U.S. patent application number 16/281642 was filed with the patent office on 2020-08-27 for lattice matched tunnel barriers for perpendicularly magnetized heusler alloys.
The applicant listed for this patent is INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to AAKASH PUSHP.
Application Number | 20200274057 16/281642 |
Document ID | / |
Family ID | 1000003925706 |
Filed Date | 2020-08-27 |
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United States Patent
Application |
20200274057 |
Kind Code |
A1 |
PUSHP; AAKASH |
August 27, 2020 |
LATTICE MATCHED TUNNEL BARRIERS FOR PERPENDICULARLY MAGNETIZED
HEUSLER ALLOYS
Abstract
A device comprising a first magnetic layer (e.g., Co.sub.2MnSi
Heusler alloy or a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy such as Mn.sub.3Ga, Mn.sub.3Ge,
etc.) and a second magnetic layer (e.g., Co.sub.2MnSi Heusler alloy
or a tetragonally distorted perpendicularly magnetized (PMA)
Heusler alloy such as Mn.sub.3Ga, Mn.sub.3Ge, etc.), and a metal
halide tunnel barrier in between the first and second magnetic
layers, wherein the metal halide tunnel barrier (e.g., NaF, NaCl,
NaBr, LiF, LiCl, and LiBr or their combination) is lattice matched
within a predetermined limit (e.g. 5%) of both the first and second
magnetic layers.
Inventors: |
PUSHP; AAKASH; (SANTA CLARA,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INTERNATIONAL BUSINESS MACHINES CORPORATION |
Armonk |
NY |
US |
|
|
Family ID: |
1000003925706 |
Appl. No.: |
16/281642 |
Filed: |
February 21, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 43/02 20130101;
H01L 43/12 20130101; G11C 11/161 20130101; H01L 43/10 20130101 |
International
Class: |
H01L 43/02 20060101
H01L043/02; H01L 43/10 20060101 H01L043/10; H01L 43/12 20060101
H01L043/12; G11C 11/16 20060101 G11C011/16 |
Claims
1. A device comprising a first magnetic layer, a second magnetic
layer, and a metal halide tunnel barrier between the first and
second magnetic layers, wherein the metal halide tunnel barrier is
lattice matched to within 5% of both the first and second magnetic
layers, and wherein the first layer, the second layer, and the
tunnel barrier are in proximity with each other so that
spin-polarized current can pass from one of the magnetic layers to
the other of the magnetic layers.
2. The device of claim 1, wherein the first magnetic layer is a
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy.
3. The device of claim 1, wherein the second magnetic layer is a
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy.
4. The device of claim 1, wherein each of the first and second
magnetic layers is a Co.sub.2MnSi Heusler alloy or a tetragonally
distorted perpendicularly magnetized (PMA) Heusler alloy.
5. The device of claim 1, wherein the metal halide tunnel barrier
includes an alkali component and at least one halide component.
6. The device of claim 5, wherein the alkali component is any of,
or a combination of, the following: Na and Li.
7. The device of claim 5, wherein the halide component is any of,
or a combination of, the following: F, Cl, and Br.
8. The device of claim 1, wherein the halide tunnel barrier is any
of, or a combination of, the following: NaF, NaCl, NaBr, LiCl, and
LiBr.
9. A method of forming the device of claim 1 comprising the steps
of: depositing the first magnetic layer; epitaxially growing the
metal halide tunnel barrier on top of the first magnetic layer, and
depositing the second magnetic layer on top of the epitaxially
grown halide tunnel barrier.
10. The method of claim 9, wherein the step of depositing the first
magnetic layer comprises depositing either a Co.sub.2MnSi Heusler
alloy layer or a tetragonally distorted perpendicularly magnetized
(PMA) Heusler alloy layer.
11. The method of claim 9, wherein the step of depositing the
second magnetic layer comprises depositing either a Co.sub.2MnSi
Heusler alloy layer or a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy layer.
