The present invention relates to a recording medium with limited using period for copyright protection suitable for recording copyrighted digital information.
Conventionally, a recording medium, in which digital information is recorded and stored, has been designed and manufactured such that the stored information is not attenuated in order to keep the recording medium in a stable condition as much as possible. Accordingly, digital information once recorded in the medium has usually been retained for years unless it was deliberately erased or overwritten.
However, as digital broadcasting has begun or digital communication has been spread, digital information has major problems concerning copyright infringement, which requires an urgent solution. The medium with the performance of storing digital information for years is likely to violate a copyright in itself unless it provides a usufructuary period of time or there is a restriction on a number of time regarding reproduction.
Japanese Laid Open Patent Publication (KOKAI) No. 10-283653 discloses an optical recording medium and an optical readout equipment enabling stored information in the medium to be erased while being read out when a usufructuary period of time expires. Japanese Laid Open Patent Publication (KOKAI) No. 8-147856 discloses a photodisk, a photomagnetic disk, and a readout equipment prohibiting unlimited use of digital data and validity in protecting a copyright.
Object of the present invention is to offer a recording medium suitable for copyright protection, in which stored digital information is attenuated in a certain period of time.
Recording medium in which stored digital information is attenuated in a certain period of time seems to offer a solution for copyright protection against the storing ability of digital information in a medium. Accurately recorded, gradually attenuated, and finally extinct digital information permits a recording medium to eliminate the possibility of infringing, etc. on a copyright in the recording of copyrighted digital information.
Storing ability of digital information in the magnetic recording medium, among others, was investigated from this viewpoint. Investigation reveals that stored digital information, which is magnetization of a crystalline region in a magnetic layer, is susceptible to thermal oscillation in a crystal lattice even at a room temperature, when the thickness or crystal grain diameter in a magnetic layer is diminished, so that the thermal oscillation disturbs magnetization direction, and extinguishes the stored magnetic information.
This suggests that with the advantage of the susceptibility, which must be eliminated in conventionally manufacturing a medium, it may possibly provide a magnetic recording medium feasible for achieving the object. Keen examination on feasibility indicates that appropriate thickness and crystal grain diameter in a magnetic layer for controlling the effect of thermal oscillation on the recording medium provides the recording medium with digital information retained for a period of time and attenuated along with time.
Magnetization M of a ferromagnetic attenuates with time t through the action of thermal energy. The following general formulae (1) and (2) represent a variation in magnetization M with time.
M(t)=M(0)exp(xe2x88x92t/xcfx84)xe2x80x83xe2x80x83(1)
1/xcfx84=foexp(xe2x88x92Kuv/kBT)xe2x80x83xe2x80x83(2)
where M(t) denotes residual magnetization at time t, and xcfx84 is time constant of magnetization attenuation. And fo signifies frequency factor (about 109/s), Ku is a constant of energy of uniaxial magnetic anisotropy in a magnetic crystal grain, v is the volume of magnetization reversal, kB is Boltzmann constant, and T is temperature (K), respectively.
Formulae (1) and (2) indicate that as the product Kuv becomes smaller, the attenuation of magnetization M with time becomes greater. Ku value in formula (2) is subject to composition of magnetic grains, and v is determined by a diameter of a magnetic crystal grain and interaction among magnetic crystal grains. Magnetization M of a cobalt-chromium alloy thin film magnetic layer which is applied in a conventional magnetic recording medium is likewise a function of Kuv/kBT, and attenuates in proportion to the reverse exponential of time t, that is, decreases linearly with time t expressed in the logarithmic scale on a chart.
While Ku value, a factor for controlling attenuation, is subject to the composition of a magnetic layer, v value depends on microstructure of the magnetic layer, including thickness, crystal grain diameter, and segregation of impurities at grain boundaries. Hence, manufacturing condition of the magnetic layer controls v value.
Findings reveal that the thickness of a magnetic layer of 200 xc3x85 or less, or the diameter of a crystal grain of 100 xc3x85 or less provides a magnetic recording medium with stored digital information attenuated by 50% or more in a year. In order to obtain a desired magnetization attenuation, it is preferable to form the thickness of a magnetic layer with 200 xc3x85 or less and crystal grain diameter of 100 xc3x85 or less. Attenuation of 1% or less in a day requires the thickness of the magnetic layer from 100 xc3x85 to 200 xc3x85 and the diameter of the crystal grain from 50 xc3x85 to 100 xc3x85.
A first recording medium for copyright protection of the present invention comprises a non-magnetic substrate and an intelligent layer. The intelligent layer is formed above or on the non-magnetic substrate, and stores digital information. The digital information is attenuated spontaneously by 50% or more in a year.
A second recording medium for copyright protection of the present invention comprises a non-magnetic substrate and an intelligent layer. The intelligent layer is formed above or on the non-magnetic substrate, and stores digital information. The digital information is attenuated spontaneously by 50% or more in one month.
A third recording medium for copyright protection of the present invention comprises a non-magnetic substrate and an intelligent layer. The intelligent layer is formed above or on the non-magnetic substrate, and stores digital information. The digital information is attenuated spontaneously by 5% or less in a day and 50% or more in one month.
A fourth recording medium for copyright protection of the present invention stores magnetization at a crystalline region in a magnetic layer. Magnetization in crystalline region stands for digital information, and the magnetic layer stands for the intelligent layer in a first, second, or third recording medium.
A fifth recording medium for copyright protection of the present invention takes advantage of thermal oscillation in a crystal lattice of the magnetic layer as set forth in a fourth recording medium, magnetization of the crystalline region being attenuated spontaneously.
A sixth recording medium for copyright protection of the present invention has the thickness of the magnetic layer of 200 xc3x85 or less, thereby controlling the thermal oscillation in the crystal lattice of the magnetic layer as set forth in a fifth recording medium.
A seventh recording medium for copyright protection of the present invention has a diameter of a crystal gain of 100 xc3x85 or less, thereby controlling the thermal oscillation in the crystal lattice of the magnetic layer as set forth in the fifth recording medium.
An eighth recording medium for copyright protection of the present invention has the thickness of the magnetic layer of 200 xc3x85 or less, and the diameter of the magnetic crystal grain from 50 xc3x85 to 100 xc3x85 to control thermal oscillation in the crystal lattice of the magnetic layer as set forth in the fifth recording medium, magnetization of a crystalline region being attenuated by 5% or less in a day and 50% or more in a month or more.
A ninth recording medium for copyright protection of the present invention has the magnetization of the crystalline region attenuated by 5% or less in a day and 50% or more in a month or more at 23xc2x0 C. when the digital information is recorded at a frequency of 100 MHz in the magnetic layer as set forth in the fifth recording medium.
A tenth recording medium for copyright protection of the present invention has the magnetization of a crystalline region attenuated by 5% or less in a day and 50% or more in a month or more at 23xc2x0 C. when magnetization of the crystalline region in the magnetic layer as set forth in the fifth recording medium is saturated in the medium cut into chips with 8 mm long and 8 mm wide in a certain direction.
An eleventh recording medium for copyright protection of the present invention is made of plastics for a non-magnetic substrate as set forth in any of the preceding recording medium.