U.S. patent application number 10/664207 was filed with the patent office on 2004-04-15 for magnetic devices with a ferromagnetic layer having perpendicular magnetic asisotropy and an antiferromagnetic layer for perpendicularly exchange biasing the ferromagnetic layer.
Invention is credited to Fullerton, Eric Edward, Maat, Stefan, Parkin, Stuart Stephen Papworth, Takano, Kentaro.
Application Number | 20040070890 10/664207 |
Document ID | / |
Family ID | 25095164 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040070890 |
Kind Code |
A1 |
Fullerton, Eric Edward ; et
al. |
April 15, 2004 |
Magnetic devices with a ferromagnetic layer having perpendicular
magnetic asisotropy and an antiferromagnetic layer for
perpendicularly exchange biasing the ferromagnetic layer
Abstract
The invention is a magnetic device that includes a
ferromagnetic/antiferro- magnetic (F/AF) structure wherein the
ferromagnetic layer is perpendicularly exchange biased by the
antiferromagnetic layer. The invention has application to
perpendicular magnetic recording disks and magnetic tunnel junction
devices used as read heads for disk drives and memory cells in
magnetic memory arrays.
Inventors: |
Fullerton, Eric Edward;
(Morgan Hill, CA) ; Maat, Stefan; (San Jose,
CA) ; Parkin, Stuart Stephen Papworth; (San jose,
CA) ; Takano, Kentaro; (San Jose, CA) |
Correspondence
Address: |
DANIEL E. JOHNSON
IBM CORPORATION, ALMADEN RESEARCH CENTER
INTELLECTUAL PROPERTY LAW DEPT. C4TA/J2B
650 HARRY ROAD
SAN JOSE
CA
95120-6099
US
|
Family ID: |
25095164 |
Appl. No.: |
10/664207 |
Filed: |
September 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10664207 |
Sep 16, 2003 |
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09772468 |
Jan 29, 2001 |
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6650513 |
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Current U.S.
Class: |
360/324.2 ;
428/811.1; 428/828; G9B/5.114; G9B/5.241 |
Current CPC
Class: |
G11B 5/3903 20130101;
G01R 33/093 20130101; H01F 10/3218 20130101; G11B 5/3909 20130101;
B82Y 25/00 20130101; B82Y 10/00 20130101; H01F 10/3286 20130101;
Y10T 428/1114 20150115; G11B 5/66 20130101; H01F 10/3254 20130101;
H01F 10/30 20130101 |
Class at
Publication: |
360/324.2 ;
428/694.00T |
International
Class: |
G11B 005/127 |
Claims
1. A magnetic device comprising: a layer of ferromagnetic material;
a layer of antiferromagnetic material interfacially exchange
coupled with the ferromagnetic layer, whereby the ferromagnetic
layer exhibits perpendicular exchange bias.
2. The device according to claim 1[Claim Reference] wherein the
antiferromagnetic material is selected from the group consisting of
a cobalt oxide, a nickel oxide, an oxide of an alloy of cobalt and
nickel, and a platinum-manganese alloy.
3. The device according to claim 1[Claim Reference] wherein the
layer of ferromagnetic material has in-plane magnetic anisotropy,
and wherein the layer of antiferromagnetic material is
interfacially exchange coupled with the ferromagnetic layer at the
edges of the ferromagnetic layer, whereby the ferromagnetic layer
exhibits perpendicular exchange bias at said edges.
4. The device according to claim 1[Claim Reference] wherein the
layer of ferromagnetic material has perpendicular magnetic
anisotropy.
5. The device according to claim 4[Claim Reference] wherein the
ferromagnetic material having perpendicular magnetic anisotropy is
selected from the group consisting of a cobalt-platinum-chromium
alloy, an iron-platinum alloy, one or more cobalt-platinum
bilayers, and one or more cobalt-palladium bilayers.
6. The device according to claim 5[Claim Reference] where the
cobalt-platinum-chromium alloy includes one or more of B, Nb and
Ta.
7. The device according to claim 5[Claim Reference] wherein any one
of the cobalt layers in said bilayers includes one or more of the
elements selected from the group consisting of B, Ta, Cr, O, Cu and
Ag.
8. The device according to claim 1[Claim Reference] wherein the
device is a magnetic recording medium.
9. The device according to claim 8[Claim Reference] further
comprising a substrate, an underlayer on the substrate, and a
capping layer, and wherein the exchange-coupled ferromagnetic layer
and antiferromagnetic layer are located between the underlayer and
the capping layer.
10. The device according to claim 1[Claim Reference] wherein the
device is a magnetic tunnel junction read head.
11. The device according to claim 10[Claim Reference] wherein the
ferromagnetic layer is the sensing ferromagnetic layer of the
magnetic tunnel junction and has two side edges, and wherein the
antiferromagnetic layer comprises two portions, each portion being
in contact with a respective side edge of the ferromagnetic
layer.
12. The device according to claim 1[Claim Reference] wherein the
device is a magnetic tunnel junction memory cell having an
insulating tunnel barrier layer.
13. The device according to claim 12[Claim Reference] wherein the
ferromagnetic layer has its magnetic moment pinned in a direction
perpendicular to the barrier layer of the magnetic tunnel junction
by being perpendicularly exchange biased by the antiferromagnetic
layer.
14. A magnetic device comprising: a substrate; and a bilayer of a
ferromagnetic layer and an antiferromagnetic layer on the
substrate, the ferromagnetic layer having perpendicular magnetic
anisotropy with its magnetic moment oriented generally
perpendicular to the plane of the ferromagnetic layer and being
perpendicularly biased by the antiferromagnetic layer.
15. The device according to claim 14[Claim Reference] wherein the
material of the antiferromagnetic layer is selected from the group
consisting of a cobalt oxide, a nickel oxide, an oxide of an alloy
of cobalt and nickel, and a platinum-manganese alloy.