12. The method of claim 9, wherein the step of depositing the first
magnetic layer comprises depositing either a Co.sub.2MnSi Heusler
alloy layer or a tetragonally distorted perpendicularly magnetized
(PMA) Heusler alloy layer, and the step of depositing the second
magnetic layer comprises depositing either another Co.sub.2MnSi
Heusler alloy layer or another tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy layer.
13. The method of claim 9, wherein the metal halide tunnel barrier
includes an alkali component and at least one halide component.
14. The method of claim 13, wherein the alkali component is any of,
or a combination of, the following: Na and Li.
15. The method of claim 13, wherein the halide component is any of,
or a combination of, the following: F, Cl, and Br.
16. The method of claim 9, wherein the step of epitaxially growing
the halide tunnel barrier comprises any of, or a combination of,
the following: epitaxially growing a NaF tunnel barrier,
epitaxially growing a NaCl tunnel barrier, epitaxially growing a
NaBr tunnel barrier, epitaxially growing a LiF tunnel barrier,
epitaxially growing a LiCl tunnel barrier, and epitaxially growing
a LiBr barrier.
17. The method of claim 9, wherein the step of epitaxially growing
the halide tunnel barrier is done via RF sputtering.
18. The method of claim 9, wherein the step of epitaxially growing
the halide tunnel barrier is done via evaporation.
19. A device comprising: a first magnetic layer; a metal halide
tunnel barrier underlying the first magnetic layer; and a second
magnetic layer underlying the tunnel barrier, wherein: the first
magnetic layer, the metal halide tunnel barrier, and the second
magnetic layer are in proximity with each other, such that they
form a magnetic tunnel junction, the metal halide tunnel barrier is
composed of any one of, or a combination of, NaF, NaCl, NaBr, LiF,
LiCl, and LiBr, and at least one of the magnetic layers is formed
from a Heusler alloy.
20. The device of claim 19, wherein the metal halide tunnel barrier
and the Heusler layer are lattice matched to within 5%.
21. The device of claim 19, wherein the first magnetic layer is a
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy.
22. The device of claim 19, wherein the second magnetic layer is a
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy.
23. The device of claim 19, wherein each of the first and second
magnetic layers is a Co.sub.2MnSi Heusler alloy or a tetragonally
distorted perpendicularly magnetized (PMA) Heusler alloy.
24. A method of forming the device of claim 19 comprising the steps
of: depositing the second magnetic layer made of a tetragonally
distorted perpendicularly magnetized (PMA) Heusler alloy;
epitaxially growing the metal halide tunnel barrier on top of the
second magnetic layer, the metal halide tunnel barrier composed of
any one of or a combination of NaF, NaCl, NaBr, LiF, LiCl, and
LiBr, and depositing the first magnetic layer made of a
tetragonally distorted perpendicularly magnetized (PMA) Heusler
alloy on top of the epitaxially grown metal halide tunnel barrier,
wherein the metal halide tunnel barrier is lattice matched to
within 5% of both the first and second magnetic layers.
25. The method of claim 24, wherein the step of epitaxially growing
the metal halide tunnel barrier is done via RF sputtering.
26. The method of claim 24, wherein the step of epitaxially growing
the metal halide tunnel barrier is done via evaporation.
Description
BACKGROUND OF THE INVENTION
Field of Invention
[0001] The present invention relates generally to the field of
tunnel barriers. More specifically, the present invention is
related to the use of a halide tunnel barrier that is lattice
matched to perpendicularly magnetized Heusler alloys.
Discussion of Related Art
[0002] MgO is the standard tunnel barrier for Magnetic Tunnel
Junctions (MTJs) that are essential for STT-MRAM (Spin Transfer
Torque-Magnetic Random Access Memory) since it enhances the spin
polarization of the magnetic electrodes along its 001 orientation
due to spin-filtering effect.
