Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
2000-12-28
2004-05-25
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
C360S059000
Reexamination Certificate
active
06741524
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thermally-assisted recording and/or reproducing device, and more particularly, to an improved and novel thermally-assisted recording device capable of heating a magnetic or other kinds of recording media by electron beams to write and/or read data to the medium with an extremely high density.
Personal computer (PC) systems and audio and/or video (AV) systems require a peripheral storage unit which has a large capacity and also is inexpensive. Currently, most of such peripheral storage units are magnetic or optical recording devices. The magnetic recording devices include a fixed magnetic hard disc drive (HDD) and magnetic tape recording device. Many of the PC systems adopt an HDD and an optical disc drive or magnetic tape recording device. Generally, various data including OS (operating system) and other software are stored in the HDD to which random access is made, while the optical disc drive or magnetic tape recording device is used for long-term storage of important data. Conventionally, the AV systems for storage of a large amount of moving picture information use mainly the magnetic tape recording device as the peripheral storage unit. With the larger capacity of the recent HDD and optical recording device, however, it has become expected that the HDD and optical recording device are employed in the AV systems for their speedy accessibility which is not possible with the conventional magnetic tape recording device. The magnetic and optical recording devices for use in the PC systems and AV systems are required to have a larger capacity and higher speed and be more inexpensive. With the conventional peripheral storage units, however, it is said that problems will arise as in the following.
First, the magnetic recording device will be considered. The magnetic recording device to magnetically write and read information has constantly been evolved as a large capacity, high speed and inexpensive information storage means. Among others, the recent hard disc drive (HDD) has shown remarkable improvements. Specifically, as proved on the product level, its recording density is over 10 Gbpsi (gigabits per square inch), internal data transfer rate is over 100 Mbps (megabits per second) and price is as low as several yens/MB (megabytes). The high recording density of HDD is due to a combination of improvements of a plurality of elements such as signal processing technique, servo control mechanisms, head, medium, HID, etc. Recently, however, it has become apparent that the thermal agitation of the medium inhibits the higher density of HDD.
The high density of magnetic recording can be attained by making smaller the recording cell (recording bit) size. However, as the recording cell is made smaller, the signal magnetic field intensity from the medium is reduced. So, to assure a predetermined signal-to-noise ratio (S/N ratio), it is indispensable to reduce the medium noise. The medium noise is caused mainly by a disordered magnetic transition. The magnitude of the disorder is proportional to a magnetic transition unit of the medium. The magnetic medium uses a layer formed from polycrystalline particles (will be referred to as “multiparticle layer” or “multiparticle medium” herein). In case a magnetic exchange interaction works between magnetic particles, the magnetic transition unit of the multiparticle layer is composed of a plurality of exchange-coupled magnetic particles.
Heretofore, when a medium is to have the recording density is several hundreds Mbpsi to several Gbpsi for example, the lower noise of the medium has been attained mainly by reducing the exchange interaction between the magnetic particles and making smaller the magnetic transition unit. In the latest magnetic medium of 10 Gbpsi in recording density, the magnetic transition unit is of only 2 or 3 magnetic particles. Thus, predictably, the magnetic transition unit will be reduced to only one magnetic particle in near future.
Therefore, to assure a predetermined S/N ratio by further reducing the magnetic transition unit, it is necessary to make smaller the size of the magnetic particles. Taking the volume of a magnetic particle as V, a magnetic energy the particle has can be expressed as KuV where Ku is an anisotropy energy density the particle has. When V is made smaller for a lower medium noise, KuV becomes smaller with a result that the thermal energy each particle has at a temperature near the room temperature will disturb information written in the medium, which is the “thermal agitation” referred to herein and has become the problem as mentioned above.
According to the analysis made by Sharrock et al., the ratio between magnetic energy and thermal energy of a particle, KuV/kT where k is Boltzman's constant and t is absolute temperature, is required to be greater than 100 or so in order to keep the reliability of the record life. If the particle size is decreased for a lower medium noise with the anisotropy energy density Ku being maintained at (2 to 3)×10
6
erg/cc of the CoCr group alloy conventionally used as a magnetic layer in the recording medium, it will be difficult to assure a thermal agitation resistance.
More specifically, the multiparticle layer of Co, Cr, Ta and Pt used in the current magnetic recording medium has a Ku value of about (2 to 4)×10
6
erg/cc. With a particle size of 10 nm&phgr;-10 nmt or so, the magnetic energy of each particle will be under 100 times of the thermal energy each particle has at the room temperature and there will take place a noticeable destruction of written information due to the thermal agitation. Improvement of the medium material and increasing the anisotropy energy density Ku may look like an approach to the solution of the problem, but a larger value of Ku will be accompanied by a larger coercive force, which will make the information writing to the medium more difficult.
Recently, magnetic layer materials having a Ku value of more than 10
7
erg/cc such as CoPt, FePd, etc. have been attracting much attention from all the field of industries concerned. However, simply increasing the Ku value for compatibility between the small particle size and thermal agitation resistance will lead to another problem. The problem concerns the recording sensitivity. Specifically, as the Ku value of the magnetic layer of a medium is increased, the recording coercive force Hc
0
of the medium (Hc
0
=Ku/Isb; Isb is a net magnetization of the magnetic layer of the medium) will increase and the necessary magnetic field for saturation recording increase proportionally to Hc
0
.
A recording magnetic field developed by a recording head and applied to the medium depends upon a current supplied to a recording coil as well as upon a recording magnetic pole material, magnetic pole shape, spacing, medium type, layer thickness, etc. Since the tip of the recording magnetic pole is reduced in size as the recording density is higher, however, the magnetic field developed by the recording head is limited in intensity.
Even a combination of a single-pole head which will develop a largest magnetic field and a vertical medium backed with a soft magnetic material for example can develop a magnetic field whose largest possible intensity is on the order of 10 kOe (Oe: oersted). On the other hand, to assure a sufficient thermal agitation resistance with a necessary particle size of about 5 nm for a future high-density, low-noise medium, it is necessary to use a magnetic layer material having a KU value of 10
7
erg/cc or more. In this case, however, since the magnetic field intensity necessary for write to the medium at a temperature approximate to the room temperature is over 10 kOe, no write to the medium is possible. Therefore, if the Ku value of the medium is simply increased, there will arise a problem of the write to the medium being impossible.
As having been described in the foregoing, in the magnetic recording using the conventional multiparticle medium, the lower noise, thermal agitation resistance and higher r
Akiyama Junichi
Ichihara Katsutaro
Kikitsu Akira
Tanaka Kuniyoshi
Tanaka Tsutomu
Dinh Tan
Kabushiki Kaisha Toshiba
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