Title: Magnetoresistive element and method for producing the same, as well as magnetic head, magnetic memory and magnetic recording device using the same
Abstract: The present invention provides a method for producing a magnetoresistive element including a tunnel insulating layer, and a first magnetic layer and a second magnetic layer that are laminated so as to sandwich the tunnel insulating layer, wherein a resistance value varies depending on a relative angle between magnetization directions of the first magnetic layer and the second magnetic layer. The method includes the steps of: (i) laminating a first magnetic layer, a third magnetic layer and an Al layer successively on a substrate; (ii) forming a tunnel insulating layer containing at least one compound selected from the group consisting of an oxide, nitride and oxynitride of Al by performing at least one reaction selected from the group consisting of oxidation, nitriding and oxynitriding of the Al layer; and (iii) forming a laminate including the first magnetic layer, the tunnel insulating layer and a second magnetic layer by laminating the second magnetic layer in such a manner that the tunnel insulating layer is sandwiched by the first magnetic layer and the second magnetic layer. The third magnetic layer has at least one crystal structure selected from the group consisting of a face-centered cubic crystal structure and a face-centered tetragonal crystal structure and is (111) oriented parallel to a film plane of the third magnetic layer. According to this production method, it is possible to produce a magnetoresistive element with excellent properties and thermal stability.
Patent Number: 6,943,041 Issued on 09/13/2005 to Sugita, et al.
| Inventors:
|
Sugita; Yasunari (Osaka, JP);
Odagawa; Akihiro (Tsuchiura, JP);
Matsukawa; Nozomu (Nara, JP);
Kawashima; Yoshio (Neyagawa, JP);
Morinaga; Yasunori (Suita, JP)
|
| Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
719412 |
| Filed:
|
November 21, 2003 |
Foreign Application Priority Data
| Apr 23, 2002[JP] | 2002-120433 |
| Current U.S. Class: |
438/3; 438/238 |
| Intern'l Class: |
H01L 021/00 |
| Field of Search: |
438/3,238-240,381
|
References Cited [Referenced By]
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| 6005753 | Dec., 1999 | Fontana et al.
| |
| 6052263 | Apr., 2000 | Gill.
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| 6169303 | Jan., 2001 | Anthony.
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| 6201259 | Mar., 2001 | Sato et al.
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| 6210818 | Apr., 2001 | Saito.
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| 6226197 | May., 2001 | Nishimura.
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| 6312840 | Nov., 2001 | Kumagai et al.
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| 6387548 | May., 2002 | Hasegawa et al.
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| 6418001 | Jul., 2002 | Nakatani.
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| 2001/0053053 | Dec., 2001 | Saito et al.
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| 2002/0039264 | Apr., 2002 | Ohsawa et al.
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| 2003/0197984 | Oct., 2003 | Inomata et al.
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| 10-162326 | Jun., 1998 | JP.
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| 2002/-204004 | Jul., 2002 | JP.
| |
| 00/74154 | Dec., 2000 | WO.
| |
| 02/088765 | Nov., 2002 | WO.
| |
Other References
Johnson. M, "Symposium on Spin Tunneling and Injection Phenomena", J. Appln.
Phys. 79(8), Apr. 15, 1996, pp. 4724-4729.
Ji Hyung Yu et al. "Magnetic Tunnel Junctions with High Magnetoresistance and
Small Bias Voltage Dependence Using Expitaxial NiFe (111) Ferromagnetic Bottom
Electrodes", Journal of Applied Physics, vol. 93, No. 10, pp. 8555-8557, May 15, 2003.
Koichiro Inomata, MRAM Technology Progress and Prospect Materials Integration
vol. 13, No. 12, P13-18, 2000 (Japanese only).
T.Miyazaki et al., "Giant Magnetic Tunneling Effect in Fe/Al2O3/Fe
Junction", Journal of Magnetism and Magnetic Materials, 139 (1995) L231-L234.
Ping Shang et al., "High-resolution electron microscopy study of tunneling junctions
with AIN and AIon barriers", Journal of Applied Physics, vol. 89, No. 11, pp. 6874-6876,
Jun. 1, 2001.
Yasunari Sugita et al., "Tunneling Magnetoresistance Enhancement for Pt-Added
Magnetic Tunnel Junctions", Japanese Journal of Applied Physics, vol. 41, No. 10A,
pp. L1072-1074, Oct. 1, 2002.
Nozomu Matsuka et al., "Thermally stable exchange-biased magnetic tunnel junctions
over 400° C" Applied Physics Letter, vol. 81, No. 25, pp. 4784-4786, Dec.
16, 2002.
|
Primary Examiner: Tsai; H. Jey
Attorney, Agent or Firm: Merchant & Gould P.C.
Claims
1. A method for producing a magnetoresistive element comprising a tunnel insulating
layer, and a first magnetic layer and a second magnetic layer that are laminated
so as to sandwich the tunnel insulating layer,
wherein a resistance value varies depending on a relative angle between magnetization
directions of the first magnetic layer and the second magnetic layer, the method
comprising the steps of:
(i) laminating a first magnetic layer, a third magnetic layer and an Al layer
successively on a substrate;
(ii) forming a tunnel insulating layer containing at least one compound selected
from the group consisting of an oxide, nitride and oxynitride of Al by performing
at least one reaction selected from the group consisting of oxidation, nitriding
and oxynitriding of the Al layer;
(iii) forming a laminate comprising the first magnetic layer, the tunnel insulating
layer and a second magnetic layer by laminating the second magnetic layer in such
a manner that the tunnel insulating layer is sandwiched by the first magnetic layer
and the second magnetic layer; and
(iv) heat treating the laminate at not less than 350° C.,
wherein the third magnetic layer comprises a magnetic material containing at
least one element selected from the group consisting of Fe, Co, and Ni,
the magnetic material further contains at least one element selected from the
group consisting of Rh, Pd, Ag, Ir, Pt, and Au, and
the third magnetic layer has at least one crystal structure selected from the
group consisting of a face-centered cubic crystal structure and a face-centered
tetragonal crystal structure and is (111) oriented parallel to a film plane of
the third magnetic layer.
