Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
1998-01-06
2001-05-08
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S336000, C428S690000, C428S690000, C428S690000, C428S900000, C369S013010
Reexamination Certificate
active
06228482
ABSTRACT:
TECHNICAL FIELD
The present invention relates to magneto-optic (MO) recording media, including media useful in near-field, air-incident magneto-optic recording applications employing magnetic field modulation.
BACKGROUND INFORMATION
In magneto-optic recording, data is represented as a magnetized domain on a magnetizable recording medium. Each domain is a stable magnetizable region representative of a data bit. Data is written to the medium by applying a focused beam of high intensity light in the presence of a magnetic field. The recording medium typically includes a substrate, a magneto-optic recording layer, a reflective layer, and two or more dielectric layers that together form the MO stack. In substrate-incident recording, the beam passes through the substrate before it reaches the recording layer. The reflective layer in a substrate-incident recording medium is formed on a side of the recording layer opposite the substrate. The reflective layer reflects the beam back to the recording layer, increasing overall exposure and absorption. In near-field, air-incident recording, the beam does not pass through the substrate. Instead, a solid immersion lens (SIL) is used to transmit the beam across an extremely thin air gap and through the top of the recording medium to the recording layer. The SIL transmits the beam by evanescent coupling across the air gap. In an air-incident recording medium, the reflective layer is formed adjacent the substrate and the thin air gap forms one of the layers in the MO stack from an optical performance standpoint. The recording beam heats a localized area of the recording medium above its Curie temperature. The area is allowed to cool in the presence of a magnetic field. The magnetic field overcomes the demagnetizing field of the perpendicular anisotropy recording medium, causing the localized area to acquire a particular magnetization. The direction of the magnetic field and the resulting magnetization determine the data represented at the domain. With beam modulation recording techniques, the magnetic field is maintained in a given direction for period of time as the beam is selectively modulated across the recording medium to achieve desired magnetization. According to magnetic field modulation (MFM) recording techniques, the beam is continuously scanned across the recording medium while the magnetic field is selectively modulated to achieve desired magnetization. Alternatively, the beam can be pulsed at a high frequency in coordination with modulation of the magnetic field. Examples of various MFM recording techniques are described in The Physical Principles of Magneto-optical Recording, by Masud Mansuripur, Cambridge University Press 1995.
To read the recorded data, the drive applies a lower intensity plane-polarized beam to the recording medium. Upon transmission through and/or reflection from the recording medium, the plane-polarized beam experiences a Kerr rotation in polarization. The Kerr angle of rotation varies as a function of the magnetization of the localized area. An optical detector translates the Kerr rotation angle into an appropriate bit value.
SUMMARY
The present invention is directed to a magneto-optic recording medium having a reduced demagnetizing field threshold. The recording medium incorporates a thin magneto-optical recording layer that allows writing and erasure of data using relatively small magnetic fields. Consequently, this recording medium is particularly useful for high-frequency magnetic field modulation recording applications. To protect the thin recording layer against reactants, the recording medium may include dielectric layers that encapsulate the recording layer and are formed from a material that is substantially non-reactant with the recording layer. It is also desirable that the dielectric layers exhibit only a small degree of surface roughness, preventing significant nonuniformities in the domain walls of the recording layer, and thereby contributing to the reduced demagnetizing field threshold. An example of a suitable material is doped, amorphous silicon carbide. In an air-incident embodiment, it is also desirable that the other layers in the MO stack, along with the recording layer, be selected and optimized in thickness to provide a recording medium that yields consistent performance for variations in air gap thickness. Such variations can result from relative movement of the recording medium and recording head during operation. The reduced demagnetizing field threshold facilitates use of the medium with recording techniques employing relatively small magnetic fields. For effective recording and/or erasure, the magnetic field applied by a magneto-optic drive must be sufficient to overcome the demagnetizing field threshold of the magnetic recording medium. In some applications, however, it may be desirable to employ relatively small magnetic fields to write and erase data. In near-field, air-incident recording, for example, the applied magnetic fields used for write and erasure processes may be significantly smaller than in substrate-incident applications.
Substrate-incident applications ordinarily make use of beam modulation recording techniques in which the applied magnetic field is fixed in a given direction for a long period of time. Beam modulation recording thereby allows relatively large fields, typically on the order of 200 to 350 Oerstad (Oe), to be used for writing and erasing. In the near-field scheme, however, it is desirable to use magnetic field modulation recording techniques. For example, near-field recording is expected to provide extremely large storage densities that require high data rates. The increased inductance associated with higher magnetic fields can undermine data rate capabilities. Due to this inductance and the magnetic response time of the recording medium, it may be necessary to use smaller magnetic fields for the high frequency modulation necessary for high-speed recording. For a near-field optical drive, the magnetic field used for magnetic field modulation may be on the order of 60 to 150 Oe, well below the demagnetizing fields of many other recording techniques using existing media.
In accordance with the present invention, there is provided a recording medium having a magneto-optic recording layer with a reduced thickness, resulting in a reduced demagnetizing threshold. The demagnetizing field threshold of a magnetic body varies with changes in the geometry of that body. For example, the demagnetizing field threshold of a magneto-optic thin film can be reduced by reducing its thickness. By reducing the thickness of the recording layer and tuning associated layers appropriately, the recording medium of the present invention provides effective results for recording techniques involving relatively small magnetic fields, such as magnetic field modulation.
The present invention provides, in one embodiment, a magneto-optic recording medium comprising a magneto-optic recording layer having a thickness of less than or equal to approximately 15 nm.
In another embodiment, the present invention provides a magneto-optic recording medium comprising in order a substrate, a first dielectric layer, a single magneto-optic recording layer, and a second dielectric layer, wherein the recording layer has a thickness of less than or equal to approximately 15 nm.
In a further embodiment, the present invention provides a magneto-optic recording medium comprising in order a substrate, a first dielectric layer, a single magneto-optic recording layer, and a second dielectric layer, wherein the recording layer is selected to provide a demagnetizing field threshold of less than or equal to approximately 150 Oe.
Other advantages, features, and embodiments of the present invention will become apparent from the following detailed description and claims.
REFERENCES:
patent: 4737947 (1988-04-01), Osato et al.
patent: 5282095 (1994-01-01), Spruit et al.
patent: 5573847 (1996-11-01), Treves
patent: 0 498 089 (1992-08-01), None
patent: 0 619 577 (1994-10-01), None
Aspen Frank E.
Schroeder Mark L.
Imation Corp.
Kiliman Leszek
Levinson Eric D.
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