16. The device according to claim 14[Claim Reference] wherein the
material of the ferromagnetic layer is selected from the group
consisting of a cobalt-platinum-chromium alloy, an iron-platinum
alloy, one or more cobalt-platinum bilayers, and one or more
cobalt-palladium bilayers.
17. The device according to claim 14[Claim Reference] wherein the
device is a perpendicular magnetic recording disk and further
comprising an underlayer located between the substrate and the
bilayer, and wherein the antiferromagnetic layer is a layer of
nickel-oxide directly on the underlayer.
18. The device according to claim 14[Claim Reference] wherein the
ferromagnetic layer comprises a cobalt-platinum-chromium alloy
directly on the antiferromagnetic layer.
19. The device according to claim 14[Claim Reference] wherein the
device is a magnetic tunnel junction memory cell having a pinned
ferromagnetic layer, a free ferromagnetic layer and an insulating
tunnel barrier layer between the pinned and free layers, wherein
the material of the antiferromagnetic layer is a platinum-manganese
(PtMn) alloy, and wherein the ferromagnetic layer is the pinned
layer with its magnetic moment pinned in a direction perpendicular
to the tunnel barrier layer by being perpendicularly exchange
biased by the PtMn alloy antiferromagnetic layer.
Description
BACKGROUND OF INVENTION
[0001] This invention relates in general to magnetic devices, such
as magnetic recording disks and heads and magnetic tunnel
junctions, and more particularly to magnetic devices that use
exchange-coupled antiferromagnetic/ferromagnetic (AF/F)
bilayers.
[0002] The exchange biasing of a ferromagnetic (F) film by an
adjacent antiferromagnetic (AF) film is a phenomenon that has
proven to have many useful technological applications in magnetic
devices, and was first reported by W. H. Meildejohn and C. P. Bean,
Phys. Rev. 102, 1413 (1959). Whereas the magnetic hysteresis loop
of a ferromagnetic single layer film is centered about zero field,
a F/AF bilayer often will show an asymmetric magnetic hysteresis
loop which is shifted from zero magnetic field in the plane of the
film by an exchange-bias field, H.sub.B. The direction of the
exchange bias field within the plane of the film can be varied. In
particular, this direction can be set during the growth of the AF
film and is determined by the orientation of the magnetic moment of
the F film when the AF film is deposited on top of the F film. The
direction of the exchange bias field can also be changed by heating
the F/AF bilayer after growth above the so-called blocking
temperature, T.sub.B, of the AF film. The blocking temperature is
typically close to but below the Nel or magnetic ordering
temperature of the AF film. In addition to an offset of the
magnetic hysteresis loop of the F film, the F film in a F/AF
bilayer typically shows an increased coercivity below T.sub.B. The
detailed mechanism which determines the magnitude of the exchange
bias field and the increased coercive field is still a matter of
considerable debate but it is generally agreed that these effects
arise from an interfacial interaction between the F and AF films.
See, for example the articles by J. Nogus and I. K. Schuller, J.
Magn. Magn. Mat. 192, 203 (1999); and A. and K. Takano, J. Magn.
Magn. Mat. 200, 552 (1999). Exchange-biased thin films have found
several important applications, especially for magnetic recording
read heads. In particular, exchange biasing can be used to locally
harden or stiffen the magnetic response of, for example, the edges
of a ferromagnetic sensing film in an anisotropic magnetoresistive
read head. Exchange-biased ferromagnetic films can also be used to
provide local magnetic fields such as those required for optimally
biasing magnetoresistive read sensors. Exchange biasing is also an
integral component of giant magnetoresistive spin-valve heads. More
recently, exchange biasing has been used to engineer magnetic
tunnel junction devices for use in magnetic recording read sensors
applications and in magnetic random access memories, as described
for example in IBM's U.S. Pat. No. 5,650,958.
[0003] Although exchange biasing has been widely studied and has
found important technological applications, the phenomenon of
exchange biasing has only previously been observed in F/AF thin
film systems in which the moment of the ferromagnetic film lies in
the plane of the film. For many applications, particularly for
advanced magnetic recording media and advanced magnetic recording
read heads, it would be extremely useful to be able to exchange
bias ferromagnetic films whose moments lie perpendicular to the
plane of the film.
[0004] What is needed is a means of providing perpendicular
exchange bias for perpendicularly magnetized ferromagnetic
films.
SUMMARY OF INVENTION
[0005] The invention is a magnetic device that includes a
ferromagnetic/antiferromagnetic (F/AF) structure wherein the
ferromagnetic layer has perpendicular magnetic anisotropy by being
exchange coupled with the antiferromagnetic layer. When used in
perpendicular magnetic recording disks, the F/AF structure can be a
Co layer and a CoO AF layer located on top of the Co layer where
the Co layer is the top layer in a series of Co/Pt bilayers, or a
NiO AF layer and a Co layer located on top of the NiO layer, where
the Co layer is the bottom layer in a series of Co/Pt bilayers.
When used in a magnetic tunnel junction read head, the F/AF
structure is the ferromagnetic free layer that is longitudinally
biased at its edges by being perpendicularly exchange coupled to AF
insulating oxide layers at these edges. When used in magnetic
tunnel junction memory cells the F/AF structure is the fixed
magnetic layer and a AF conducting layer where the AF layer
exhibits perpendicular exchange biasing to the adjacent fixed
ferromagnetic layer.
[0006] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF DRAWINGS
[0007] FIG. 1A: Co/Pt multilayer with perpendicular anisotropy
exchange biased with a CoO layer.
[0008] FIG. 1B: NiO antiferromagnetic layer exchange biased with
Co/Pt multilayer with perpendicular anisotropy.
[0009] FIG. 1C: PtMn antiferromagnetic layer exchange biased with
Co/Pt multilayer with perpendicular anisotropy.