2. The method for producing a magnetoresistive element according to claim 1,
wherein the magnetic material has a composition represented by the formula Fe
xCo
y,
where x and y are values satisfying the following equations:
3. The method for producing a magnetoresistive element according to claim 1,
wherein the magnetic material has a composition represented by the formula Fe
x′Ni
y′,
where x′ and y′ are values satisfying the following equations:
4. The method for producing a magnetoresistive element according to claim 1,
wherein the magnetic material has a composition represented by the formula M
pZ
q,
where M is at least one element selected from the group consisting of Fe, Co
and Ni,
Z is at least one element selected from the group consisting of Rh, Pd, Ag, Ir,
Pt and Au, and
p and q are values satisfying the following equations:
5. The method for producing a magnetoresistive element according to claim 1,
wherein an antiferromagnetic layer is laminated between the substrate and the
first magnetic layer in the step (i).
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for producing magnetoresistive elements.
The invention also relates to magnetoresistive elements, as well as magnetic heads,
magnetic memories and magnetic recording devices, which are magnetic devices using
the same.
2. Description of the Related Art
With the recent developments in advanced communication networks, there is a
demand for devices capable of handling a large volume of information at high speeds.
For example, as large-capacity, high-speed devices, expectations are growing for
magnetic heads and magnetic memories (MRAMs) that utilize the tunneling magnetoresistance
effect (TMR effect).
The TMR effect is the phenomenon in which the resistance value varies depending
on a relative angle between the magnetization directions of a pair of magnetic
layers laminated with a tunnel insulating layer interposed therebetween. Magnetoresistive
elements (TMR elements) utilizing this phenomenon have a ratio of change in the
magnetoresistance (MR ratio) in a minute magnetic field that is by far larger than
elements utilizing the anisotropic magnetoresistance effect (AMR effect) or the
giant magnetoresistance effect (GMR effect). Therefore, extensive developments
are under way to apply TMR elements to next-generation magnetic heads and MRAMs.
Each of the layers constituting a TMR element is extremely thin, and is on the
order of several nm to several tens of nm. In order to achieve a TMR element with
excellent magnetoresistance properties (MR properties), it is important to control
these layers. Particularly, the state of a tunnel insulating layer is considered
to have a significant effect on the MR properties of the element.
For example, in the case of using a TMR element for a device such as a magnetic
head, it is preferable to realize a large MR ratio and minimize (e.g., 10 Ω·μm
2
or less) the junction resistance value (resistance value per unit area when
a current is supplied in a direction perpendicular to the film plane direction
of the element). When the junction resistance value is small, it is possible to
suppress, for example, the generation of shot noise, which is the phenomenon of
electrons being transmitted randomly through the tunnel insulating layer (shot
noise causes a reduction in the S/N (signal-to-noise ratio) of the element). The
junction resistance value can be reduced by, for example, decreasing the thickness
of the tunnel insulating layer. However, simply decreasing the thickness of the
tunnel insulating layer possibly may reduce the resulting MR ratio. In general,
the interface between the tunnel insulating layer and a magnetic layer in contact
therewith is not completely smooth, exhibiting a roughness at the atomic level
in the sub-nanometer to several nanometer range. That is, regions in which the
thickness is locally large and regions in which the thickness is locally small
are present in the tunnel insulating layer. Therefore, there is the possibility
that with a decrease in the thickness of the tunnel insulating layer, a leakage
current may be generated in the region in which the thickness is locally small.
Since a leakage current does not contribute to the MR effect, this causes a reduction
in the MR ratio, although the apparent junction resistance value is reduced.
In addition, TMR elements have the problem that the resulting MR ratio decreases
with an increase in the applied bias voltage. In the case of a MRAM using TMR elements,
for example, a bias voltage of about 400 mV generally is applied. In this state,
the resulting MR ratio is about half of that in the state in which no bias voltage
is applied. Such "bias voltage dependence of the MR ratio" is considered to be
attributed to, for example, lattice defects in the tunnel insulating layer, impurities
contained in the tunnel insulating layer, elementary excitations on the interface
between the tunnel insulating layer and the magnetic layer and mismatches in the
band structure. Among them, lattice defects in the tunnel insulating layer, mismatches
in the band structure and the like are believed to be due partly to a roughness
on the interface between the tunnel insulating layer and the magnetic layer.
In the case of using TMR elements for devices such as magnetic heads and MRAMs,
the elements are required to have thermal stability capable of withstanding the
process of manufacturing the devices. For example, heat treatment at about 200°
C. to 300° C. is necessary in the manufacturing process of the elements themselves.
When used for magnetic heads, the elements need to be stable at the operating environment
temperature (e.g., about 120° C. to 170° C.) of the magnetic heads. Research
is also carried out to fabricate MR elements on CMOSs for use as MRAM devices.
Heat treatment at even higher temperatures (e.g., 400° C. to 450° C.)
is necessary in the manufacturing process of CMOSs.