[0003] On the other hand, cubic Heusler alloys (for example
Co.sub.2MnSi, Co2FeSi, etc.) have already demonstrated
exceptionally high spin polarization as MTJ electrodes due to their
half-metallic properties, regardless of the choice of tunnel
barriers (high spin polarization has been demonstrated for
amorphous AlO.sub.x tunnel barriers as well as crystalline MgO
tunnel barriers).
[0004] Perpendicularly magnetized anisotropy (PMA) materials are
essential for scaling STT-MRAM technology beyond 30 nm. Several
tetragonally distorted Heusler alloys naturally exhibit PMA.
However, these tetragonally distorted Heusler alloys do NOT exhibit
half-metallic behavior and thus will require proper choice of
tunnel barriers in order to offer high spin-polarization leading to
large tunneling magnetoresistance (TMR).
[0005] Tsunegi et al. in their paper titled "Large Tunnel
Magnetoresistance in Magnetic Tunnel Junctions Using a Co.sub.2MnSi
Heusler Alloy Electrode and a MgO Barrier" disclose that a large
tunnel magnetoresistance (TMR) ratio of 753% has been observed at 2
K in a magnetic tunnel junction (MTJ) using a Co.sub.2MnSi Heusler
alloy electrode and a crystalline MgO tunnel barrier. Tsunegi et
al. report that the observed TMR ratio is the largest reported to
date in MTJs using a Heusler alloy electrode. Tsunegi et al. also
report a large TMR ratio of 217% that is observed at room
temperature (RT), wherein this TMR at RT is much larger than that
of MTJs using an amorphous Al-oxide tunnel barrier. It is further
noted within the paper that the temperature dependence of the TMR
ratio is still large because of inelastic tunneling in the
antiparallel magnetic configuration.
[0006] Zhang et al. in their paper titled "Bias Dependence of Spin
Transfer Torque in Co.sub.2MnSi Heusler Alloy Based Magnetic Tunnel
Junctions" investigate the bias dependence of the anti-damping like
and field-like spin transfer torque components in both symmetric
(Co.sub.2MnSi/MgO/Co.sub.2MnSi) and asymmetric
(Co.sub.2MnSi/MgO/CoFe) structure Heusler-based MTJs using spin
transfer torque ferromagnetic resonance. It is noted in this paper
that while the damping like torque is linear with respect to bias
for both MTJ structures, the asymmetric MTJ structure has an
additional linear component to the ordinarily quadratic field like
torque bias dependence and that these results can be accounted for
by a free electron tunneling model. It is further noted in this
paper that the low damping and low saturation magnetization
properties of Heusler alloys are more likely to lead significant
improvements to spin torque switching efficiency rather than their
half metallic character.
[0007] As noted above, Co.sub.2MnSi--MgO tunneling interface is
well studied in the prior art. FIGS. 1A-D depict the lattice
structures of such a Co.sub.2MnSi|MgO tunneling interface. FIGS.
1A-B illustrate the lattice mismatch between Co.sub.2MnSi and MgO
(with respect to MgO) to be (3.995-4.212)/4.212=.about.-5.15%.
FIGS. 1C and 1D illustrate the oxygen (O) on cobalt (Co) and O on
manganese (Mn) lattice structures, respectively. Even in the
preferred configuration of O on Mn, where O is bonded with Mn,
there is a 5% strain at the interface because of lattice mismatch,
because the Mn--Mn distance=3.996 .ANG. in Co.sub.2MnSi, whereas
the O--O distance is 4.212 .ANG. in MgO.
[0008] Cabailh et al. in their paper titled "Thin Film NaCl Growth
in Ag Sub-Surface Using Evaporation" study the surface work
function (WF) and substrate temperature dependence of the NaCl
thin-film growth on Ag(001) using noncontact atomic force
microscopy and Kelvin probe force microscopy. In Cabailh et al.,
the NaCl film is composed of large crystalline islands in the
sub-monolayer range, where such islands decrease in density and
increase in size with increasing temperature during deposition.
