U.S. patent application number 10/010785 was filed with the patent office on 2003-05-15 for antiferromagnetically coupled thin films for magnetic recording.
Invention is credited to Doerner, Mary Frances, Fullerton, Eric E., Margulies, David Thomas T., Tang, Kai.
Application Number | 20030090831 10/010785 |
Document ID | / |
Family ID | 21747417 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030090831 |
Kind Code |
A1 |
Doerner, Mary Frances ; et
al. |
May 15, 2003 |
ANTIFERROMAGNETICALLY COUPLED THIN FILMS FOR MAGNETIC RECORDING
Abstract
The applicants disclose an antiferromagnetically coupled layer
structure for magnetic recording wherein the top ferromagnetic
structure is a bilayer structure including a relatively thin first
sublayer of ferromagnetic material in contact with the
coupling/spacer layer. The first sublayer has a higher magnetic
moment than the second sublayer. The layer structure of the
invention results improved manufacturability and improved
performance. A preferred embodiment of a layer structure according
to the invention includes: a bottom ferromagnetic layer preferably
of CoCr; an antiferromagnetic coupling/spacer layer preferably of
Ru; and a top ferromagnetic structure including a thin first
sublayer of material preferably of CoCr, CoCrB or CoPtCrB, and a
thicker second sublayer of material preferably of CoPtCrB with a
lower moment than the first sublayer.
Inventors: |
Doerner, Mary Frances;
(Santa Cruz, CA) ; Fullerton, Eric E.; (Morgan
Hill, CA) ; Margulies, David Thomas T.; (Gilroy,
CA) ; Tang, Kai; (San Jose, CA) |
Correspondence
Address: |
G. Marlin Knight
P.O. Box 1320
Pioneer
CA
95666
US
|
Family ID: |
21747417 |
Appl. No.: |
10/010785 |
Filed: |
November 9, 2001 |
Current U.S.
Class: |
360/97.11 ;
428/837; G9B/5.241; G9B/5.288 |
Current CPC
Class: |
Y10T 428/1107 20150115;
Y10T 428/12861 20150115; Y10T 428/24942 20150115; B82Y 25/00
20130101; G11B 5/66 20130101; G11B 5/7373 20190501; Y10T 428/12465
20150115; H01F 10/3295 20130101; G11B 5/7369 20190501; G11B 5/012
20130101; Y10S 428/90 20130101; Y10T 428/24975 20150115; G11B
2005/001 20130101; G11B 5/7379 20190501; H01F 10/3218 20130101;
Y10T 428/12847 20150115 |
Class at
Publication: |
360/97.01 ;
428/694.0MM; 428/694.0EC; 428/694.0TS |
International
Class: |
G11B 005/012 |
Claims
1. A thin film magnetic recording medium comprising: an underlayer;
a bottom ferromagnetic layer deposited on the underlayer; a
nonmagnetic coupling/spacer layer adjacent to the bottom
ferromagnetic layer; a first sublayer of ferromagnetic CoCrB or
CoPtCrB adjacent to the coupling/spacer layer, the first sublayer
having a first thickness and a first moment; and a second sublayer
of ferromagnetic material with a second moment and a second
thickness with the second moment being lower than the first moment
and the second thickness being greater than the first thickness,
and the first and second sublayers of ferromagnetic material being
antiferromagnetically coupled to the bottom ferromagnetic
layer.
2. The thin film magnetic recording medium of claim 1 wherein the
second sublayer of ferromagnetic material comprises CoPtCrB.
3. The thin film magnetic recording medium of claim 1 further
comprising a seed layer of RuAI deposited prior to the underlayer
and wherein the underlayer is adjacent to the seed layer.
4. The thin film magnetic recording medium of claim 3 further
comprising a preseed layer deposited prior to the seed layer and
wherein the seed layer is adjacent to the pre-seed layer.
5. The thin film magnetic recording medium of claim 4 wherein the
pre-seed layer is CrTa or AITi.