[0010] FIGS. 2A and 2B: Out of plane hysteresis loops of a [Co(4
.ANG.)/Pt(5 .ANG.)].sub.5 multilayer sample with (2A) and without
(2B) a Co+CoO cap measured at 10.degree. K and 300.degree. K
[0011] FIG. 3: Temperature dependence of the coercivity H.sub.C
(squares) and loop shift H.sub.B (circles) of the out of plane
hysteresis loops of the [Co(4 .ANG.)/Pt(5 .ANG.)].sub.5 multilayers
with and without (triangles) Co+CoO (15 .ANG.) cap.
[0012] FIG. 4: Temperature dependence of the coercivity H.sub.C
(squares) and loop shift H.sub.B (circles) of the out of plane
hysteresis loops of a [Co(4 .ANG.)/Pt(5 .ANG.)] film with a Co+CoO
(15 .ANG.) cap.
[0013] FIG. 5: Normalized magnetization perpendicular to the plane
of the field versus perpendicular magnetic field for three
Si/SiO.sub.2/150 .ANG.Pt/500 .ANG.NiO/[x .ANG.Co/7 .ANG.Pt].sub.N
samples for Co thicknesses of .about.7, 8 and 9 .ANG. and N is
1.
[0014] FIG. 6: Perpendicular exchange bias field (filled circles)
and coercivity (open squares) of a family of Si/SiO.sub.2/150
.ANG.Pt/500 .ANG.NiO/[x .ANG.Co/7 .ANG.Pt].sub.N multilayers as a
function of thickness of the Co layer for N=1.
[0015] FIG. 7: Normalized perpendicular magnetization versus
perpendicular magnetic field for a series of samples with structure
Si/SiO.sub.2/50 .ANG.Ta/150 .ANG.Pt/PtMn(t.sub.PtMn)/[7 .ANG.Co/5
.ANG.Pt].sub.3/20 .ANG.Pt for t.sub.PtMn varying from 40 to 300
.ANG..
[0016] FIG. 8: Perpendicular exchange bias field H.sub.B and
coercive field H.sub.C as a function of PtMn thickness for a series
of films with the structure Si/SiO.sub.2/50 .ANG.Ta/150
.ANG.Pt/PtMn(t.sub.PtMn)/[7 .ANG.Co/5 .ANG.Pt].sub.3/20
.ANG.Pt.
[0017] FIG. 9: Dependence of exchange bias field on Pt thickness
for a series of structures of the form Si/SiO.sub.2/50 .ANG.Ta/150
.ANG.Pt/300 .ANG.PtMn/[7 .ANG.Co/t.sub.Pt .ANG.Pt].sub.N/20 .ANG.Pt
for N=1 to 4 bilayers and for Pt thickness t.sub.Pt varying from 2
to 16 .ANG..
[0018] FIGS. 10A, 10B and 10C: Sectional schematic views of
structures for perpendicular magnetic recording media.
[0019] FIG. 11: A schematic sectional view of a prior art MTJ MR
read head illustrating he arrangement of the various layers of
material, including the structure for providing longitudinal
biasing of the sensing ferromagnetic layer.
[0020] FIG. 12: A schematic sectional view of an MTJ MR read head
according to the present invention illustrating the arrangement of
the various layers of material, including the AF/F bilayer
structure for providing longitudinal biasing of the sensing
ferromagnetic layer without the need for an insulating layer
between the longitudinal bias layer and the sensing ferromagnetic
layer.
[0021] FIG. 13: A prior art MTJ memory cell showing the AF/F
bilayer structure for biasing the moment of the pinned
ferromagnetic layer in the plane of the layer.
[0022] FIG. 14: An MTJ memory cell according to the present
invention showing the AF/F bilayer structure for biasing the moment
of the pinned ferromagnetic layer perpendicular to the plane of the
layer.
DETAILED DESCRIPTION
[0023] Numerous F/AF structures have been studied, where the AF
layer is typically an alloy of Mn, for example, Mn--Fe, Mn--Ir,
Mn--Pt, Mn--Pd, Mn--Pd--Pt, Cr--Mn, or Cr--Mn--Pt alloys, or NiO or
CoO or and oxide of Ni and Co, and where the F layer is typically
formed from either Co, Ni, or Fe or a binary or ternary alloy of
these elements. However, exchange biasing has only previously been
reported for F layers that are magnetized in the plane of the F
layer. This limits the type of magnetic thin film structures that
can be engineered to take advantage of exchange biasing to those in
which the magnetic moment of the F layer lies within the plane of
the thin film structure, i.e., those in which the F layer has an
in-plane magnetic easy anisotropy axis.
[0024] In the present invention, the principle of perpendicular
exchange biasing and coercivity enhancement has been demonstrated,
i.e., exchange biasing for AF/F structures for which the F layer
has an out-of-plane ferromagnetic easy axis and for a variety of
well-known AF materials. The magnitude of the perpendicular
exchange bias effect is comparable to that found in similar F/AF
structures for which the F layer has an in-plane magnetic easy
axis.
[0025] The use of perpendicular exchange bias, the subject of this
invention, will be described with reference to two important
applications, namely that of improved high density magnetic
recording media and that of improved magnetic tunnel junction
recording read heads
[0026] Perpendicular Exchange Bias Materials
[0027] Perpendicular exchange bias materials Ferromagnetic Co films
can show a magnetic anisotropy easy axis, which is either in-plane
or perpendicular to the plane film, depending on their thickness.