Each island in Cabailh et al. is composed of a large base island 2
monolayers (ML) thick (for T>343 K), which collects impinging
NaCl molecules that form ad-islands on top. Kelvin probe force
microscopy (KPFM) measurements in Cabailh et al. show a reduction
of the silver WF by
.DELTA..phi.=.phi..sub.Ag-.phi..sub.NaCl/Ag=0.69.+-.0.03 eV with no
dependence on the film thickness (1-6 ML), in agreement with recent
theoretical calculations.
[0009] Kiguchi et al. in their paper titled "Thin Film LiCl Growth
on Cu Sub-Surface Using Evaporation" study the growth of thin film
LiCl on a Cu(001) substrate using reflection high-energy electron
diffraction and Auger electron spectroscopy. Kiguchi et al. noted
that at a substrate temperature of 300 K, a single-domain LiCl film
([100].sub.film//[110].sub.substrate) grows in a layer-by-layer
fashion, while a double-domain LiCl film
([100].sub.film//[100].sub.substrate and
[100].sub.film//[110].sub.substrate) grows at substrate
temperatures higher than 470 K.
[0010] Embodiments of the present invention are an improvement over
prior art systems and methods.
SUMMARY OF THE INVENTION
[0011] In one embodiment, the present invention provides a device
comprising a first magnetic layer (e.g., Co.sub.2MnSi Heusler alloy
or a tetragonally distorted perpendicularly magnetized (PMA)
Heusler alloy) and a second magnetic layer (e.g., Co.sub.2MnSi
Heusler alloy or a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy), and a metal halide tunnel barrier
(e.g., NaF, NaCl, NaBr, LiF, LiCl, and LiBr) in between the first
and second magnetic layers, wherein the halide tunnel barrier is
lattice matched to within 5% of both the first and second magnetic
layers, and wherein the first layer, the second layer, and the
tunnel barrier are in proximity with each other so that
spin-polarized current can pass from one of the magnetic layers to
the other of the magnetic layers.
[0012] In another embodiment, the present invention provides a
method comprising: depositing a first magnetic layer (e.g.,
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy); epitaxially
growing a metal halide tunnel barrier (e.g., NaF, NaCl, NaBr, LiF,
LiCl, and LiBr) on top of the first magnetic layer, and depositing
a second magnetic layer (e.g., Co.sub.2MnSi Heusler alloy or a
tetragonally distorted perpendicularly magnetized (PMA) Heusler
alloy) on top of the epitaxially grown metal halide tunnel barrier,
wherein the metal halide tunnel barrier is lattice matched to
within 5% of both the first and second magnetic layers.
[0013] In yet another embodiment, the present invention provides a
device comprising: a first magnetic layer (e.g., Co.sub.2MnSi
Heusler alloy or a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy); a metal halide tunnel barrier
underlying the first magnetic layer; and a second magnetic layer
(e.g., Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy) underlying the
tunnel barrier, wherein the first magnetic layer, the metal halide
tunnel barrier, and the second magnetic layer are in proximity with
each other, such that they form a magnetic tunnel junction, the
metal halide tunnel barrier is composed of any one of or a
combination of NaF, NaCl, NaBr, LiF, LiCl, and LiBr, and at least
one of the magnetic layers is formed from a Heusler alloy.
[0014] In another embodiment, the present invention provides a
method comprising: depositing a first magnetic layer made of a
tetragonally distorted perpendicularly magnetized (PMA) Heusler
alloy; epitaxially growing a metal halide tunnel barrier on top of
the first magnetic layer, the metal halide tunnel barrier composed
of any one of or a combination of NaF, NaCl, NaBr, LiF, LiCl, and
LiBr, and depositing a second magnetic layer made of a tetragonally
distorted perpendicularly magnetized (PMA) Heusler alloy on top of
the epitaxially grown metal halide tunnel barrier, wherein the
metal halide tunnel barrier is lattice matched to within 5% of both
the first and second magnetic layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present disclosure, in accordance with one or more
various examples, is described in detail with reference to the
following figures. The drawings are provided for purposes of
illustration only and merely depict examples of the disclosure.