However, the MR properties of conventional TMR elements tend to deteriorate
by heat treatment at about 300° C. to 350° C. This is presumably due
to the diffusion of impurities into the tunnel insulating layer, an increase in
the interface roughness and the like. The tunnel insulating layer has a very small
thickness, so that it is susceptible to such effect. In order to apply TMR elements
to devices such as magnetic heads and MRAMs, it is therefore important to develop
TMR elements whose MR properties tend less to deteriorate when the temperature
of the elements is increased by heat treatment and the like.
SUMMARY OF THE INVENTION
In view of this situation, it is an object of the present invention to provide
a TMR element excellent in properties and thermal stability, and a method for producing
such a TMR element. It is also an object of the present invention to provide a
magnetic head, a magnetic memory and a magnetic recording device that are excellent
in properties and thermal stability. It should be noted that in the present specification,
the TMR element simply may be referred to as "magnetoresistive element" or "MR element".
In order to achieve the forgoing objects, the present invention provides a method
for producing a magnetoresistive element including a tunnel insulating layer, and
a first magnetic layer and a second magnetic layer that are laminated so as to
sandwich the tunnel insulating layer,
wherein a resistance value varies depending on a relative angle between magnetization
directions of the first magnetic layer and the second magnetic layer. The method
includes the steps of:
(i) laminating a first magnetic layer, a third magnetic layer and an Al layer
successively on a substrate;
(ii) forming a tunnel insulating layer containing at least one compound selected
from the group consisting of an oxide, nitride and oxynitride of Al by performing
at least one reaction selected from the group consisting of oxidation, nitriding
and oxynitriding of the Al layer; and
(iii) forming a laminate including the first magnetic layer, the tunnel insulating
layer and a second magnetic layer by laminating the second magnetic layer in such
a manner that the tunnel insulating layer is sandwiched by the first magnetic layer
and the second magnetic layer,
wherein the third magnetic layer has at least one crystal structure selected
from the group consisting of a face-centered cubic crystal structure and a face-centered
tetragonal crystal structure and is (111) oriented parallel to a film plane of
the third magnetic layer. The method for determining whether the layer is (111)
oriented parallel is described later in the examples.
In the production method of the present invention, the third magnetic layer may
include a magnetic material containing at least one element selected from the group
consisting of Fe, Co and Ni.
In the production method of the present invention, the magnetic material may
have
a composition represented by the formula Fe
xCo
y, where x
and y are values satisfying the following equations:
In the production method of the present invention, the magnetic material may
have
a composition represented by the formula Fe
x′Ni
y′,
where x′ and y′ are values satisfying the following equations:
In the production method of the present invention, the magnetic material further
may contain at least one element selected from the group consisting of Rh, Pd,
Ag, Ir, Pt and Au.
In the production method of the present invention, the magnetic material may
have
a composition represented by the formula M
pZ
q, where M is
at least one element selected from the group consisting of Fe, Co and Ni, Z is
at least one element selected from the group consisting of Rh, Pd, Ag, Ir, Pt and
Au, and p and q are values satisfying the following equations:
In the production method of the present invention, an antiferromagnetic layer
may be laminated between the substrate and the first magnetic layer in the step (i).
The production method of the present invention further may include the step of:
(a) heat treating the laminate, after the step (iii).
Next, the present invention provides a magnetoresistive element including:
a tunnel insulating layer containing at least one compound selected from the group
consisting of an oxide, nitride and oxynitride of Al; a first magnetic layer and
a second magnetic layer that are laminated so as to sandwich the tunnel insulating
layer; and a third magnetic layer disposed between the first magnetic layer and
the tunnel insulating layer. A resistance value varies depending on a relative
angle between magnetization directions of the first magnetic layer and the second
magnetic layer, and the third magnetic layer has at least one crystal structure
selected from the group consisting of a face-centered cubic crystal structure and
a face-centered tetragonal crystal structure and is (111) oriented parallel to
a film plane of the third magnetic layer.
In the magnetoresistive element of the present invention, the third magnetic
layer
may include a magnetic material containing at least one element selected from the
group consisting of Fe, Co and Ni.
In the magnetoresistive element of the present invention, the magnetic material
may have a composition represented by the formula Fe
xCo
y,
where x and y are values satisfying the following equations:
In the magnetoresistive element of the present invention, the magnetic material
may have a composition represented by the formula Fe
x′Ni
y′,
where x′ and y′ are values satisfying the following equations:
In the magnetoresistive element of the present invention, the magnetic material
further may contain at least one element selected from the group consisting of
Rh, Pd, Ag, Ir, Pt and Au.
In the magnetoresistive element of the present invention, the magnetic material
may have a composition represented by the formula M
pZ
q, where
M is at least one element selected from the group consisting of Fe, Co and Ni,
Z is at least one element selected from the group consisting of Rh, Pd, Ag, Ir,
Pt and Au, and p and q are values satisfying the following equations:
The magnetoresistive element of the present invention further may include an
antiferromagnetic layer.
In the magnetoresistive element of the present invention, the antiferromagnetic
layer is disposed on a side opposite a plane of the first magnetic layer facing
the tunnel insulating layer and is (111) oriented parallel to a film plane of the
antiferromagnetic layer.
Next, the present invention provides a magnetic head including the above-described
magnetoresistive element and a shield for limiting an introduction of a magnetic
field other than a magnetic field to be detected by the magnetoresistive element
to the magnetoresistive element.
Alternatively, the magnetic head of the present invention also may
include the above-described magnetoresistive element and a magnetic flux guiding
portion for guiding a magnetic field to be detected by the magnetoresistive element
to the magnetoresistive element.
Next, the present invention provides a magnetic memory that may include the
above-described magnetoresistive element and an information recording conductive
line for recording information on the magnetoresistive element and an information
reading conductive line for reading the information.