6. The thin film magnetic recording medium of claim 4 wherein the
pre-seed layer is CrTi.
7. The thin film magnetic recording medium of claim 3 wherein the
underlayer includes CrTi.
8. The thin film magnetic recording medium of claim 1 wherein the
bottom magnetic layer is CoCr.
9. The thin film magnetic recording medium of claim 8 wherein the
nonmagnetic coupling/spacer layer is ruthenium.
10. The thin film magnetic recording medium of claim 9 wherein the
second sublayer of ferromagnetic material includes CoPtCrB.
11. A thin film magnetic recording medium comprising: a pre-seed
layer of CrTi; a seed layer of RuAI deposited on the pre-seed
layer; an underlayer of CrTi deposited on the seed layer; a bottom
ferromagnetic layer of CoCr deposited on the underlayer; a
nonmagnetic coupling/spacer layer of Ru adjacent to the bottom
ferromagnetic layer; a first sublayer of ferromagnetic CoCr, CoCrB
or CoPtCrB adjacent to the coupling/spacer layer, the first
sublayer having a first thickness and a first moment; and a second
sublayer of ferromagnetic CoPtCrB with a second moment and a second
thickness with the second moment being lower than the first moment
and the second thickness being greater than the first thickness,
and the first and second sublayers being antiferromagnetically
coupled to the bottom ferromagnetic layer.
12. A method of fabricating a magnetic thin film storage medium
comprising the steps of: depositing an underlayer of a chromium
alloy; depositing a bottom ferromagnetic layer of a cobalt alloy on
the underlayer; depositing a nonmagnetic coupling/spacer layer on
the bottom ferromagnetic layer; depositing a first sublayer of
ferromagnetic CoCrB or CoPtCrB on the coupling/spacer layer, the
first sublayer having a first thickness and a first moment; and
depositing a second sublayer of ferromagnetic material with a
second moment and a second thickness with the second moment being
lower than the first moment and the second thickness being greater
than the first thickness, and the first and second sublayers of
ferromagnetic material being antiferromagnetically coupled to the
bottom ferromagnetic layer.
13. The method of claim 12 wherein the second sublayer of
ferromagnetic material comprises CoPtCrB.
14. The method claim 12 further comprising the step of depositing a
seed layer of RuAI deposited prior to the step of depositing the
underlayer and wherein the underlayer is deposited on the seed
layer.
15. The method of claim 14 further comprising the step of
depositing a pre-seed layer of CrTi, CrTa or AITi prior to the seed
layer and wherein the seed layer is deposited on the pre-seed
layer.
16. The method of claim 12 wherein the bottom magnetic layer is
CoCr.
17. The method of claim 16 wherein the nonmagnetic coupling/spacer
layer is ruthenium.
18. The method of claim 17 wherein the second sublayer of
ferromagnetic material includes CoPtCrB.
19. A disk drive comprising: a magnetic transducer including a read
and a write head; a spindle; and a magnetic thin film disk mounted
on the spindle, the magnetic thin film disk including: a bottom
ferromagnetic layer deposited on the underlayer; a nonmagnetic
coupling/spacer layer adjacent to the bottom ferromagnetic layer; a
first sublayer of ferromagnetic CoCrB or CoPtCrB adjacent to the
coupling/spacer layer, the first sublayer having a first thickness
and a first moment; and a second sublayer of ferromagnetic material
with a second moment and a second thickness with the second moment
being lower than the first moment and the second thickness being
greater than the first thickness, and the first and second
sublayers being antiferromagnetically coupled to the bottom
ferromagnetic layer.
20. The disk drive of claim 19 wherein the bottom ferromagnetic
layer is CoCr.
21. The disk drive of claim 19 wherein the second sublayer is
CoPtCrB.
22. The disk drive of claim 19 wherein the magnetic thin film disk
further comprises a seed layer of RuAI under the underlayer.