In particular, whereas Co films thicker than .about.10 .ANG.
exhibit an in-plane easy axis, thinner Co films are known to show
an out-of-plane easy axis. To obtain stronger out-of-plane magnetic
anisotropy, thin Co films can be grown in a multilayered structure,
where the thin Co layers are separated by a noble metal, such as Au
or Pt. This is because the out-of-plane magnetic anisotropy, for
such ultra-thin Co layers, is a result of an interfacial magnetic
anisotropy which depends sensitively on the interface between the
Co layer and the adjacent layers. The anisotropy of the bulk or
volume of the Co film is typically in-plane although this can also
have an out-of-plane component depending on the crystal structure
and texture of the Co film. The same is true for Co--X alloys where
X is, for example, Cr, Pt and Pd or a combination of these and
other elements
[0028] FIG. 1A shows the structure of a sputtered multilayer of
[CO/Pt].sub.N where N is the number of Co/Pt repetitions, in which
N was varied from 1 to 10. Co/Pt multilayers are candidates for
perpendicular magnetic recording media and have also been used for
perpendicular magneto-optic recording. The multilayer was deposited
using dc magnetron sputtering in 3 mTorr argon at 150.degree. C. on
a Si (100) substrate covered with 20 .ANG.Si.sub.3N.sub.4. A seed
or buffer layer of 150 .ANG.Pt was deposited first on the substrate
prior to the multilayer deposition. On top of the Co/Pt multilayer
was deposited a thin metallic Co layer, which was allowed to
oxidize in the ambient atmosphere at room temperature to form CoO,
which is an antiferromagnet. The magnetic properties of the films
were measured in a SQUID magnetometer for temperatures in the range
10-300.degree. K with the magnetic field aligned either in the
plane or perpendicular to the plane of the Co layers.
[0029] FIG. 2A shows the magnetization M normalized with respect to
the saturation magnetization Ms of a [Co(4 .ANG.)/Pt(5
.ANG.)].sub.5 multilayered thin film sample with a Co/CoO capping
layer versus out-of-plane magnetic field. The sample exhibits a
square out-of-plane hysteresis loop with a coercive field
H.sub.C.about.460 Oe at 300.degree. K. After cooling the sample tol
10.degree. K in a positive magnetic field of magnitude 4 kOe, the
magnetization versus out-of-plane magnetic field hysteresis loop is
observed to be shifted to a negative magnetic exchange bias field
H.sub.B=-0.9 kOe and has a significantly increased coercive field,
H.sub.C=3.1 kOe. The hysteresis loop of a similar sample without
the Co/CoO cap layer showed no evidence of exchange bias after a
similar field-cooling treatment, as shown in FIG. 2B. To determine
the magnitude of the coercive field enhancement for the
exchange-biased sample the temperature dependence of the coercive
field of both [Co(4 .ANG.)/Pt(5 .ANG.)].sub.5 multilayered samples
with and without Co/CoO cap layers were measured. At 10.degree. K
the coercive field of the sample without the Co/CoO cap layer was
only .about.1.2 kOe, about 2.5 times smaller than the sample with
the Co/CoO cap layer. Thus the presence of the antiferromagnetic
CoO layer is shown to give rise to both a perpendicular exchange
bias field and an increased coercivity to the perpendicularly
magnetized Co/Pt ferromagnetic multilayer.
[0030] The interfacial exchange-bias energy per unit area .sigma.
of the [Co(4 .ANG.)/Pt(5 .ANG.)].sub.5/Co/Coo sample at 10.degree.
K is calculated to be .about.0.4 erg/cm.sup.2 using the
relation
.sigma.=M.sub.StH.sub.B
[0031] where M.sub.S is the magnetic moment per unit volume,
H.sub.B the exchange-bias field and t the thickness of the
ferromagnetic layer. Here M.sub.S is taken to be that of bulk Co at
10.degree. K (.about.1420 emu/cm.sup.3) and t is the magnetic
thickness of the [Co(4 .ANG.)/Pt(5 .ANG.)].sub.5/11 .ANG.Co/4
.ANG.Coo multilayer, i.e., .about.31 .ANG.. Note that the value of
the exchange-bias energy is comparable to that observed for
longitudinal biasing using thicker Co layers in Co/CoO bilayers, as
described by T. Ambrose and C. L. Chien, Phys. Rev. Lett. 76, 1743
(1996).
[0032] The temperature dependence of H.sub.C and H.sub.B are shown
in FIG. 3. The temperature at which H.sub.B vanishes is defined as
the blocking temperature, T.sub.B. From FIG. 3, a blocking
temperature of .about.220.degree. K can be inferred, which is
approximately 70.degree. K below the bulk Neel temperature of CoO
(293.degree. K). For the sample without the CoO layer, H.sub.C was
found to depend linearly on temperature and coincides with the
H.sub.C of the biased sample at .about.250.degree. K, which is the
same as the Nel temperature of a thin CoO film. So, dearly the
enhanced coercivity below 250.degree. K arises from the interaction
between the Co/Pt multilayer and the AF CoO layer. Due to the
interfacial nature of the AF/F exchange coupling, the biasing
effect should scale approximately with the inverse thickness of the
ferromagnetic layer, which is well documented for longitudinal
biasing for F layers above some critical thickness. If this
dependence is assumed to also hold true for perpendicular exchange
biasing, then the magnitude of H.sub.C and H.sub.B for
perpendicular exchange biasing should be enhanced significantly if
the CoO layer is coupled only to a single bilayer of Co(4
.ANG.)/Pt(5 .ANG.), instead of five bilayers. Indeed, this
assumption was verified, as indicated by the temperature dependence
of H.sub.C and H.sub.B shown in FIG. 4 (obtained after cooling the
sample to 10.degree. K in a field of 4 kOe). Note that at
10.degree. K, for example, the magnitude of the exchange bias field
is .about.2.4 kOe and the coercive field is .about.9 kOe, which are
significantly higher than values of H.sub.B and H.sub.C found for
the multilayer sample of FIG. 3.