In the magnetic memory of the present invention, a plurality of the magnetoresistive
elements may be disposed in the form of a matrix.
Next, the present invention provides a magnetic recording device including
one of the above-described magnetic heads and a magnetic recording medium capable
of reading magnetic information with the magnetic head.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1E are schematic cross-sectional views showing the respective
steps of an example of the method of producing a magnetoresistive element according
to the present invention.
FIGS. 2A and 2B are schematic cross-sectional views for illustrating the difference
between a conventional magnetoresistive element and the magnetoresistive element
of the present invention.
FIG. 3 is a schematic cross-sectional view showing an example of the magnetoresistive
element of the present invention.
FIG. 4 is a schematic cross-sectional view showing another example of the magnetoresistive
element of the present invention.
FIG. 5 is a cross-sectional view showing an example of the magnetoresistive
element of the present invention that includes electrodes.
FIG. 6 is a cross-sectional view showing an example of the magnetic head of
the present invention.
FIG. 7 is a cross-sectional view showing another example of the magnetic head
of the present invention.
FIG. 8A is a schematic view showing yet another example of the magnetic head
of the present invention.
FIG. 8B is a cross-sectional view obtained by cutting the magnetic head shown
in FIG. 8A on plane A shown in FIG. 8A.
FIG. 9 is a schematic view showing an example of the magnetic recording device
of the present invention.
FIGS. 10A and 10B are schematic views showing another example of the magnetic
recording device of the present invention.
FIG. 11 is a schematic view showing an example of the magnetic memory of the
present invention.
FIGS. 12A and 12B are schematic views showing basic examples of the operations
in the magnetic memory of the present invention.
FIGS. 13A and 13B are schematic views showing basic examples of the operations
in the magnetic memory of the present invention.
FIGS. 14A and 14B are schematic views showing basic examples of the operations
in the magnetic memory of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described below with reference
to the drawings. In the following embodiments, the same reference numerals are
applied to the same parts, and redundant explanations are thus omitted in some cases.
First, the method for producing a MR element of the present invention is described.
According to the method of producing a MR element of the present invention,
it is possible to obtain a TMR element with excellent properties and thermal stability.
FIGS. 1A to
1E show an example of the method of producing a MR element
of the present invention. First, as shown in FIGS. 1A to
1C, a lower electrode
layer
2, a first magnetic layer
3, a third magnetic layer
4
and an Al layer
5 are laminated successively on a substrate
1 (step
(i)). At this time, the third magnetic layer (smooth interface layer)
4
is a layer that has at least one crystal structure selected from the group consisting
of a face-centered cubic crystal (fcc structure) and a face-centered tetragonal
crystal (fct structure) and that is (111) oriented parallel to its own film plane.
Accordingly, it is possible to achieve an Al layer
5 having the (111) orientation
with respect to its own film plane by laminating the Al layer
5 on the third
magnetic layer
4. In general, Al is most stable and dense when its crystal
structure is the fcc structure having the (111) orientation. Therefore, it is possible
to achieve an Al layer
5 having a more stable and dense crystal structure
than in the case in which the Al layer
5 is directly laminated on the first
magnetic layer
3 without providing the third magnetic layer
4.
Next, as shown in FIG. 1D, a tunnel insulating layer
6 containing an
Al oxide (Al—O) is formed by oxidizing the Al layer
5 (step (ii)).
As described above, because of the presence of the third magnetic layer
4,
the Al layer
5 has the fcc structure with the (111) orientation, as well
as a stable and dense crystal structure. Therefore, the tunnel insulating layer
6 formed by oxidizing the Al layer
5 can be a dense tunnel insulating
layer having few crystal defects. It should be noted that when a composition is
denoted with hyphens, such as in the case of "Al—O", in this specification,
there is no particular limitation on the composition ratio of the contained elements.
Next, as shown in FIG. 1E, a second magnetic layer
7 is laminated such
that the tunnel insulating layer
6 is sandwiched by the first magnetic layer
3 and the second magnetic layer
7. In this manner, a laminate
10
including the first magnetic layer
3, the tunnel insulating layer
6
and the second magnetic layer
7 is formed (step (iii)). Thereafter, for
example, an upper electrode layer may be laminated further, or micro-fabrication
may be performed, as required. As described above, the tunnel insulating layer
6 can be a dense tunnel insulating layer having few crystal defects, so
that it is possible to obtain a TMR element with excellent properties and thermal stability.
In addition, the following effects seem to be achieved by laminating the third
magnetic layer
4:
FIG. 2A is a schematic cross-sectional view showing an example of a conventional
TMR element. In general, the surface of a lower electrode layer
52 has the
roughness reflecting a roughness of the substrate surface or the crystal grain
boundaries of the lower electrode layer itself (taking the periodicity in the film
plane direction with respect to the film plane of the lower electrode layer, i.e.,
with respect to the film plane of the element as "d" and the height of the projections
in a direction perpendicular to the film plane as "h"). When a magnetic layer
53
is laminated on such a lower electrode layer
52, and a tunneling insulating
layer
56 is formed, the surface of the magnetic layer
53, that is,
the interface between the magnetic layer
53 and the tunnel insulating layer
56 will have the same roughness. Moreover, the same roughness results on
the surface of the tunnel insulating layer
56 itself. For this reason, it
is difficult to form the tunnel insulating layer
56 as a dense tunnel insulating
layer with few crystal defects, resulting in the possibility that its properties
and the thermal stability of the element may deteriorate. Although not explicitly
shown in FIG. 2A, a roughness attributed to the atomic steps of the magnetic layer
53 itself also can be caused, so that there is the possibility that a roughness
that is more minute than the periodicity d and the height h shown in FIG. 2A further
may be caused on the interface between the magnetic layer
53 and the tunnel
insulating layer
56. In the descriptions of the method for producing a MR
element of the present invention, "surface" means the face of each of the layers
of the element that is on a side opposite to the substrate.