23. The disk drive of claim 22 wherein the magnetic thin film disk
further comprises a pre-seed layer of CrTi under the seed layer.
Description
FIELD OF THE INVENTION
[0001] The invention relates to magnetic thin film media with
antiferromagnetically coupled ferromagnetic layers and more
particularly to materials used for the plurality of thin films in
such media.
BACKGROUND OF THE INVENTION
[0002] A typical prior art head and disk system 10 is illustrated
in block form in FIG. 1. In operation the magnetic transducer 20 is
supported by the suspension 13 as it flies above the disk 16. The
magnetic transducer 20, usually called a "head" or "slider," is
composed of elements that perform the task of writing magnetic
transitions (the write head 23) and reading the magnetic
transitions (the read head 12). The electrical signals to and from
the read and write heads 12, 23 travel along conductive paths
(leads) 14 which are attached to or embedded in the suspension 13.
The magnetic transducer 20 is positioned over points at varying
radial distances from the center of the disk 16 to read and write
circular tracks (not shown). The disk 16 is attached to a spindle
18 that is driven by a spindle motor 24 to rotate the disk 16. The
disk 16 comprises a substrate 26 on which a plurality of thin films
21 are deposited. The thin films 21 include ferromagnetic material
in which the write head 23 records the magnetic transitions in
which information is encoded.
[0003] The conventional disk 16 includes substrate 26 of glass or
AIMg with an electroless coating of Ni.sub.3P that has been highly
polished. The thin films 21 on the disk 16 typically include a
chromium or chromium alloy underlayer and at least one
ferromagnetic layer based on various alloys of cobalt. For example,
a commonly used alloy is CoPtCr. Additional elements such as
tantalum and boron are often used in the magnetic alloy. A
protective overcoat layer is used to improve wearability and
corrosion resistance. Various seed layers, multiple underlayers and
laminated magnetic films have all been described in the prior art.
The laminated magnetic films have included multiple ferromagnetic
layers that are ferromagnetically coupled and more recently
antiferromagnetic coupling has been proposed. Seed layers have been
suggested for use with nonmetallic substrate materials such as
glass. Typically the seed layer is a relatively thin layer which is
the first crystalline film deposited in the structure and is
followed by the underlayer. Materials proposed for use as seed
layers include chromium, titanium, tantalum, MgO, tungsten, CrTi,
FeAI, NiAI and RuAI. The use of preseed layers 31 is relatively
recent practice. The pre-seed layer is a noncrystalline thin film
which provides a base for growing the subsequent crystalline films
that is superior to the substrate for this purpose. It is known
that substantially improved SNR can be achieved by the use of a
laminated magnetic layer of two (or more) separate magnetic layers
that are separated by a nonmagnetic spacer layer. The reduced media
noise is believed due to reduced exchange coupling between the
magnetic layers. The use of lamination for noise reduction has been
extensively studied to find the favorable spacer layer materials,
including Cr, CrV, Mo and Ru, and spacer thicknesses, from a few
Angstroms upward, that result in the best decoupling of the
magnetic layers and the lowest media noise.
[0004] In U.S. Pat. No. 6,280,813 to Carey, et al. (which is
commonly assigned with the present application) a layer structure
is described that includes at least two ferromagnetic films
antiferromagnetically coupled together across a nonferromagnetic
coupling/spacer film. In general, it is said that the exchange
coupling oscillates from ferromagnetic to antiferromagnetic with
increasing coupling/spacer film thickness and that the preferred 6
Angstrom thickness of the ruthenium coupling/spacer layer was
selected because it corresponds to the first antiferromagnetic peak
in the oscillation for the particular thin film structure.