[0033] Since the Nel temperature of CoO is approximately room
temperature (.about.290.degree. K), this is not a useful material
for most applications, particularly for magnetic recording
applications. However, the Nel temperature of the related
antiferromagnetic oxide, NiO, is well above room temperature and
just like CoO is well known to give rise to longitudinal exchange
bias. Similarly, the family of oxides Co.sub.1-xNi.sub.xO are known
to give rise to longitudinal exchange bias and have varying Nel
temperatures intermediate between those of CoO and NiO, as
described by M. J. Carey and A. E. Berkowitz, Appl. Phys. Lett. 60,
3060 (1992).
[0034] FIG. 1B shows the structure of a perpendicular exchange
biased system with NiO antiferromagnetic bias layers prepared by dc
magnetron sputtering at ambient temperature (.about.40.degree. C.)
on Si(100) wafers covered with -5000 .ANG.thick SiO.sub.2 layers
(prepared by wet oxidation of the Si wafer). First a seed or buffer
layer of 150 .ANG.Pt was deposited, followed by a 500 .ANG. thick
NiO layer which was grown by reactive sputtering from a metallic Ni
target in an Ar(65%)-O.sub.2(35%) gas mixture. The composition of
NiO was measured using the Rutherford backscattering (RBS)
technique on a companion sample comprised solely of NiO deposited
on a silicon wafer on which any silicon oxide was removed by a HF
acid etch. On top of the NiO layer was grown a multilayer of
[CO/Pt].sub.N for which the thickness of the Co and Pt layers and
the number of Co/Pt bilayer repeats were varied. The structure was
capped with a thin 20 .ANG. Pt layer to protect the Co/Pt
multilayer from oxidation. FIG. 5 shows normalized magnetization
versus perpendicular field loops measured at room temperature for
three typical samples comprised of Si/SiO.sub.2/150 .ANG.Pt/500
.ANG.NiO/[x .ANG.Co/7 .ANG.Pt].sub.N for Co thicknesses of
.about.7, 8 and 9 .ANG. and N is 1. The magnetic hysteresis loop
shows a substantial perpendicular exchange bias field of .about.200
The exchange bias field was set by heating the sample to
200.degree. C. for 30 minutes and cooling the sample in a field of
1 kOe to room temperature. The as-deposited sample showed a much
smaller exchange bias field of .about.25 Oe. For this sample the
NiO is polycrystalline but crystalline NiO films were prepared by
reactive sputtering of NiO on Ag underlayers, which were prepared
on hydrogen terminated silicon wafers. Using Si(111) and Si(100)
wafers [111] and [100] oriented NiO films were prepared,
respectively. Perpendicular exchange bias was found for both
crystalline and polycrystalline NiO layers.
[0035] FIG. 6 shows the perpendicular exchange bias field (filled
circles) and coercivity (open squares) of a family of
Si/SiO.sub.2/150 .ANG.Pt/500 .ANG.NiO/[x .ANG.Co/7 .ANG.Pt].sub.N
multilayers as a function of the thickness of the Co layer for N=1.
The multilayer no longer exhibits a perpendicular easy axis of
magnetization for Co layers thicker than .about.11 .ANG.. These
results clearly establish perpendicular exchange bias at room
temperature.
[0036] CoO, NiO and Co.sub.1-xNi.sub.xO are Insulating
antiferromagnetic layers. However, perpendicular exchange bias is
not solely a property of insulating antiferromagnetic layers. FIG.
7 shows the structure of a perpendicular exchange biased system
with PtMn as the antiferromagnetic layer. These structures were
deposited by dc magnetron sputtering in 3 mTorr argon at ambient
temperature on Si(100) wafers covered with .about.5000 .ANG.
SiO.sub.2. First 50 .ANG.Ta/150 .ANG.Pt underlayers were deposited
to provide a suitable template layer for the PtMn layer. The PtMn
films had a composition of .about.Pt.sub.48Mn.sub.52 as determined
by RBS measurements on companion films. [Co/Pt]N multilayers were
grown on top of the PtMn layers with varying Co and Pt thicknesses
and varying number of Co/Pt bilayers. Magnetic hysteresis loops are
shown in FIG. 7 for a set of films with [7 .ANG.CO/5 .ANG.Pt].sub.3
ferromagnetic layers and PtMn layers with thickness t.sub.PtMn
which was varied from 40 to 300 .ANG.. FIG. 7 shows that
significant exchange bias fields are observed at room temperature.
No exchange bias field was found in the as-deposited films. In
order to obtain longitudinal exchange bias with PtMn
antiferromagnetic layers the Pt.sub.1-xMn.sub.x alloy has to have a
composition in the range x-0.45-0.49 and the PtMn alloy has to be
chemically ordered. The chemical ordering is accomplished by
annealing the structure at an elevated temperature for a suitable
length of time. The structures here were annealed at 280.degree. C.
for 60 minutes in a perpendicular field of .about.1000 Oe and were
subsequently cooled to ambient temperature in the same field. The
perpendicular exchange bias fields and coercive fields for these
structures are shown in FIG. 8 as a function of PtMn thickness.
Exchange bias fields of up to .about.200 Oe are observed at room
temperature.
[0037] The strength of the perpendicular exchange bias field
depends on the thickness of the Co and Pt layers and the number of
Co/Pt bilayers. FIG. 9 shows the dependence of exchange bias field
on the thickness of Pt for a series of structures of the form
Si/SiO.sub.2/50 .ANG.Ta/150 .ANG.Pt/300 .ANG.PtMn/[7
.ANG.Co/t.sub.Pt .ANG. Pt].sub.N/20 .ANG.Pt for N=1 to 4 bilayers
and for Pt thickness t.sub.Pt varying from 2 to 16 .ANG.. The Co/Pt
multilayer has a perpendicular easy axis for t.sub.Pt<.about.10
.ANG.. For thicker Pt layers the easy anisotropy axis is in the
plane of the film. For thinner Pt layers the exchange bias field
H.sub.B is increased to more than 300 For N=1 for thin Pt the
coercive field is quite small in the range of 50-100 Oe depending
on the Pt thickness. We conclude that substantial perpendicular
exchange bias fields can be readily obtained using PtMn
antiferromagnetic layers.