These drawings are provided to facilitate the reader's
understanding of the disclosure and should not be considered
limiting of the breadth, scope, or applicability of the disclosure.
It should be noted that for clarity and ease of illustration these
drawings are not necessarily made to scale.
[0016] FIGS. 1A and 1B illustrate the lattice mismatch between
Co.sub.2MnSi and MgO (with respect to MgO) to be about 5.15%.
[0017] FIGS. 1C and 1D illustrate the oxygen (O) on cobalt (Co) and
O on manganese (Mn) lattice structures, respectively.
[0018] FIGS. 2A and 2B illustrate the lattice mismatch between
Co.sub.2MnSi and NaCl (with respect to NaCl) to be about 0.18%.
[0019] FIGS. 3 and 4 depict non-limiting examples structures formed
according to the teachings of the present invention.
[0020] FIGS. 5 and 6 depict non-limiting examples of the present
invention invention's method to manufacture a halide-based
tunneling interface.
[0021] FIGS. 7A through 7D depict a regular cubic Heusler
structure, regular tetragonal Heusler structure, inverse cubic
Heusler structure, inverse tetragonal Heusler structure,
respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] While this invention is illustrated and described in
preferred embodiments, the invention may be produced in many
different configurations. There is depicted in the drawings, and
will herein be described in detail, a preferred embodiment of the
invention, with the understanding that the present disclosure is to
be considered as an exemplification of the principles of the
invention and the associated functional specifications for its
construction and is not intended to limit the invention to the
embodiment illustrated. Those skilled in the art will envision many
other possible variations within the scope of the present
invention.
[0023] Note that in this description, references to "one
embodiment" or "an embodiment" mean that the feature being referred
to is included in at least one embodiment of the invention.
Further, separate references to "one embodiment" in this
description do not necessarily refer to the same embodiment;
however, neither are such embodiments mutually exclusive, unless so
stated and except as will be readily apparent to those of ordinary
skill in the art. Thus, the present invention can include any
variety of combinations and/or integrations of the embodiments
described herein.
[0024] FIGS. 2A-B illustrate the lattice structures of a
non-limiting example of a Co.sub.2MnSi|NaCl tunneling interface.
According to the present invention, the best choice for tunnel
barrier for PMA Heusler alloys that offer spin filtering properties
are rock salt structures such as NaCl, LiCl, NaF, etc. and
combinations thereof in their <001> orientation. FIGS. 2A-B
depict an example lattice structure of the Co.sub.2MnSi|NaCl
tunneling interface that may be used between two magnetic layers.
FIGS. 2A-B illustrate the lattice mismatch between Co.sub.2MnSi and
NaCl (with respect to NaCl) to be (3.995-3.988)/3.988=.about.0.18%
(significantly lower than the lattice mismatch that was shown in
FIGS. 1A-B). The lattice constant of Co.sub.2MnSi is 5.65 .ANG.,
whereas the lattice constant of NaCl is 5.64 .ANG.. NaCl is the
classic rock salt structure, the same symmetry as that of MgO. It
is because of this symmetry that the 001 orientation of a rock salt
structure should offer spin-filtering and, thus, a high TMR.
[0025] FIGS. 7A-D depict a regular cubic Heusler structure, regular
tetragonal Heusler structure, inverse cubic Heusler structure,
inverse tetragonal Heusler structure, respectively. The cubic
Heusler compounds X.sub.2YZ can have regular or inverse structure,
as shown in FIGS. 7A and 7C, with four sites that form four FCC
sublattices--site Z (occupied by atom Z), site II that is
octahedrally coordinated by Z, and two equivalent sites I that are
tetrahedrally coordinated by Z. In the regular structure in FIG.