Materials that are appropriate for use as the nonferromagnetic
coupling/spacer films include ruthenium (Ru), chromium (Cr),
rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. Because
the magnetic moments of the two antiferromagnetically coupled films
are oriented antiparallel, the net remanent magnetization-thickness
product (Mrt) of the recording layer is the difference in the Mrt
values of the two ferromagnetic films. This reduction in Mrt is
accomplished without a reduction in the thermal stability of the
recording medium because the volumes of the grains in the
antiferromagnetically coupled films add constructively. An
embodiment of the structure includes two ferromagnetic CoPtCrB
films, separated by a Ru spacer film having a thickness selected to
maximize the antiferromagnetic exchange coupling between the two
CoPtCrB films. The top ferromagnetic layer is designed to have a
greater Mrt than the bottom ferromagnetic layer, so that the net
moment in zero applied magnetic field is low, but nonzero. The
Carey '813 patent also states that the antiferromagnetic coupling
is enhanced by a thin (5 Angstroms) ferromagnetic cobalt interface
layer added between the coupling/spacer layer and the top and/or
bottom ferromagnetic layers. The patent mentions, but does not
elaborate on the use CoCr interface layers.
SUMMARY OF THE INVENTION
[0005] The applicants disclose an antiferromagnetically coupled
layer structure for magnetic recording wherein the top
ferromagnetic structure is a bilayer structure including a
relatively thin first sublayer of ferromagnetic material in contact
with the coupling/spacer layer. The first sublayer has a higher
magnetic moment than the second sublayer. The second sublayer has a
lower magnetic moment and is much thicker than the first sublayer
with a composition and thickness selected to provide the Mrt when
combined with first sublayer that is needed for the overall
magnetic structure. The layer structure of the invention results
improved manufacturability and improved performance. A preferred
embodiment of a layer structure according to the invention is:
[0006] a pre-seed layer preferably of CrTi
[0007] a seed layer preferably of RuAI;
[0008] an underlayer preferably of CrTi;
[0009] a bottom ferromagnetic layer preferably of CoCr;
[0010] an antiferromagnetic coupling/spacer layer preferably of Ru;
and
[0011] a top ferromagnetic structure including:
[0012] a thin first sublayer of material preferably of CoCr, CoCrB
or CoPtCrB, and
[0013] a thicker second sublayer of material preferably of CoPtCrB
with a lower moment than the first sublayer.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 is a symbolic illustration of the prior art showing
the relationships between the head and associated components in a
disk drive.
[0015] FIG. 2 is an illustration of a layer structure for a
magnetic thin film disk according to the invention.
[0016] FIG. 3 is an illustration of a magnetic layer stack
including a bilayer upper ferromagnetic layer structure for a
magnetic thin film disk according to the invention.
[0017] FIG. 4 is a graph of coercivity (Hcr) versus interface
sublayer thickness for four different cobalt alloys used
experimentally in thin film structures according to the
invention.
[0018] FIG. 5 is a graph of KuV/kT versus interface sublayer
thickness for four different cobalt alloys used experimentally in
thin film structures according to the invention.
DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED
EMBODIMENTS
[0019] FIG. 2 illustrates the layer structure 21 of a thin film
magnetic disk 16 according to the invention. The layers under the
underlayer 33 may be any of several combinations of seed layers 32
and pre-seed layers 31 as noted in more detail below. The
applicants' most preferred structure includes a pre-seed layer of
CrTi.sub.50, a seed layer of RuAI (B2 crystal structure) and a
CrTi.sub.10 underlayer. The substrate 26 can be any prior art
substrate material with glass being preferred.