On the other hand, in the TMR element of the present invention, the third magnetic
layer
4 is disposed between the tunnel insulating layer
6 and the
first magnetic layer
3, as shown in FIG. 2B (FIG. 2B is a schematic cross-sectional
view showing an example of the TMR element of the present invention). As described
above, the third magnetic layer has at least one crystal structure selected from
the group consisting of the fcc structure and the fct structure and is (111) oriented
parallel to its own film plane. Therefore, the surface can be smoothed to the order
of the atomic size (order of sub-nanometer). Accordingly, the interface between
the third magnetic layer
4 and the tunnel insulating layer
6 can
be smoothed by laminating the third magnetic layer
4 on the first magnetic
layer
3 and thereafter forming the tunnel insulating layer
6. The
surface of the tunnel insulating layer
6 also can be smoothed, so that the
interface between the second magnetic layer and the tunnel insulating layer
6
can be smoothed when the second magnetic layer is further laminated on the tunnel
insulating layer
6. Accordingly, it is possible to obtain a TMR element
with excellent properties and thermal stability.
As described above, in a TMR element obtained by the production method of the
present invention, it is possible to achieve a dense tunnel insulating layer having
few crystal defects, and the interface between the tunnel insulating layer and
a magnetic layer in contact therewith can be smoothed, so that the thickness of
the tunnel insulating layer can be reduced even further. Therefore, it is possible
to obtain a TMR element having both a small junction resistance value and a large
MR ratio.
Since a dense tunnel insulating layer having few crystal defects can be achieved,
it is possible to obtain a TMR element with little loss in the MR ratio when a
bias voltage is increased (i.e., with small bias voltage dependence).
Further, the (111) plane, which is the finest plane among the crystal planes
of the third magnetic layer, is parallel to the film plane of the third magnetic
layer (i.e., the film plane of the element), so that it is possible to suppress
the thermal diffusion of the atoms from layers other than the tunnel insulating
layer (e.g., the lower electrode layer, the first magnetic layer and others, and
an antiferromagnetic layer in the case where the element contain the same) to the
tunnel insulating layer. When the crystal grain size of the magnetic material contained
in the first magnetic layer is increased by heat treatment and the like, it is
also possible to inhibit such an effect from being exerted on the tunnel insulating
layer. Accordingly, it is possible to obtain a TMR element with excellent thermal stability.
Although the Al layer
5 is oxidized in the example shown in FIG.
1, at least one reaction selected from the group consisting of oxidizing, nitriding
and oxynitriding may be performed on the Al layer
5. It is possible to form
a tunnel insulating layer
6 containing at least one compound selected from
the group consisting of an oxide, nitride and oxynitride of Al, and in this case
also a TMR element similar to the example shown in FIG. 1 can be obtained. The
method for forming each layer, the method for oxidizing the Al layer and the like
will be discussed later.
Next, the third magnetic layer
4 is described more specifically.
The third magnetic layer
4 has at least one crystal structure selected
from the group consisting of the fcc structure and the fct structure and is (111)
oriented parallel to its own film plane. There is no particular limitation on its
composition, thickness and the like as long as it is magnetically coupled to the
first magnetic layer
3.
The thickness of the third magnetic layer laminated in the step (i) is, for example,
in the range of at least 1 nm and at most 10 nm, preferably, in the range of at
least 1.5 nm and at most 5 nm. The surface roughness of the third magnetic layer
4 is, for example, in the range of at least 0.5 nm and at most 1 nm, using
the arithmetical mean roughness R
a prescribed in JIS (Japanese Industrial
Standards) B 0601-1994. For example, "d" is in the range of at least 15 nm and
at most 70 nm and "h" is in the range of at least 3 nm and at most 8 nm, using
the periodicity d and the height h shown in FIGS. 2A and 2B. As discussed above,
the surface roughness of the third magnetic layer
4 is reflected in the
surface roughness on the interface between the third magnetic layer
4 and
the tunnel insulating layer
6.
In the production method of the present invention, the third magnetic layer
4
may include a magnetic material containing at least one element selected from the
group consisting of Fe, Co and Ni. In this case, the magnetic coupling between
the first magnetic layer
3 and the third magnetic layer
4 can be
strengthened further, so that it is possible to obtain a TMR element with better properties.
More specifically, in the production method of the present invention, the third
magnetic layer
4 may include a magnetic material having a composition represented
by the formula Fe
xCo
y. Alternatively, the third magnetic
layer
4 may include a magnetic material having a composition represented
by the formula Fe
x′Ni
y′. In these formulas,
x, y, x′ and y′ are values satisfying the following equations:
It should be noted that in this specification, the values used for representing
compositions are based on the atomic composition ratios, unless stated otherwise.
Such a magnetic material tends to have an fcc structure, and the third magnetic
layer
4 thus can have the fcc structure with higher reliability in the step
(i). Accordingly, it is possible to obtain a TMR element with better properties
and thermal stability.
