Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-12-27
2003-08-05
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S690000, C428S900000
Reexamination Certificate
active
06602620
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording apparatus and a magnetic recording medium for magnetically recording and reproducing information and a manufacturing method thereof.
A magnetic recording medium in a form of a rigid magnetic disk apparatus is widely used for a personal data file, a communication server, a large-scale computer file and the like. Also, a magnetic recording medium in a form of a magnetic tape apparatus is widely used for image or audio files for personal use or broadcasting. This is because a magnetic recording medium constituted by an ensemble of magnetic crystal grains shows a very high magnetization reversal rate, and therefore, a very high recording data transfer rate in an order of several hundreds Mbps or more, and also can attain a high recording density of about several tens Gb/in
2
. As for the magnetic recording medium, a higher transfer rate and a higher recording density are expected in a situation toward forthcoming multimedia era that amount of information continues to increase remarkably.
An areal recording density of the magnetic recording medium, particularly of a hard disk drive (HDD), has been improved by 60% or more per year over the past five years or more, and currently reaches several Gb/in
2
. Such improvement in areal density is owing to innovation and improvement of various elemental technologies such as use of a magnetoresistive reproducing system, use of a recording magnetic pole material having a high saturation magnetic flux density, improvement in processing of magnetic head of a narrow track width, use of a magnetic head having a narrower gap, miniaturization and high-precision processing of a slider, high-precision servo technology, and development of novel modulation/demodulation technology represented by PRML. In addition, with respect to a magnetic recording medium itself, there is advanced progress in elemental technologies such as smoothing and flattening of medium surface leading to low flying height operation of a magnetic head, reduction in magnetization transition width due to increase in coercivity and decrease in thickness of a magnetic layer, and medium noise reduction due to decrease in exchange interaction between magnetic grains and reduction in magnetic grain size.
In the aforementioned conventional so-called multigrain magnetic recording medium, it is supposed that, if isolation of magnetic grains and reduction in magnetic grain size are advanced to ensure low noise, the recording density will be limited because of thermal disturbance. Hereinafter, the thermal disturbance will be described.
For improvement of a recording density, it is necessary to reduce a recording cell size on a medium, which brings about reduction in signal magnetic field intensity generated from the medium. In order to meet an S/N ratio required for a recording system, noise must be reduced corresponding to reduction in signal intensity. The medium noise is mainly caused by fluctuation of a magnetization transition, and the fluctuation is proportional to a size of a magnetization reversal unit made of magnetic grains. Therefore, in order to reduce the medium noise, it is required to isolate magnetic grains by disrupting exchange interaction between magnetic grains, i.e., to reduce the fluctuation of the magnetization transition to an order of a size of single magnetic grain, and to reduce magnetic grain size.
Magnetic energy that a single isolated magnetic grain has is given by a product of magnetic anisotropy energy density and volume of the grain. To reduce a medium thickness in order to reduce a magnetization transition width and to reduce a magnetic grain size in order to meet a requirement for low noise significantly lowers the volume of magnetic grain, and further significantly lowers magnetic energy of the grain. If the magnetic energy of a certain grain is several hundred times of thermal energy at an operating temperature (at least at room temperature) for a magnetic memory, resistance against thermal disturbance is considered to be sufficient. However, if the magnetic energy of the grain is less than a hundred times of thermal energy, there is possibility that the magnetization direction of the magnetic grain is reversed by thermal disturbance and recorded information is lost. Because of the thermal disturbance, it is thought that the areal density of HDD will be limited to about 40 to 50 Gb/in
2
.
A conventional multigrain magnetic medium such as CoCr-based medium has a structure in which a Cr-rich non-magnetic grain boundary is segregated between magnetic grains in order to lower exchange coupling between magnetic grains. However, in a method of fabricating a magnetic film by conventional sputtering, diameters of magnetic grains cannot be adjusted directly, and it is difficult to reduce the magnetic grain size uniformly. Thus, there is large distribution in grain diameter and intergranular distance, and grains are arranged irregularly. Therefore, even if exchange interaction between grains is severed to isolate grains, medium noise is not sufficiently lowered, which inhibits improvement in recording density. Specifically, when distribution in grain diameter is expressed by full width at half maximum (FWHM) of distribution of grain diameters, a value of about ±50% is exhibited in a typical medium, and a value of ±25% or more is exhibited even in a medium in which distribution is controlled by low-speed sputtering or the like. For example, a typical medium of 20 nm in average grain diameter has a number of grains between 10 nm and 30 nm. This means that there are considerable grains of less than 10 nm in grain diameter, which are strongly affected by thermal disturbance. Distribution in intergranular distance is more significant: the distribution is ±70% in FWHM in a typical medium, and is ±45% or more even in a well controlled medium. That is, a typical medium of 2 nm in intergranular distance has a number of grains of 0.6 nm to 3.4 nm in intergranular distance. This means that there are considerable grains in an exchange coupled state.
There has been proposed some solutions to overcome the problem of thermal disturbance. One solution is use of a magnetic material with high magnetic anisotropy. However, if the magnetic anisotropy becomes higher, the recording saturation magnetic field required for a medium is increased, and it is required to further increase saturation magnetic flux density of a magnetic pole material for recording head. This cannot be a practical solution because currently available soft magnetic film material, including laboratory level, is hard to meet the above requirements.
Another solution is light thermal assisted recording. In this method, a highly anisotropic magnetic material is employed, and a recording portion is heated by light irradiation during recording. This lowers the anisotropy of magnetic grains and the recording saturation magnetic field, and therefore, recording can be performed with an available recording head. However, this method is impractical because it requires providing an optical system in a drive unit having almost no extra space, including a space between disks. In addition, this method increases power-consumption, and brings about additional heat generation.
As another technical seed to avoid the problem of thermal disturbance for overcoming the HDD recording density limit, there has been proposed a near field optical recording employing SIL or evanescent light. However, optical recording cannot achieve high transfer rate like magnetic recording as long as a heat mode process is employed. On the other hand, there has been proposed a method employing a photon mode material in order to attain a ultrahigh transfer rate and ultrahigh density, but such a method is in a research level and not realized at all.
The foregoing methods cannot give a proper solution to thermal disturbance that prevents higher recording density of magnetic media.
Currently, it is considered that effective methods to solve thermal distur
Ichihara Katsutaro
Kikitsu Akira
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Rickman Holly
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