7A, two X atoms (circle with "=" within labeled X(I)) have
identical environments--they are located in sites I in the same x-y
plane. In this structure, the Y atom (circle with "+" within) in
site II and the Z atom (smallest circle that is gray) are located
in another x-y plane. In the inverse structure shown in FIG. 7C,
two X atoms have different environments--one X atom (circle with
"=" within and labeled X(I)) is located in site I in one x-y plane
with a Y atom (circle with "+ within), while another X atom [circle
with "*" within and labeled X(II)] is located in site II in another
x-y plane with a Z atom (smallest circle that is gray). Regular
(FIG. 7B) and inverse (FIG. 7D) tetragonal Heusler structures can
be obtained from regular and inverse cubic structures,
respectively, by stretching (or compressing) parent cubic structure
along the z-axis. The tetragonal unit cells shown in FIGS. 7B and
7D are rotated 45.degree. around the z axis relative to the parent
cubic structures shown in FIGS. 7A and 7D, respectively. Most PMA
Heuslers have a tetragonal lattice constant, a (defined in FIG.
7B)<3.99 .ANG.. As one non-limiting example, for perfect lattice
matching to a PMA Heusler with in-plane lattice constant a, an
appropriate choice of spacer would be (using Vegard's law):
(Na.sub.1-x,Li.sub.x)Cl or Na(Cl.sub.1-y,F.sub.y), where
x.about.(3.99-a)/0.36 or y.about.(3.99-a)/1.01.
[0026] FIG. 3 depicts a non-limiting example of a structure formed
according to the teachings of the present invention. Device 300
comprising: a first magnetic layer 302, a tunnel barrier 304
underlying the first magnetic layer 302 (made from, for example, a
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy); and a second
magnetic layer 306 (made from, for example, a Co.sub.2MnSi Heusler
alloy or a tetragonally distorted perpendicularly magnetized (PMA)
Heusler alloy) underlying the tunnel barrier 304, wherein the first
magnetic layer 302, the tunnel barrier 304, and the second magnetic
layer 306 are in proximity with each other, such that they form a
magnetic tunnel junction. Tunnel barrier 304 may be a metal tunnel
barrier and is composed of any one of, or a combination of, NaF,
NaCl, NaBr, LiF, LiCl, and LiBr. At least one of the magnetic
layers 302, 306 may be formed from a Heusler alloy (ideally a
tetragonally distorted Heusler alloy that exhibits PMA). In one
embodiment, tunnel barrier 304 is lattice matched to within 5% of
both the first magnetic layer 302 and second magnetic layer 306.
Upper bound of 5% is picked in this embodiment since Co.sub.2MnSi
and MgO are lattice mismatched by .about.5%. Also, the upper bound
of 5% refers to the tunnel barrier's lattice constant being within
5% of the lattice constants associated with the first and/or second
magnetic layer.
[0027] FIG. 4 depicts another non-limiting example of a structure
formed according to the teachings of the present invention. Device
400 comprising a first magnetic layer made of a tetragonally
distorted perpendicularly magnetized (PMA) Heusler alloy 402
(nonlimiting examples include, Mn.sub.3Ga, Mn.sub.3Ge); a second
magnetic layer made of a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy 406 (nonlimiting examples include,
Mn.sub.3Ga, Mn.sub.3Ge); and a metal halide tunnel barrier 404
composed of any one of, or a combination of, NaF, NaCl, NaBr, LiF,
LiCl, and LiBr. Metal halide tunnel barrier 404 is disposed between
the first magnetic layer made of a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy 406 and the second
magnetic layer made of a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy 402. The first magnetic layer 402
made of a tetragonally distorted perpendicularly magnetized (PMA)
Heusler alloy, the metal halide tunnel barrier 404, and the second
magnetic layer made of a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy 406 are in proximity with each
other, such that they form a magnetic tunnel junction.
[0028] Tetragonally distorted Heusler Alloys, such as Mn.sub.3Ga,
Mn.sub.3Ge, etc., are used in the first and second magnetic layer
where the halide tunnel barrier with appropriate stoichiometry
modification/engineering (using Vegard's law) will lattice match
the tetragonally distorted PMA Heuslers to within a pre-determined
upper limit, e.g., 5%. One of the ways of obtaining the correct
stoichiometry of the barrier would be: (Na.sub.1-x,Li.sub.x)Cl or
Na(Cl.sub.1-y,F.sub.y), where x.about.(3.99-a)/0.36 or
y.about.(3.99-a)/1.01, where a is the in-plane lattice constant of
the tetragonally distorted PMA Heusler as indicated in FIGS. 7B and
7D.