[0038] First Embodiment of Invention: Perpendicular Magnetic
Media
[0039] The areal density, i.e., the number of magnetic bits per
unit area of magnetic media, in magnetic disk drives has
significanUy increased in recent years and is continuing to
increase very rapidly. Whereas today the vast majority of magnetic
recording disk drives use longitudinally magnetized magnetic media,
perpendicularly magnetized magnetic media may have technological
advantages for ultra-high density magnetic recording for areal
densities above .about.100 Gbit/in.sup.2. Useful magnetic thin film
materials for perpendicular recording include CoCrPt and FePt
alloys and Co/Pd, Co/Pt and Fe/Pt multilayers. For improved
recording performance, the above alloys and multilayers are often
doped with additional elements. For example, CoCrPtX alloys are
commonly used for longitudinal recording, where X=B, Nb, and Ta as
the typical dopants. Such structures are also proposed for
perpendicular recording. Similarly, the Co layers in Co/Pd and
Co/Pt multilayers are often replaced by Co--Y alloys where Y=B, Ta,
Cr, O, Cu, Ag, Pt, Pd. In the following we will refer to CoCrPt and
FePt alloys and Co/Pd and Co/Pt multilayers but the results and
discussion apply equally to CoCrPtX and FePtX alloys and CoY/Pd and
CoY/Pt alloys.
[0040] As the size of the magnetic bits shrink the magnetic bits
become more susceptible to demagnetization from thermal
fluctuations, the so-called superparamagnetic effect. The magnetic
anisotropy of the magnetic material may be increased to stabilize
the magnetic bits from the superparamagnetic effect but the
magnetic anisotropy of suitable magnetic materials is limited.
Moreover, it may be difficult to find suitable magnetic materials
with the required combination of properties including flatness,
thickness, magnetic moment per unit area and magnetic anisotropy.
Alternative methods to increase the magnetic stability of magnetic
media against thermal fluctuations are needed. It has been
demonstrated for longitudinally magnetized magnetic media that one
method to increase the coercivity and stability of the
ferromagnetic layer without increasing its magnetic moment is by
exchange coupling the ferromagnet to an antiferromagnet using the
well known in-plane exchange bias effect.
[0041] The magnetic stability of the magnetic media in a
perpendicular magnetic recording device can be enhanced by contact
with an AF layer that is exchange coupled to the media. FIGS.
10A-10C illustrate improved magnetic media according to this
invention. The enhanced magnetic stability can be achieved by
locating an AF layer 310 at the top of the ferromagnetic layer 305,
as shown in FIG. 10A and as demonstrated in FIG. 1A, or by locating
the AF layer 310' at the bottom of the ferromagnetic layer 305', as
shown in FIG. 10C and demonstrated in FIGS. 1B and 1C. The AF can
also be located within the ferromagnetic media layer (306 and 308)
itself, as shown in FIG. 10B. Combining the different structures
shown in FIGS. 10A-10C is also possible. The choice of the AF layer
material and its location within the structure will depend on the
ferromagnetic layer material.
[0042] As described above exchange coupling of a F layer to an AF
layer leads to both an increase of the coercivity of the F layer as
well as an offset of the magnetic hysteresis loop of the F layer.
For magnetic media applications it is advantageous to have a
coercivity enhancement without any loop shift. This is commonly
observed for many F/AF coupled magnetic system when the layer
thickness of the AF layer is below a critical thickness. This
phenomenon, coercivity enhancement without a loop shift, is also
observed for systems that show a high amount of interfacial F/AF
spin-frustration. This condition occurs at spin-compensated AF
surfaces, i.e., AF surfaces that exhibit no net moment. To minimize
energy the F spins may undergo a spin-flop transition with respect
to the AF spins. The resulting increased coercivity of the F layer
has been calculated recently by T. C. Schulthess and W. H. Butler,
J. Appl. Phys. 85, 5510 (1999) using a micromagnetic model. An
experimental example for longitudinal biasing without loop-shift
are FeRh.sub.0.95Ir.sub.0.05/NiFe bilayers studied by S. Yuasa, M.
Nyvlt, T. Katayama, and Y. Suzuki, J. Appl. Phys. 83, 6813 (1998).
Typical ferromagnetic layers, 305, 305', 306 and 308, in which data
is recorded for both longitudinal and perpendicular magnetic
recording, are alloys of Co--Pt--Cr. These films must be deposited
on suitable underlayers to promote their proper crystallographic
structure and orientation. For perpendicular recording the
Co--Pt--Cr layer must have the c(easy magnetic axis) of its hcp
structure oriented normal to the Co--Pt--Cr film plane. In the
media shown in FIG. 10A this is achieved by the proper choice of
the underlayer 301 structure and orientation. Common examples are
(0002) oriented layers of hcp materials (for example, Ti or
non-magnetic Co--Cr alloys) or (111) oriented layers of fcc
materials such as Pt. The media is covered with an overcoat or
capping layer 320 (or 320' or 320" in FIGS. 10C and 10B,
respectively) that protects the media from wear and oxidation.
Thus, if the AF layer is located below or within the media (AF
layers 310' and 310" In FIGS. 10C and 10B, respectively), the AF
layer must itself have suitable structural characteristics to
promote the proper growth of the media layer in the required
crystallographic orientation. For Co--Pt--Cr alloy media layers,
the underlayers must have the correct crystallographic symmetry to
promote c-axis, small-grain growth. This can be achieved by the
(111) growth orientation of fcc underlayers or by the c-axis growth
of hcp underlayers. For Fe--Pt alloy recording layers, the required
symmetry of the AF layer would be (001) orientation of cubic AF
materials to promote the growth the FePt perpendicular to the film.