[0020] The magnetic layer stack 34 is composed of a plurality of
layers which are further illustrated in FIG. 3. A magnetic layer
stack 34 according to the invention has at least four distinct
layers. The bottom magnetic layer 44 is a ferromagnetic material of
the type used in the prior art of thin film disks. Co-pending
bearing application Ser. No. 09/631,908 filed Aug. 04, 2000 and
which is commonly assigned with the present application describes
considerations in selecting the material for the bottom magnetic
layer 34. Examples of materials suitable for bottom magnetic layer
44 include CoCr, CoPtCr and CoPtCrB with CoCr being preferred by
the applicants with 10 to 14 at.% chromium. The thickness of the
bottom magnetic layer 44 must be chosen in relation to the
thickness of the top magnetic layer structure 40 as indicated
above, but 10-30 Angstroms is a preferred range. The
coupling/spacer layer 43 is a nonmagnetic material with a thickness
that is selected to antiferromagnetically couple the top magnetic
layer structure 40 with the bottom magnetic layer 44. Ruthenium is
the preferred material for the coupling/spacer layer 43, but the
prior art indicates that suitable materials include chromium (Cr),
rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. The
thickness of the exchange/spacer layer 43 is according to the prior
art; for example, approximately 6 Angstroms is a preferred target
thickness for a ruthenium coupling/spacer layer 43. The top
magnetic layer structure 40 according to the invention is a bilayer
structure including two distinct ferromagnetic materials. The
interface (first) sublayer 42 is a thin layer of material with a
relatively high moment, that is, a moment higher than the second
sublayer. The preferred materials for the interface sublayer 42 are
CoCr, CoCrB and CoPtCrB. The preferred thickness is approximately
3-15 Angstroms. The interface sublayer material is selected to have
a higher magnetic moment than the second sublayer. As is known in
the art, the moment of the cobalt alloys is decreased in varying
degrees by the amounts of the alloying elements, e.g., platinum,
chromium and boron, that are included. Therefore, even though the
first and second sublayers may contain the same elements, the
atomic percentages will be different to achieve the goal of a
higher moment in the first sublayer.
[0021] The second sublayer has a lower magnetic moment and is also
much thicker than the first sublayer with a composition and
thickness selected to provide the Mrt when combined with first
sublayer that is needed for the overall magnetic structure. The top
ferromagnetic layer structure, i.e., the first and second sublayer,
is designed to have a greater Mrt than the bottom ferromagnetic
layer, so that the net moment in zero applied magnetic field is
low, but nonzero.
[0022] The layer structure of the invention results in improved
manufacturability and improved performance. Unlike some prior art
designs which required an interface layer on each side of
exchange/spacer layer, the applicants' design uses only one
interface sublayer which results in a simplification of the
manufacturing process. The interface sublayer of the invention also
improves the SNR by lowering the intergranular exchange in the top
magnetic layer structure in comparison to that obtained with a
cobalt interface layer. This in turn allows interface sublayer of
the invention to be somewhat thicker than a cobalt interface
sublayer. As extremely thin as the interface sublayer must be, a
wider tolerance in thickness is an improvement in
manufacturability.
[0023] FIG. 4 is a graph of coercivity (Hcr) versus interface
sublayer thickness for four different cobalt alloys used
experimentally in thin film structures according to the invention.
The magnetic layer stack for each of the samples included a bottom
magnetic layer of either CoCr11 or CoCr14, a coupling/spacer layer
of ruthenium and a top magnetic layer of CoPt12Cr19B8. The absolute
values of coercivity of the magnetic layer stacks varied somewhat
between the base films, but the trend according to the thickness of
the interface sublayer is the significant aspect of the data. The
four compositions of interface sublayers used in the experiment
were CoCr11, CoCr16, CoCr14B6 and CoCr16B4. The coercivity of each
of the test samples was increased by the inclusion of the interface
sublayer. The CoCr interface sublayers increased the coercivity the
most at a thickness of between 3 and 4 Angstroms, with less
improvement with increased thickness. The CoCrB interface sublayers
showed a more constant contribution from about 3 Angstroms to 7
Angstroms in thickness. Therefore, the CoCrB interface sublayers
would allow a wider range of thicknesses to be used in
manufacturing process control, but the CoCr alloys provide a
stronger effect under optimum conditions. FIG. 5 is a graph of the
thermal stability parameter KuV/kT versus interface sublayer
thickness for the four different cobalt alloy interface sublayers
discussed above. The graph shows that KuV/kT was increased
significantly with each alloy even at a thickness between 3 and 4
Angstroms. Although not shown in the data in FIGS. 4 and 5, CoPtCrB
with a composition in the ranges listed below is also a suitable
material for the interface sublayer.
