Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2003-07-23
2004-12-21
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013420, C369S275200, C369S288000
Reexamination Certificate
active
06834026
ABSTRACT:
TECHNICAL FIELD
This invention relates to thermally-assisted magnetic recording (TAMR) disk drives, in which data is written while the magnetic recording layer is at an elevated temperature, and more particularly to a TAMR disk that has a ferromagnetic recording layer exchange-coupled to an antiferromagnetic-to-ferromagnetic switching layer.
BACKGROUND OF THE INVENTION
Magnetic recording disk drives use a thin film inductive write head supported on the end of a rotary actuator arm to record data in the recording layer of a rotating disk. The write head is patterned on the trailing surface of a head carrier, such as a slider with an air-bearing surface (ABS) to allow the slider to ride on a thin film of air above the surface of the rotating disk. The write head is an inductive head with a thin film electrical coil located between the poles of a magnetic yoke. When write current is applied to the coil, the pole tips provide a localized magnetic field across a gap that magnetizes regions of the recording layer on the disk so that the magnetic moments of the magnetized regions are oriented into one of two distinct directions. The transitions between the magnetized regions represent the two magnetic states or binary data bits. The magnetic moments of the magnetized regions are oriented in the plane of the recording layer in longitudinal or horizontal recording, and perpendicular to the plane in perpendicular or vertical recording.
The magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data bits are written precisely and retain their magnetization state until written over by new data bits. The data bits are written in a sequence of magnetization states to store binary information in the drive and the recorded information is read back with a use of a read head that senses the stray magnetic fields generated from the recorded data bits. Magnetoresistive (MR) read heads include those based on anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) such as the spin-valve type of GMR head, and more recently magnetic tunneling, such as the magnetic tunnel junction (MTJ) head. Both the write and read heads are kept in close proximity to the disk surface by the slider's ABS, which is designed so that the slider “flies” over the disk surface as the disk rotates beneath the slider.
The areal data density (the number of bits that can be recorded on a unit surface area of the disk) is now approaching the point where magnetic grains that make up the data bits are so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, a minimal stability ratio of stored magnetic energy per grain, K
U
V, to thermal energy, k
B
T, of K
U
V/k
B
T>>60 will be required where K
U
and V are the magneto-crystalline anisotropy and the magnetic switching volume, respectively, and k
B
and T are the Boltzman constant and absolute temperature, respectively. Because a small number of grains of magnetic material per bit are required to prevent unacceptable media noise, the switching volume V will have to decrease, and accordingly K
U
will have to increase. However, increasing K
U
also increases the switching field, H
0
, which is proportional to the ratio K
U
/M
S
, where M
S
is the saturation magnetization (the magnetic moment per unit volume). (The switching field H
0
is the field required to reverse the magnetization direction, which for most magnetic materials is very close to but slightly greater than the coercivity or coercive field H
C
of the material.) Obviously, H
0
cannot exceed the write field capability of the recording head, which currently is limited to about 9 kOe for longitudinal recording, and perhaps 15 kOe for perpendicular recording.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted magnetic recording (TAMR), wherein the magnetic material is heated locally to near or above its Curie temperature during writing to lower the coercivity enough for writing to occur, but high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature). Several approaches for heating the media in TAMR have been proposed, including use of a laser beam or ultraviolet lamp to do the localized heating, as described in “Data Recording at Ultra High Density”,
IBM Technical Disclosure Bulletin
, Vol. 39, No. 7, July 1996, p. 237; “Thermally-Assisted Magnetic Recording”,
IBM Technical Disclosure Bulletin
, Vol. 40, No. 10, October 1997, p. 65; and IBM's U.S. Pat. No. 5,583,727. A read/write head for use in a TAMR system is described in U.S. Pat. No. 5,986,978, wherein a special optical channel is fabricated adjacent to the pole or within the gap of a write head for directing laser light or heat down the channel. IBM's pending application Ser. No. 09/608,848 filed Jun. 29, 2000 describes a TAMR disk drive wherein the thin film inductive write head includes an electrically resistive heater located in the write gap between the pole tips of the write head for locally heating the magnetic recording layer.
Generally, K
U
and M
S
of a magnetic material decrease with temperature according to K
U
(T)~M
S
(T)
n
(e.g., n=3 for cubic materials), such that H
0
=&agr;K
U
/M
S
also decreases steadily with increasing temperature, where &agr;≅1 for isotropic media and &agr;≅2 for highly oriented media. Therefore by heating the media during the write process and letting it cool to room temperature, the write field constraint of the head can be circumvented while at the same time retaining the long time thermal stability of the stored magnetizations representing the recorded data bits. However, for materials with a very high magneto-crystalline anisotropy this requires writing at close to the Curie temperature of the media. In IBM's pending application Ser. No. 09/874,100 filed Jun. 4, 2001, a technique is described that uses a bilayer of two ferromagnetic materials. The first ferromagnetic layer is formed of a high-coercivity (or magnetically “hard”) material that has a room temperature coercivity too high for writing with a conventional write head and a low Curie temperature. The second ferromagnetic layer directly above or below the first layer is formed of a low-coercivity (or magnetically “soft”) material with a coercivity suitable for writing with a conventional write head and a high Curie temperature. During writing the bilayer is heated to a temperature of about or slightly above the Curie temperature of the first layer, thereby reducing or eliminating the coercivity of the first layer. The bit pattern is then recorded in the second layer. The two layers then cool, and as the first layer cools to below its Curie temperature, it becomes ferromagnetic again and the bit pattern is “copied” from the second layer into the first layer by magnetic exchange interaction. Upon further cooling the anisotropy of the first layer returns to its original high value, thus providing the desired long-term stability of the recorded data bits.
A practical implementation of this type of TAMR disk uses a layer of low coercivity material suitable for writing with a conventional write head and with a high Curie temperature T
CL
(e. g., a granular CoPtCrB alloy), and a layer of high coercivity material incapable of being written to by a conventional write head and with a low Curie temperature T
CH
(e. g., chemically-ordered high anisotropy FePt).
The switching field H
0
of such a bilayer material is best approximated by
H
0
=
α
·
K
H
t
H
+
K
L
t
L
M
H
t
H
+
M
L
t
L
where K
H
and K
L
are the anisotropy constants of the high and low coercivity layers, respectively, M
H
and M
L
are the saturation magnetiza
Fullerton Eric E.
Maat Stefan
Thiele Jan-Ulrich
Berthold Thomas R.
Neyzari Ali
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