Multilayer media, such as Co/Pd or Co/Pt multilayers, do not in
general require a preferred crystallographic axis and therefore can
be grown on any crystallographic orientation of the AF layer and
maintain the perpendicular anisotropy.
[0043] When the AF layer is grown on top of the media layer (layer
310 in FIG. 10A) the orientation of the AF layer will not effect
the growth of the media layer and thus the AF layer can be chosen
to optimize the exchange interaction between the ferromagnetic and
AF layers. However, a disadvantage of this structure is an
increased spacing between the media and recording head leading to
increased spacing loss.
[0044] Second Embodiment of Invention: Improved Longitudinally
Biased Magnetic Tunnel Junction Read Head
[0045] MTJ devices have been proposed as memory cells for
solid-state memory and for use in magnetoresistive (MR) sensors,
especially for magnetic recording read heads. A magnetic tunnel
junction (MTJ) device is comprised of two ferromagnetic layers
separated by a thin insulating tunnel barrier layer and is based on
the phenomenon of spin electron tunneling. One of the ferromagnetic
layers has a higher saturation field in one direction of an applied
magnetic field, typically due to its higher coercivity than the
other ferromagnetic layer. The insulating tunnel barrier layer is
thin enough that quantum mechanical tunneling occurs between the
ferromagnetic layers. The tunneling phenomenon is electron
dependent, making the magnetic response of the MTJ a function of
the relative orientations and spin polarizations of the two
ferromagnetic layers. Usually the tunneling probability of the
charge carriers is highest when the magnetic moments of the
ferromagnetic layers are parallel to one another. Thus, the
resistance of the MTJ device is lowest when the magnetic moments of
both layers are parallel, and is highest when the magnetic moments
are antiparallel. When the moments are arranged neither parallel
nor antiparallel, the tunneling probability and the resistance take
an intermediate value.
[0046] One of the problems with an MR read head lies in developing
a structure that generates an output signal that is both stable and
linear with the magnetic field strength from the recorded medium.
If some means is not used to maintain the ferromagnetic sensing
layer of the MTJ device (i.e., the ferromagnetic layer whose moment
is not fixed) in a single magnetic domain state, the domain walls
of magnetic domains will shift positions within the ferromagnetic
sensing layer, causing noise which reduces the signal to noise
ratio and which may give rise to an irreproducible response of the
head. A linear response of the head is required. The problem of
maintaining a single magnetic domain state is especially difficult
in the case of an MTJ MR read head because the sense current passes
perpendicularly through the ferromagnetic layers and the tunnel
barrier layer, and thus any metallic materials in direct contact
with the edges of the ferromagnetic layers will short circuit the
electrical resistance of the read head.
[0047] One solution is described in IBM's U.S. Pat. No. 5,729,410
as shown in FIG. 11. This patent describes an MTJ device with one
fixed ferromagnetic layer 118 and one sensing ferromagnetic layer
132 on opposite sides of the tunnel barrier layer 120. The moments
of the fixed and sensing ferromagnetic layers are oriented
longitudinally in the plane of these layers. There is also a
hard-biasing ferromagnetic layer 150 that is electrically insulated
from but yet magnetostaticly coupled (as illustrated by the arrows
153) with the sensing ferromagnetic layer 132. The magnetic tunnel
junction in the MTJ device is formed on an electrical lead 102 on a
substrate and is made up of a stack of layers. The layers in the
stack are an underlayer 112, which may be formed from several
layers, an antiferromagnetic layer 116, a fixed ferromagnetic layer
118 exchange biased with the antiferromagnetic layer so that its
magnetic moment 119 cannot rotate in the presence of an applied
magnetic field, an insulating tunnel barrier layer 120 in contact
with the fixed ferromagnetic layer 118, and a sensing ferromagnetic
layer 132 in contact with the tunnel barrier layer 120 and whose
magnetic moment 133 is free to rotate in the presence of an applied
magnetic field. The stack is generally rectangularly shaped with
parallel side edges so that the layers have contiguous edges. A
layer of hard biasing ferromagnetic material 150 is located near to
but spaced from the side edges of the sensing ferromagnetic layer
to longitudinally bias the magnetic moment of the sensing
ferromagnetic layer in a preferred direction within the plane of
this layer in the absence of an applied magnetic field. A layer of
electrically insulating material 220 isolates the hard biasing
material from the electrical lead and the sensing ferromagnetic
layer so that sense current is not shunted to the hard biasing
material but is allowed to flow perpendicularly through the layers
in the magnetic tunnel junction stack.
[0048] In the prior art MTJ MR sensor shown in FIG. 11 since the
biasing layer is formed from a metallic ferromagnetic material the
biasing layer 150 must be isolated from the MTJ sensor by an
insulating layer 220. The insulating layer 220 must have a
thickness sufficient to electrically isolate the biasing layer 150
from the MTJ and the electrical leads but must be thin enough to
permit magnetostatic coupling (shown by dashed arrow) with the
sensing ferromagnetic layer 132. This makes the fabrication of the
MTJ MR sensor more complicated than would be possible if the
longitudinal bias layer 150 could be in direct contact with the MTJ
stack.
[0049] In the present invention, as shown in FIG. 12, the
longitudinal bias layer 150' can be in direct contact with the MTJ
stack because it is formed from an insulating material. In
particular, by taking advantage of perpendicular exchange bias, the
layer 150' is formed from a layer of insulating NiO which is in
direct contact with the edges of the MTJ sensor. Preferably the NiO
layer is only in contact with the edges of the sensing
ferromagnetic layer 132', but because the moment of the
ferromagnetic layer 118' is fixed by exchange bias with the
conducting antiferromagnetic bias layer 116', the layer 150' can
also be in contact with edges of the fixed ferromagnetic layer
118'. The layer 150' can also be In contact with the edges of the
antiferromagnetic bias layer 116'. Most importantly because the
layer 150' is formed from an insulating antiferromagnetic material
the layer 150' can be in contact with both the free and fixed
ferromagnetic layers without electrically shorting these layers.