In the production method of the present invention, the third magnetic layer
4
may contain at least one element selected from the group consisting of Fe, Co and
Ni, and at least one element selected from the group consisting of Rh, Pd, Ag,
Ir, Pt and Au. Such a magnetic material has atomic radii of Rh, Pd, Ag, Ir, Pt
and Au that are larger than those of Fe, Co and Ni, and is easily (111) oriented,
so that the surface of the third magnetic layer
4 can be smoothed with higher
reliability in the step (i). Moreover, since the energy state of the surface can
be better stabilized as compared to a magnetic layer consisting only of at least
one element selected from the group consisting of Fe, Co and Ni, it is possible
to achieve a more stabilized and denser Al layer
5 in the step (i). Accordingly,
it is possible to form a denser tunnel insulating layer
6 having fewer crystal
defects in the step (ii).
Furthermore, since Rh, Pd, Ag, Ir, Pt and Au are less reactive with
oxygen, nitrogen and the like, it is possible to thermally stabilize the third
magnetic layer more effectively, thereby obtaining a MR element with better thermal
stability. In addition, the selective oxidation (nitriding, oxynitriding) of the
Al layer
5 can be performed in the step (ii), while preventing the oxidation
(nitriding, oxynitriding) of the third magnetic layer
4 as much as possible,
so that it is possible to obtain a TMR element with better properties.
In the production method of the present invention, the third magnetic layer
4
may include a magnetic material having a composition represented by the formula
M
pZ
q, where M is at least one element selected from the group
consisting of Fe, Co and Ni, Z is at least one element selected from the group
consisting of Rh, Pd, Ag, Ir, Pt and Au, and p and q are values satisfying the
following equations:
Inclusion of such a magnetic material in the third magnetic layer
4
makes it possible to obtain a TMR element with better properties and thermal stability.
Particularly, it is preferable that p and q satisfy the relations p+q=1,
0.6≦p≦0.95, 0.05≦q≦0.4, and it is more preferable that
they satisfy the relations p+q=1, 0.6≦p≦0.9, 0.1≦q≦0.4.
When q is greater than 0.5, the thickness of the third magnetic layer
4
may be at most 2 nm, for example. Since Rh, Pd, Ag, Ir, Pt and Au are nonmagnetic
elements, there is the possibility that the MR properties of the element possibly
may deteriorate if the thickness of the third magnetic layer is too large.
In the production method of the present invention, an antiferromagnetic layer
may be laminated between the substrate and the first magnetic layer in the step
(i). For instance, in the example shown in FIGS. 1A to
1E, an antiferromagnetic
layer may be laminated between the lower electrode layer
2 and the first
magnetic layer
3. Such a production method makes it possible to obtain a
spin valve TMR element having one of the first magnetic layer
3 and the
second magnetic layer
7 as a pinned magnetic layer and the other of these
magnetic layers as a free magnetic layer. When an antiferromagnetic layer is laminated
between the lower electrode layer and the first magnetic layer, an exchange coupling
magnetic field is generated between the first magnetic layer and the antiferromagnetic
layer. Accordingly, it is possible to obtain a spin valve TMR element having the
first magnetic layer as a pinned magnetic layer (magnetic layer whose magnetization
direction is fixed by the antiferromagnetic layer) and the second magnetic layer
2 as a free magnetic layer (magnetic layer whose magnetization can be rotated
relatively easily with respect to the first magnetic layer). In addition, the thickness
of the antiferromagnetic layer laminated in the step (i) is, for example, in the
range of at least 5 nm and at most 50 nm.
In the case of a spin valve MR element, a relative angle between the magnetization
directions of the pinned magnetic layer and the free magnetic layer can be changed
more easily, so that it is possible to achieve a TMR element more suitable for
devices operating with minute magnetic fields. Further, it is possible to achieve
a smaller TMR element exhibiting a larger MR ratio.
There is no particular limitation on the material used for the antiferromagnetic
layer, and antiferromagnetic alloys containing Mn (Mn-based antiferromagnetic alloys)
may be used, for example. As the Mn-based antiferromagnetic alloys, for example,
alloys may have a composition represented by the formula Z-Mn (where Z is at least
one element selected from the group consisting of Pt, Pd, Ir, Fe, Ru and Rh). In
particular, alloys having a composition of Fe—Mn, Rh—Mn, Ir—Mn,
Pt—Mn, Pt—Pd—Mn, Ni—Mn and the like are preferable.
These antiferromagnetic alloys are likely to have the fcc structure or the
fct structure, and tend to be (111) oriented parallel to their own film plane when
used as the antiferromagnetic layer. Therefore, by laminating such an antiferromagnetic
layer, it is possible to reduce the surface roughness of the substrate or the surface
roughness of each layer attributed to the crystal grain boundaries of the lower
electrode layer. Accordingly, it is possible to obtain a TMR element with better
properties and thermal stability.
Particularly, during the lamination of the antiferromagnetic layer
between the substrate and the first magnetic layer in the step (i), the antiferromagnetic
layer may be laminated after laminating an underlayer of Ni—Fe, Pt or the
like. Ni—Fe, Pt and the like are likely to have the fcc structure, so that
the antiferromagnetic layer can have the fcc structure with higher reliability.
Accordingly, it is possible to obtain a TMR element with better properties and
thermal stability.
Additionally, since Pt—Mn, Pt—Pd—Mn, Ni—Mn
and the like are changed into the fct structure by heat treatment at 250°
C. or above, the exchange coupling magnetic field with the first magnetic layer
can be improved further by heat treatment. Therefore, it is possible to obtain
a TMR element with better properties. Alternatively, it is possible to laminate
the antiferromagnetic layer on the second magnetic layer (i.e., on a side opposite
to the substrate side of the tunnel insulating layer), instead of laminating it
between the substrate and the first magnetic layer (i.e., instead of laminating
it between the substrate and the tunnel insulating layer). In this case, it is
possible to obtain a spin valve TMR element having the second magnetic layer as
the pinned magnetic layer and the first magnetic layer as the free magnetic layer.
