between magnetic tracks formed using this method, while having a low saturation magnetization, have a high coercive force, so that a state of insufficient magnetization occurs, and when writing information in the magnetic track portions, write bleeding occurs.
Further, methods have been disclosed in which, in so-called patterned media manufacturing in which a magnetic recording pattern is positioned with a constant regularity at each bit, the magnetic recording pattern is etched by ion irradiation, or the magnetic layer is formed by amorphization (see Non-patent Reference 1 and Patent Reference 5). However, in this method also, there are problems such as the occurrence of contamination of the magnetic recording media in manufacturing processes and reduced smoothness of the surface, as well as such problems as inadequate elimination of magnetization in the magnetic layer by the ion irradiation.    Patent Reference 1: Japanese Unexamined Patent Application No. 2004-164692    Patent Reference 2: Japanese Unexamined Patent Application No. 2004-178793    Patent Reference 3: Japanese Unexamined Patent Application No. 2004-178794    Patent Reference 4: Japanese Unexamined Patent Application No. 5-205257    Patent Reference 5: U.S. Pat. No. 6,331,364    Non-patent Reference 1: IEICE Technical Report, MR2005-55 (2006-02), pp. 21-26 (The Institute of Electronics, Information and Communication Engineers)
In the embossing method of manufacturing, depression/protrusion shapes are formed in the substrate, and magnetic layers and a protective layer are formed thereupon, so that the depression/protrusion shapes are transferred without modification to the surface, and it is not easy to realize a flat surface.
On the other hand, in the case of discrete track magnetic recording media prepared using the magnetic layer machining method, a magnetic layer for recording is formed on the substrate surface, and thereafter magnetic patterns are formed. Consequently after magnetic patterns have been formed by an imprint method, used in semiconductors and other fields, the portion which is to serve as a nonmagnetic portion, is subjected to for example dry etching and burying with SiO2 and carbon nonmagnetic material or similar are performed to flatten the surface, and the surface is then covered with a protective film layer, after which a lubricating layer is formed. Such magnetic etched-type discrete track media entails complicated manufacturing processes, and not only may be the source of contamination, but cannot attain a flat surface.
In general in magnetic recording media with such a structure, the thinner the protective film layer, the shorter is the distance between head and magnetic layer, so that signal entry into and exit from the head is greater, and higher recording densities are possible. Also, the pit density within tracks is determined by the flying height of the head traveling over the protective film layer surface, with a depression/protrusion shape. Hence preservation of stable head flight while achieving high recording densities is a vital problem. To this end, a depression/protrusion pattern is sought which brings the head into as close proximity to the magnetic layer as possible while maintaining stable head flight, and yet which prevents mutual interference between signals of adjacent tracks.
However, there have been no proposals of manufacturing technology for discrete track media with a flat surface, and which poses no risk of contamination in manufacturing processes, nor have there been proposals of technology for the manufacture of magnetic recording media in which write bleeding does not occur when writing information to the magnetic track portions.
In the manufacture of so-called patterned media, it has been proposed that the magnetic layer be amorphized by ion irradiation to form the magnetic recording patterns; however, there have been the problems that elimination of magnetic properties in the magnetic layer is inadequate, and that write bleeding occurs. This is thought to occur because, although magnetic layer crystals are temporarily amorphized by ions implanted into the magnetic layer, in subsequent processes, and due to the heat at the time of ion irradiation, a portion of the amorphous structure is recrystallized, and as a result there is recovery of the magnetic characteristics of the magnetic layer subjected to ion irradiation.
This invention was devised in light of the above circumstances of the prior art, and has, as an object, the provision, for magnetic recording devices, engineering difficulties in respect to which are being faced as recording densities rise, of magnetic recording media in which, while maintaining recording/reproduction characteristics comparable or superior to those of the prior art, enables higher recording densities, and reduces to the utmost the coercive force and remanent magnetization of the nonmagnetic portions which magnetically separate magnetic patterns, to eliminate write bleeding in the event of magnetic recording, and to thereby enable increases in the areal recording density, as well as the provision of a manufacturing method for such media, and a magnetic recording/reproduction device.
In particular, this invention has as an object the provision, for discrete track magnetic recording devices in which depressions and protrusions are formed after depositing a magnetic layer on a nonmagnetic substrate, of magnetic recording media with simplified manufacturing processes, and with low risk of contamination, and with excellent head flying characteristics, compared with magnetic layer machining methods of the prior art, through the elimination of processes for removal of the magnetic layer, as well as a method of manufacture of such media, and a magnetic recording/reproduction device.