Note that even though the magnetic moment of the ferromagnetic
layer 132' is in the plane of the layer, its magnetic moment along
the sides of the layer will point perpendicularly to these edge
surfaces as indicated by the arrow 133'. Thus the antiferromagnetic
oxide layer 150' can provide a perpendicular exchange biasing field
to longitudinally stabilize the in-plane magnetic moment of the
free layer 132'.
[0050] The perpendicular biasing antiferromagnetic layer can be
formed from any insulating antiferromagnetic material which
exhibits the perpendicular exchange bias effect. For example this
layer can be formed from an oxide of Ni--Co for which the blocking
temperature is intermediate between that of CoO and NiO. Similarly,
this layer could be formed from an insulating antiferromagnetic
iron oxide, for example, .alpha.-Fe.sub.2O.sub.3. The direction of
the perpendicular exchange bias field, shown by the arrow 151',
must be set so that it is along the same direction as the magnetic
moment of the sensing ferromagnetic layer in the absence of an
applied magnetic field. Since the moment of the fixed ferromagnetic
layer 118' is fixed by the longitudinal exchange bias layer 116' it
is important that the blocking temperature of the layer 116' is
much higher than that of the perpendicular exchange bias layer
150'. Thus the direction of the longitudinal exchange bias field
provided by the layer 116' is first set by heating the MTJ device
above the blocking temperature of layer 116' and cooling the sensor
in an applied field large enough to orient the moment of layer 118'
in the direction 119'. The direction of the longitudinal bias field
from layer 116' will then be set along the direction 119'. In a
second step the direction of the perpendicular exchange bias field
provided by the regions 150' is set by heating the MTJ sensor above
the blocking temperature of the perpendicular exchange bias layer
150' but well below the blocking temperature of the longitudinal
exchange bias layer 116'. The MTJ sensor is then cooled in a
magnetic field large enough to align the moment of the sense layer
along the direction 153'. Suitable materials for the longitudinal
exchange bias layer 116' include Pt--Mn and Ni--Mn alloys which
have sufficiently high blocking temperatures compared to the
blocking temperature of NiO which forms the perpendicular exchange
bias layer 150'. The layer 116' could also be formed from an Ir--Mn
alloy layer.
[0051] Third Embodiment of Invention: Magnetic Tunnel Junction
Memory Storage Cell with Perpendicularly Exchange Biased Pinned
Ferromagnetic Layer and Perpendicularly Magnetized Free Layer
[0052] Magnetic tunnel junction structures can be used as
non-volatile memory storage cells as described in IBM's U.S. Pat.
No. 5,650,958. The MTJ memory storage cell in the prior art is
comprised of two ferromagnetic layers each with their moments in
the plane of these layers. A prior art MTJ storage cell is shown in
FIG. 13. The cell is comprised of metallic lower and upper wires 11
and 50 to provide electrical current to the MTJ cell for reading
the state of the cell. The MTJ cell is further comprised of lower
10 and upper 30 ferromagnetic electrodes which are separated by a
thin insulating tunneling barrier 20. The barrier is a layer of
Al.sub.2O.sub.3 which is formed by first depositing a thin layer of
Al metal and then oxidizing it. The lower MTJ electrode includes an
underlayer 12 which is used to promote the growth of the
antiferromagnetic layer 16, which is used to exchange bias the
ferromagnetic layer 18. Layers 16 and 18 form a AF/F exchange
biased bilayer structure that enables the moment of the F layer 18
to be fixed along a particular direction in the plane of the film
18 by means of exchange biasing with the AF layer 16. The arrow 90
indicates the direction. The direction of the exchange bias field
is fixed by cooling the device from a temperature above the
blocking temperature of the AF and cooling the device through the
blocking temperature to at least the operating temperature of the
device in a magnetic field large enough to saturate the magnetic
moment of the F layer in the direction of the magnetic field. The
upper ferromagnetic electrode 30 is a ferromagnetic layer 32 and an
overlayer 34. The direction of the moment of the F layer 32 is
indicated by the arrow 80 and is in the plane of the layer 32 but
can be either parallel or antiparallel to that of the moment of the
pinned F layer 90. The memory cell has two magnetic states in which
the moments of the pinned layer 18 and that of the free F layer 32
are either parallel or antiparallel and in the plane of the MTJ
device.
[0053] As the size of the MTJ device is shrunk laterally, which is
required for scaling to ever greater memory capacities, the total
moments of each of the F layers becomes smaller and smaller if the
thickness of these layers is maintained approximately constant.
Thus, for small enough MTJ devices, the magnetic anisotropy of
these layers is not sufficient to maintain the magnetic moments in
the required orientation because of the well-known
superparamagnetic effect whereby thermal fluctuations will cause
the magnetic moments to fluctuate. One way to avoid this problem is
to increase the thickness of the F layers but, for sufficiently
narrow MTJ devices, this means that the moments of the F layers
will prefer to align themselves perpendicular to the plane of the
layers. What is needed is a means of pinning the magnetic moment of
the fixed F layer perpendicular to the plane of this layer.
[0054] FIG. 14 shows a MTJ memory cell according to the present
invention in which the magnetic moments of the fixed ferromagnetic
layer 18' and the free ferromagnetic layer 32' are oriented
perpendicular to the plane of the layers, as shown by the arrows
90' and 80' respectively. The moment of the fixed ferromagnetic
layer is pinned by exchange biasing to an AF layer 16' which
displays perpendicular exchange bias. Since current must be passed
perpendicularly through the ferromagnetic layers of the MTJ device
for reading the state of the cell, the AF layer 16' must be
conducting. Thus a suitable AF material is the PtMn alloy as
described above and shown in FIG.
[0055] While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
* * * * *