The other layers are described below.
There is no particular limitation on the thickness of the Al layer
5
laminated in the step (i). It may be set arbitrarily, depending on the required
properties of the TMR element, and it is, for example, in the range of at least
0.1 nm and at most 10 nm. The thickness of the laminated Al layer
5 can
be used directly as the thickness of the tunnel insulating layer
6.
There is no particular limitation on the materials used for the first magnetic
layer
3 and the second magnetic layer
7 that are laminated in the
step (i) and the step (iii), as long as they are magnetic materials exhibiting
ferromagnetic properties. For example, magnetic materials made of Co, Fe, Ni, Co—Fe,
Ni—Fe, Ni—Co—Fe or the like may be used. It is also possible,
as required, to laminate a plurality of magnetic films made of different magnetic
materials. The thickness of the first magnetic layer
3 and the second magnetic
layer
7 that are laminated may be, for example, in the range of at least
1 nm and at most 20 nm.
In the case of producing a spin valve MR element, a magnetic material with excellent
soft magnetic properties, for example, may be used for the magnetic layer serving
as the free magnetic layer. More specifically, a permalloy (e.g., Ni
81Fe
19:
composition ratio by wt %), Co
0.9Fe
0.1, (Co
0.9Fe
0.1)
0.8B
0.2
or the like may be used, for example. A magnetic material with large magnetic anisotropy,
for example, may be used for the magnetic layer serving as the pinned magnetic
layer. More specifically, examples include Co
0.5Fe
0.5, Co
0.5Pt
0.5,
Fe
0.5Pt
0.5. Alternatively, since the magnetization direction
of the pinned magnetic layer can be fixed by the antiferromagnetic layer and the
like in a spin valve TMR element, the above-described magnetic material with excellent
soft magnetic properties may be used for the magnetic layer serving as the pinned
magnetic layer.
One of the first magnetic layer
3 and the second magnetic layer
7
may include a laminated film structure (so-called laminated ferrimagnetic structure)
in which a pair of magnetic films are laminated with a nonmagnetic film interposed
therebetween. At this time, when the pair of magnetic films are magnetically coupled
via the nonmagnetic film such that their magnetization directions are antiparallel
with respect to each other, it is possible to reduce a leakage magnetic field generating
from the end of the element, thereby obtaining a TMR element with better properties.
Particularly, when the magnetic layer serving as the pinned magnetic layer includes
the above-described laminated ferrimagnetic structure, it is possible to increase
the magnetic anisotropy of the pinned magnetic layer further, that is, to achieve
a pinned magnetic layer whose magnetization direction is less likely to change
by the magnetic field that is applied from the outside to the element.
There is no particular limitation on the material used for the nonmagnetic
film as long as it is a nonmagnetic material, and Ru, Cr, Cu or the like may be
used, for example. The film thickness may be, for example, in the range of at least
0.4 nm and at most 1.5 nm. The magnetic film may be a film containing, for example,
Fe, Co or Ni. The thickness of the magnetic film may be, for example, in the range
of at least 1 nm and at most 10 nm.
There is no particular limitation on the substrate
1 as long as it is
nonmagnetic, and Si, AlTiC, Al
2O
3 (e.g., sapphire) may be
used, for example. The thickness may be, for example, in the range of at least
0.1 μm and at most 10 mm.
Additionally, in the example shown in FIGS. 1A to
1E, the lower
electrode layer
2 is laminated on the substrate
1. There is no particular
limitation on the material used for the lower electrode layer
2 as long
as it is an electrically conductive material, and a low-resistance material (e.g.,
having a linear resistivity of 100 μΩcm or lower) such as Pt, Au, Cu,
Ru, Al or TiN may be used, for example. It is also possible to laminate a plurality
of films made of different materials. In the case of laminating an upper electrode
layer, the material used for the upper electrode layer may be the same as that
used for the lower electrode layer. There is no particular limitation on the thicknesses
of the lower electrode layer and the upper electrode layer, and they may be in
the range of at least 10 nm and at most 10 μm, for example.
In the production method of the present invention, it is possible, as required,
to laminate layers other than the layers shown in FIGS. 1A to
1E. For example,
an underlayer of Ta, Nb, Zr, Pt, Cr, Ni—Fe or the like may be laminated between
the lower electrode layer
2 and the first magnetic layer
3 (between
the lower electrode layer and the antiferromagnetic layer, in the case of laminating
the antiferromagnetic layer). It is also possible to laminate a plurality of tunnel
insulating layers. In the case of laminating a plurality of tunnel insulating layers,
the above-described third magnetic layer may be laminated between at least one
of the tunnel insulating layers and a magnetic layer on the substrate side that
is adjacent thereto.
In the production method of the present invention, pulse laser deposition (PLD),
ion beam deposition (IBD), sputtering using duster ion beam, RF, DC, electron cyclotron
resonance (ECR), helicon, induction coupled plasma (ICP) or facing targets, molecular
beam epitaxy (MBE) or ion plating, for example, may be used as the method for forming
each of the layers constituting the TMR element. Other than these PVD processes,
it is also possible to use CVD processes, plating processes or sol-gel processes.
In the production method of the present invention, for example, the following
process may be used for the lamination and the subsequent oxidation of the Al layer
in the step (i) and the step (ii). First, an Al layer is formed by a PVD process
such as sputtering, MBE or IBD, using Al as the target. Next, a tunnel insulating
layer made of Al—O may be formed by oxidizing the Al layer by natural oxidation,
plasma oxidation, radical oxidation, ozone oxidation or the like.
