Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-08-30
2003-06-03
Rickman, Holly C (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S900000, C428S611000, C428S668000, C428S332000, C360S097010
Reexamination Certificate
active
06572988
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium which is suitable for the high-density magnetic recording and to a magnetic recording apparatus which uses this magnetic recording medium.
In the field of magnetic recording, there are the schemes of longitudinal recording and perpendicular recording, of which the former is generally adopted at present. The longitudinal recording is a scheme of magnetic recording in which a magnetic recording head is used to form record bits by magnetizing the magnetic recording medium in parallel to the medium surface and in such directions that N poles or S poles of adjacent bits face to each other. The perpendicular recording is a scheme of magnetic recording in which a magnetic recording head is used to form record bits by magnetizing the magnetic recording medium perpendicularly to the medium surface such that adjacent bits are magnetized in anti-parallel directions.
Both schemes are designed to read recorded information out of the recording medium based on the detection with the magnetic head of the magnetic flux straying from recorded bits. Therefore, the greater the quantity of stray magnetic flux, the higher is the detected output level of recorded information by the magnetic head. The quantity of stray magnetic flux from a bit is approximately proportional to the magnetic moment which forms the bit. Namely, for magnetization M which is the magnetic moment per unit volume of the medium and the volume V of a recorded bit, the stray magnetic flux &phgr; from a bit is approximately proportional to MV.
This means that if the area of a recorded bit on the medium decreases as a result of the raising of recording density, the stray magnetic flux from the bit will decrease and the head output level will fall. On this account, in order to accomplish the high-density recording, it is necessary to enhance the sensitivity of reproduction head to the extent of compensating the reduced stray magnetic flux and ensure a sufficient magnitude of magnetization M of the medium.
SUMMARY OF THE INVENTION
The magnitude of magnetization of the magnetic recording medium varies depending on the temperature. Generally, ferromagnetic substance has a trend of directing the magnetic moment of ferromagnetic atoms to the same direction due to the exchange interaction between magnetic moments. The magnetic moment, except for the state of 0° K, is fluctuating by receiving thermal energy. The higher the temperature, the greater is the amplitude of fluctuating. Accordingly, as the temperature rises, the thermal fluctuation energy supersedes the energy that equalizes directs to the same direction of the magnetic moment based on the exchange interaction. The magnetization which is the value of the average magnetic moment per unit volume decreases gradually with the rise of temperature, causing the transition of the medium from ferromagnetic substance to non-magnetic substance at the Curie temperature.
Therefore, even though there is ensured a sufficient magnitude of magnetization of the medium at the room temperature, if the magnetization of medium decreases sharply due to the temperature rise of the magnetic recording apparatus within its operating temperature range, the stray magnetic flux from recorded bits will also decrease sharply, resulting in a reduced output of the reproduction head.
Accordingly, it is an object of the present invention to accomplish a high recording density and provide a magnetic recording medium and a magnetic recording apparatus which are capable of producing a sufficient reproduction output throughout the operating temperature range of the magnetic recording apparatus.
At the current laboratory development stage, there is reported the accomplishment of a magnetic recording medium of the type of longitudinal recording having an areal recording density of the order of 10 Gbit per square inch (The 7th MMM-Intermag Joint Conference, session ZA, San Francisco, U.S.A., January 1998). With the bit length to track width ratio being assumed to be 20 to 1 approximately that is adopted in general, the linear recording density is evaluated to be about 400 kFCI and the bit length to be about 60 nm.
In order for the longitudinal recording scheme to achieve a high medium S/N performance of recording and reproduction at a high linear recording density, it is necessary to minimize the length of transition region between recorded bits thereby to reduce the transition noise attributable to the zig-zag domain of the transition region of the medium.
The length of transition region of the medium is generally proportional to the product of the thickness t of the magnetic recording layer of the medium and the residual magnetization Br of the recording layer. Accordingly, the smaller the product Br·t of the residual magnetization and film thickness, the smaller is the noise and more improved is the medium S/N at a high linear recording density. However, a smaller product Br·t in excess causes the stray magnetic flux from recorded bits to decrease, resulting in a reduced reproduction head output. On this account, in order to prevent the deterioration of the medium S/N and head output at a high linear recording density, the product of the residual magnetization and film thickness needs to be set in the range: 30 Gauss·&mgr;m<Br·t<80 Gauss &mgr;m.
For a linear recording density as high as around 400 kFCI, the bit length becomes about 60 nm, and assuming that magnetic crystal grains of the recording film have an average size of 15 nm or more, the bit length is filled by four crystal grains at most. The fluctuation of zig-zag domain in the transition region attributable to the distribution of crystal grain sizes and the distribution of crystal grain orientation will increase, resulting in an increased medium noise which is derived from the transition noise. Therefore, it is necessary to increase the number of crystal grains in the bit length direction and reduce the average crystal grain size below 15 nm.
However, a magnetic crystal grain with a size smaller than or equal to 5 nm is too small in its volume, causing the thermal fluctuation energy of magnetic moment to supersede the magnetic anisotropic energy of magnetic crystal grain for directing its magnetic moment to the easy axis of magnetization, and it cannot have the magnetic moment orientation stabled in the direction of easy axis of magnetization, exhibiting the property of super-paramagnetism. On this account, the size d of magnetic crystal grains needs to be in the range: 5 nm<d<15 nm.
Next, it is necessary to make the stray magnetic flux from recorded bits less dependent on the temperature variation and let the magnetic head produce a large output signal even if the temperature varies. Specifically, the medium structure needs to be designed so that the temperature-dependent variation in the quantity of stray magnetic flux from recorded bits decreases at least in the operating temperature range of the magnetic recording apparatus. For the achievement of this requirement, the inventive magnetic recording medium adopts the ferromagnetic thin film structure having a small temperature-dependent variation of saturation magnetization at least in the temperature range smaller than or equal to 350° K.
The magnitude of saturation magnetization of the medium decreases as the temperature rises, as mentioned previously, and the magnetization vanishes at the Curie temperature.
FIG. 1
is a brief graphical representation of the temperature-dependent variation of saturation magnetization. Generally, the temperature-dependent variation of saturation magnetization increases as the temperature approaches the Curie temperature. Accordingly, if the Curie temperature of the medium is close to the operating temperature range of the magnetic recording apparatus, the medium has its magnetization varied greatly in response to the temperature variation even within the operating temperature range. Therefore, the Curie temperature needs to be high enough outside the operating temperature
Futamoto Masaaki
Inaba Nobuyuki
Kirino Fumiyoshi
Yamanaka Kazusuke
Yoshida Kazuetsu
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Rickman Holly C
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