[0024] The preferred material for second sublayer magnetic film 41
is CoPtCrB which is generally described in the U.S. Pat. No.
5,523,173 to Doerner, et al. The preferred magnetic layer
composition in atomic percent is CoPt.sub.xCr.sub.yB.sub.z
where:
[0025] 10<x<16;
[0026] 14<y<20; and
[0027] 6<z<12.
[0028] The optimum platinum percentage (x) depends on the desired
coercivity for the application.
[0029] The thin film structure of the invention is intended for use
as a longitudinal recording medium. For longitudinal magnetic
recording the preferred orientation of the grains should be one of
the two c-axis in-plane orientations with the [10{overscore (1)}0]
or [11{overscore (2)}0] planes parallel to the surface. Cobalt
alloy magnetic films may be grown with the preferred orientations
of (10{overscore (1)}0) or (11{overscore (2)}0) by first depositing
an underlayer with a (112) or (200) preferred orientations
respectively. Co-pending, commonly assigned bearing U.S. patent
applications Ser. Nos. 09/295,267 and 09/547,439 describe the use
of a RuAI seed layer with a B2 crystallographic structure to obtain
an underlayer with a preferred in-plane orientation of (200) and a
cobalt alloy magnetic film with the preferred in-plane orientation
of (11{overscore (2)}0). Therefore, the embodiments of the thin
film structure of the invention that include RuAI as the seed layer
will result in a preferred orientation of (11{overscore (2)}0).
Co-pending, commonly assigned bearing U.S. patent application Ser.
No. 09/500,710 describes the use of an amorphous or nanocrystalline
CrTa or AITi as a pre-seed layer, sputter deposited onto a
nonmetallic substrate, such as glass, followed by a RuAI seed layer
with B2 structure. As stated in the referenced application, a CrTa
or AITi pre-seed layer may also be used effectively with a RuAI
seed layer to obtain smaller overall grain size, as well as, a
reduction in manufacturing costs by saving on the relatively high
cost of ruthenium. The resulting increased coercivity also allows
use of a thinner Cr alloy underlayer which also contributes to
decreased grain size. The use of a pre-seed layer of CrTa or AITi
improves grain size, grain distribution, in-plane crystallographic
orientation, coercivity and SNR.
[0030] A CrTi pre-seed layer, which is preferred over CrTa or AITi,
also allows the use of a thinner RuAI seed layer to reduce the
overall grain size, as well as, manufacturing costs from reduced
use of ruthenium. The use of the CrTi preseed layer increases
coercivity which then allows use of a thinner Cr alloy underlayer
contributing to decreased grain size. As in the case of the CrTa
and AITi pre-seed layers, the use of a pre-seed layer of CrTi
improves grain size, grain distribution, in-plane crystallographic
orientation, coercivity and SNR. In addition to providing excellent
magnetic and recording performance, the CrTi preseed layer improves
the mechanical properties. Specifically the adhesion of the thin
films to the glass surface is increased and the resistance to
scratching is improved. The preferred thickness for the CrTi
pre-seed layer is from 10 nm to 100 nm. The preferred compositional
range is from 35 at.% to 55 at.% titanium.
[0031] The thin film structures described above can be formed using
standard sputtering techniques. The films are sequentially sputter
deposited with each film being deposited on the previous film. The
atomic percent compositions given above are given without regard
for the small amounts of contamination that invariably exist in
sputtered thin films as is well known to those skilled in the
art.
[0032] The invention has been described with respect to particular
embodiments, but other uses and applications for the bilayer
ferromagnetic structure according to the invention will be apparent
to those skilled in the art.
* * * * *