Here, the natural oxidation is a process of oxidizing an Al layer in pure oxygen
gas, in a mixed gas of pure oxygen gas and another gas (e.g., inert gas or rare
gas) or in the atmosphere. In the case of oxidation in pure oxygen or in a mixed
gas of pure oxygen and another gas, the partial pressure of oxygen may be, for
example, in the range of at least 1×10
-2 Pa and at most 1×10
5
Pa. The oxidation time may be, for example, in the range of at least about
10 sec and at most about 100 min, and the temperature may be, for example, in the
range of at least 10° C. and at most 100° C.
Radical oxidation is a process in which oxygen gas is dissociated into oxygen
radicals having unpaired electrons with an RF coil, ECR plasma or the like, and
only the neutral components of the generated oxygen radicals are utilized for oxidation.
Plasma oxidation is a process in which oxygen gas is made into a plasma by substantially
the same technique as that of the radical oxidation and the oxygen radical with
a highly oxidative oxygen radical or ozone is used for oxidation. The degree of
oxidation of an Al layer using these techniques can be controlled by adjusting
the partial pressure of oxygen, the temperature, the time, the electric power used
for generating radicals or plasma and the like. The partial pressure of oxygen
may be, for example, in the range of at least 0.01 Pa and at most 10 Pa. The oxidation
time may be, for example, in the range of at least about 1 sec and at most about
100 min, and the temperature may be, for example, in the range of at least 10°
C. and at most 100° C. Under such conditions, it is possible to form a dense
tunnel insulating layer with few crystal defects.
Additionally, in the case of nitriding or oxynitriding the Al layer,
nitrogen gas or a mixed gas of oxygen gas and nitrogen gas may be used in place
of oxygen gas in the above-described processes.
The production method of the present invention further may include the step of:
(a) heat treating the laminate formed in the step (iii), after the step (iii).
Since the oxidation state in the tunnel insulating layer can be made more uniform
by performing heat treatment, it is possible to obtain a TMR element with better
properties and thermal stability. The heat treatment may be performed at a temperature
in the range of, for example, at least 150° C. and at most 500° C., in
vacuum, under reduced pressure or in inert gas or rare gas. In the case of further
laminating an upper electrode layer after the step (iii), it also may be performed
either after or before such lamination.
In addition, it is also possible to obtain a TMR element with better properties
and thermal stability by using a single crystal substrate as the substrate and
a lower electrode layer epitaxially grown on the substrate, or by smoothing the
surface of the lower electrode layer by a process such as chemical mechanical polishing
(CMP). This is because the roughness on the interface between the third magnetic
layer and the tunnel insulating layer can be smoothed more effectively.
Next, the MR element of the present invention is described.
FIG. 3 shows an example of the MR element of the present invention. The MR element
shown in FIG. 3 includes a tunnel insulating layer
6 containing at least
one compound selected from the group consisting of an oxide, nitride and oxynitride
of Al, a first magnetic layer
3 and a second magnetic layer
7 that
are laminated so as to sandwich the tunnel insulating layer
6, and a third
magnetic layer
4 disposed between the first magnetic layer
3 and
the tunnel insulating layer
6. The resistance value varies depending on
a relative angle between the magnetization directions of the first magnetic layer
3 and the second magnetic layer
7. Here, the third magnetic layer
4 has at least one crystal structure selected from the group consisting
of a face-centered cubic crystal structure and a face-centered tetragonal crystal
structure and is (111) oriented parallel to the film plane of the third magnetic
layer
4.
By forming such a MR element, it is possible to achieve a TMR element with excellent
properties and thermal stability. Such a MR element can be obtained, for example,
by the above-described production method of a MR element according to the present invention
FIG. 4 shows another example of the MR element of the present invention. The
MR element shown in FIG. 4 is a TMR element in which an antiferromagnetic layer
8 is further disposed on a side opposite to a plane of the first magnetic
layer
3 that faces the tunnel insulating layer
6 in the MR element
shown in FIG.
3. The further disposition of the antiferromagnetic layer
8 makes it possible to achieve a spin valve TMR element having the first
magnetic layer
3 as the pinned magnetic layer and the second magnetic layer
7 as the free magnetic layer. Moreover, since the third magnetic layer
4
is disposed between the tunnel insulating layer
6 and the first magnetic
layer
3, it is possible to achieve a TMR element with excellent properties
and thermal stability.
In the case of the MR element shown in FIG. 4, the antiferromagnetic layer
8
also may be (111) oriented parallel to its own film plane. This can achieve a TMR
element with better properties and thermal stability. Additionally, in the case
of the examples shown in FIGS. 3 and 4, the antiferromagnetic layer
8 also
may be disposed on the second magnetic layer
7. In this case, it is possible
to achieve a spin valve TMR element having the second magnetic layer
7 as
the pinned magnetic layer and the first magnetic layer
3 as the free magnetic layer.
In the case of the examples of the MR element shown in FIGS. 3 and 4, the material
used for each layer, the thickness, the structure and the like of each layer may
be the same as those described in the production method of a MR element of the
present invention. This can achieve a TMR element having the effects described
in the production method of a MR element of the present invention.
In the examples shown in FIGS. 3 and 4, the MR element further may include a
lower
electrode layer, an upper electrode layer, a substrate and the like. Also in this
case, it is possible to achieve a TMR element with excellent properties and thermal
stability, regardless of the surface roughness of the substrate, the state of the
crystal grain boundaries of the lower electrode layer and the like. Conversely,
the MR element does not need to include a lower electrode layer, an upper electrode
layer, a substrate and the like, as shown in the examples of the below-described
device using a MR element.
Next, devices using the MR element of the present invention are described.
In order to produce a magnetic device