[0029] FIG. 5 depicts another non-limiting example of the present
invention's method to manufacture a halide-based tunneling
interface. Method 500 comprises the steps of: depositing a first
magnetic layer (made from, for example, a Co.sub.2MnSi Heusler
alloy or a tetragonally distorted perpendicularly magnetized (PMA)
Heusler alloy such as Mn.sub.3Ga, Mn.sub.3Ge, etc.) 502,
epitaxially growing a metal halide tunnel barrier (e.g., NaF, NaCl,
NaBr, LiF, LiCl, and LiBr) on top of the first magnetic layer 504,
and depositing a second magnetic layer (made from, for example, a
Co.sub.2MnSi Heusler alloy or a tetragonally distorted
perpendicularly magnetized (PMA) Heusler alloy such as Mn.sub.3Ga,
Mn.sub.3Ge, etc.) on top of the epitaxially grown metal halide
tunnel barrier 506. The halide tunnel barrier is lattice matched
within a pre-determined upper limit, such as 5%, of both the first
and second magnetic layers.
[0030] FIG. 6 depicts another non-limiting example of the present
invention's method to manufacture another halide-based tunneling
interface. Method 600 comprises the steps of depositing a first
magnetic layer made of a tetragonally distorted perpendicularly
magnetized (PMA) Heusler alloy such as Mn.sub.3Ga, Mn.sub.3Ge, etc.
602, epitaxially growing a metal halide tunnel barrier on top of
the first magnetic layer, the metal halide tunnel barrier composed
of any one of, or a combination of, NaF, NaCl, NaBr, LiF, LiCl, and
LiBr 604, and depositing a second magnetic layer made of a
tetragonally distorted perpendicularly magnetized (PMA) Heusler
alloy such as Mn.sub.3Ga, Mn.sub.3Ge, etc. on top of the
epitaxially grown metal halide tunnel barrier 606. The metal halide
tunnel barrier is lattice matched within a pre-determined upper
limit, such as 5%, of both the first and second magnetic
layers.
[0031] In one embodiment, the halide tunnel barrier is grown on the
first magnetic layer via Radio Frequency (RF) sputtering. Radio
Frequency Sputtering is the technique involved in alternating the
electrical potential of the current in the vacuum environment at
radio frequencies to avoid a charge building up on certain types of
sputtering target materials, which over time can result in arcing
into the plasma that spews droplets creating quality control issues
on the thin films--and can even lead to the complete cessation of
the sputtering of atoms terminating the process. Since the present
invention requires insulating halides, RF sputtering would be an
effective choice.
[0032] In another embodiment, the halide tunnel barrier is grown on
the first magnetic layer via Molecular Beam Epitaxy (MBE). In one
embodiment, MBE takes place in high vacuum or ultra-high vacuum
(10.sup.-8-10.sup.-12 Torr). The deposition rate in MBE (typically
less than 3,000 nm per hour) allows the films to grow epitaxially.
The absence of carrier gases, as well as the ultra-high vacuum
environment, result in the highest achievable purity of the grown
films. Since the present invention requires insulating halides (and
since we are dealing with relatively lower melting temperatures of
.about.1,000.degree. C. as compared to MgO that has a melting
temperature of .about.2850.degree. C.), MBE will be another
effective choice.
Conclusion
[0033] Embodiments, described above, show an effective
implementation of lattice matched tunnel barriers for
perpendicularly magnetized Heusler alloys. While various preferred
embodiments have been shown and described, it will be understood
that there is no intent to limit the invention by such disclosure,
but rather, it is intended to cover all modifications falling
within the spirit and scope of the invention, as defined in the
appended claims.
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