Dynamic magnetic information storage or retrieval – Monitoring or testing the progress of recording
Reexamination Certificate
2001-02-21
2003-08-05
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
Monitoring or testing the progress of recording
C360S066000, C360S068000, C360S053000
Reexamination Certificate
active
06603617
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnetic storage devices and, more particularly, to computer disk drives. More specifically, the present invention relates to compensating for amplitude and BER (bit error rate) loss due to media thermal decay.
BACKGROUND OF THE INVENTION
Computer disk drives store digital information on magnetic disks which are coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. Typically, the digital information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk.
More specifically, during operation of a conventional disk drive, a magnetic transducer is placed above a desired track of the disk while the disk is spinning. Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track which constitute the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly-timed digital signal that can be recognized by a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Typically, transducers include separate elements for reading and writing. Such transducers are known as “dual element heads” and usually include a magneto-resistive (MR) read element or giant magneto-resistive (GMR) read element for performing the read function.
Dual element heads are advantageous because each element of the transducer can be optimized to perform its particular function. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and, thus, can read much fainter signals from the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface with no loss of read performance.
MR read elements generally include a strip of magneto-resistive material that is held between two magnetic shields. The resistance of the magneto-resistive material varies almost linearly with applied magnetic field. During a read operation, the MR strip is held near a desired track, with the varying magnetic field caused by the magnetic transitions on the track. A constant DC current is passed through the strip resulting in a variable voltage across the strip. By Ohm's law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host. GMR read elements operate in a similar manner.
FIGS.
1
(
a
)-
1
(
e
) are simplified diagrammatic representations which illustrate how data is written as transitions on a disk surface and how the transitions are read from the disk surface as data. As background, a transition is where the magnetization in the disk media changes. In general, there are two types of transitions possible; that is, where south poles face south poles and where north poles face north poles.
FIGS.
1
(
a
)-(
c
) illustrate the write process in simplified form. Specifically, FIG.
1
(
a
) illustrates a data sequence in the form of “ones” and “zeros,” which is to be stored on the disk media. FIG.
1
(
b
) illustrates the write current in the write coil for one method of storing the data sequence. In such method, the current through the write coil is reversed at each “one” and remains the same at each “zero” (see FIGS.
1
(
a
) and
1
(
b
)). Consequently, as the disk media is rotated under the write head, the disk media is magnetized as shown in FIG.
1
(
c
). It should be noted that magnetic transitions occur at each “one” and not at each “zero.” It should also be noted that FIG.
1
(
c
) represents the magnetization of the media for a portion of a track, which is shown in a linear rather than arcuate shape, as will be understood by those skilled in the art.
FIGS.
1
(
d
) and
1
(
e
) illustrate the read process in simplified form. As mentioned above, as the disk media is rotated under the read head, a constant DC current is passed through the MR strip in the read head. The magnetic transitions stored in the disk media cause the magnetic field applied to the MR strip in the read head to vary, as shown in FIG.
1
(
d
). Since the resistance of the magneto-resistive material varies almost linearly with applied magnetic field, the varying magnetic field caused by the magnetic transitions on the disk media results in a variable voltage across the strip. By Ohm's law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track, as shown in FIG.
1
(
e
). The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
The amount of information capable of being stored on a disk surface is determined, in part, by the minimum size of individual transitions. As is known to those skilled in the art, the minimum size of individual transitions is based (among other things) upon the grain size of the magnetic material forming the magnetic layer of the disk surface. In order to increase the amount of information capable of being stored on the disk surface, disk manufacturers have been continuously reducing the grain size of the magnetic material and, hence, have reduced the minimum size of individual transitions. For the magnetic layer of the disk, the remnant magnetization-thickness product has also been reduced to achieve higher linear densities and enhanced writer performance. Most of this reduction has been achieved by reducing the thickness of the magnetic layer of the disk, and hence, the grain thickness, which reduces the grain size.
Traditionally, about 500 to 1000 grains of magnetic material were required to store a bit of information. However, at present, a transition may be stored in about 100 grains of magnetic material. It is expected that the number of grains of magnetic material required to store a bit of information will continue to decrease over time. To reduce transition noise and increase the number of grains in a transition, both the diameter of the grains and the separation between the grains have been decreased. In fact, the diameter of the grains has decreased from approximately 15 nm down to approximately 9-10 nm. This has driven disk vendors to produce disks with smaller grain volumes.
As will be understood by those skilled in the art, each grain has a certain magnetic anisotropy energy associated with it. More specifically, the anisotropy energy of a grain is a fixed amount of energy required to “hold” a stored direction of magnetization in the magnetic material, and is equal to the anisotropy energy density, Ku, times the volume of the grain, V. A thermal instability ratio is defined as the anisotropy energy divided by the thermal energy, KT, and is given by the formula KuV/KT, which should be greater than 50 for adequate thermal
Faber Alan T.
Hansra Tejpal S.
Maxtor Corporation
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