U.S. patent application number 10/557665 was filed with the patent office on 2007-04-12 for spin valves using organic spacers and spin-organic light-emitting structures using ferromagnetic electrodes.
Invention is credited to Jing Shi, Valy Vardeny.
Application Number | 20070082230 10/557665 |
Document ID | / |
Family ID | 33490518 |
Filed Date | 2007-04-12 |
United States Patent
Application |
20070082230 |
Kind Code |
A1 |
Shi; Jing ; et al. |
April 12, 2007 |
Spin valves using organic spacers and spin-organic light-emitting
structures using ferromagnetic electrodes
Abstract
The spacer in a spin-valve is replaced with an organic layer,
allowing for numerous applications, including light-emitting
structures. The invention demonstrates that the spin coherence of
the organic material is sufficiently long that the carriers do not
lose their spin memory even in traversing a thicker passive
barrier. At least three methods to fabricate the organic spin-valve
devices are disclosed, in which the difficulties associated with
depositing the ferromagnetic (FM) and organic layers are
addressed.
Inventors: |
Shi; Jing; (Salt Lake City,
UT) ; Vardeny; Valy; (Salt Lake City, UT) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Family ID: |
33490518 |
Appl. No.: |
10/557665 |
Filed: |
May 24, 2004 |
PCT Filed: |
May 24, 2004 |
PCT NO: |
PCT/US04/16156 |
371 Date: |
November 22, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60472640 |
May 22, 2003 |
|
|
|
Current U.S.
Class: |
428/811 ;
257/E43.004; G9B/5.075; G9B/5.077 |
Current CPC
Class: |
H01F 10/1936 20130101;
H01L 43/12 20130101; Y10T 428/1107 20150115; G11B 5/3906 20130101;
G11B 5/29 20130101; H01F 10/005 20130101; H01F 10/3281 20130101;
H01F 10/3254 20130101; B82Y 25/00 20130101; G11B 2005/3996
20130101; H01F 10/3268 20130101; G11B 5/31 20130101; G11C 11/16
20130101; B82Y 10/00 20130101; H01F 10/3213 20130101; G01R 33/091
20130101; H01L 43/10 20130101; H01L 43/08 20130101 |
Class at
Publication: |
428/811 ;
257/E43.004 |
International
Class: |
G11B 5/39 20060101
G11B005/39 |
Claims
1. A spin-valve device, comprising: two ferromagnetic electrodes;
and an organic spacer layer between the two ferromagnetic
electrodes.
2. The spin-valve device of claim 1, wherein the electrodes and
spacer are vertically stacked.
3. The spin-valve device of claim 1, including a metallic
ferromagnetic electrode.
4. The spin-valve device of claim 3, wherein the ferromagnetic
electrode is composed of Co, Ni, Fe, or alloys thereof.
5. The spin-valve device of claim 1, including a semi-metallic
ferromagnetic electrode.
6. The spin-valve device of claim 5, wherein the ferromagnetic
electrode is ReMnO.sub.3 or CrO.sub.2.
7. The spin-valve device of claim 1, including a .pi.-conjugated
organic semiconductor ferromagnetic electrode.
8. The spin-valve device of claim 7, wherein the ferromagnetic
electrode is selected from polythiophenes, polyparaphenylenes,
polyparaphenylenevynylenes, and polyfluorenes and their block
co-polymers.
9. The spin-valve device of claim 7, wherein the ferromagnetic
electrode is 4-thiophene, 6-thiophen, or 3-PPV, such as distyryl
benzene, or other small oligomer.
10. The spin-valve device of claim 7, wherein the ferromagnetic
electrode is a porphyrine, AlQ.sub.3, PBD, dendrimer, or other
small molecule.
11. The spin-valve device of claim 1, wherein the organic spacer
layer has a thickness of 50 nanometers or greater.
12. The spin-valve device of claim 1, wherein the thickness of one
or both of the ferromagnetic electrodes is 100 nanometers or
greater.
13. The spin-valve device of claim 1, wherein the electrodes and
spacer layer are configured in a planar geometry.
14. The spin-valve device of claim 1, wherein the electrodes are of
the same material but with different widths to control the
magnetization switching in each electrode independently.
15. The spin-valve device of claim 1, wherein the electrodes and
spacer layer are configured to show an I-V response curve
characteristic of a diode.
16. The spin-valve device of claim 1, wherein one or both of the
electrodes inject electrons and holes to generate an
electroluminescence emission upon application of an externally
applied bias voltage.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from U.S. Provisional
Patent Application Ser. No. 60/472,640, filed May 22, 2003, the
entire content of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to spin vales and, more
particularly, to spin valves incorporating an organic spacer.
BACKGROUND OF THE INVENTION
[0003] Spin-valves based on the effect of giant magnetoresistance
and tunneling magnetoresistance are currently used in high-density
magnetic recording heads and magnetoresistive random-access
memories. This type of device is based on electrical resistance
having two different values; say R.sub.1 and R.sub.2 that are
dependent on an applied external magnetic field. When a magnetic
head is in proximity to the spin-valve device it can change the
resistance, or it can change voltage if an electric current runs
through the device, between the two resistance values R.sub.1 and
R.sub.2. The change in electrical resistance does not involve extra
current or voltage; it just reacts to the external magnetic field.
A spin-valve can be regarded as a switch, wherein the application
of an external magnetic field does the switching.
[0004] A conventional vertical spin-valve device can be constructed
using two thin ferromagnetic layers (each with a thickness of less
than 100 nm) and a spacer in between, which can be a metallic or
insulating thin layer (a few nm thick). When the magnetization
orientation in the two adjacent ferromagnetic electrodes is
parallel to each other, the electrical resistance measured
perpendicular to the films has value R.sub.1; alternatively, when
the two magnetization orientations of the two ferromagnetic films
are anti-parallel to each other then the resistance is R.sub.2,
which is different than R.sub.1. The magnetization of the
electrodes can be arranged to be parallel or anti-parallel to each
other by an external magnetic field. The resistance change under
the influence of the magnetic field has been dubbed
magnetoreistance or MR.
SUMMARY OF THE INVENTION
[0005] Broadly according to this invention, the spacer in a
spin-valve is replaced with an organic layer. However, the
thickness of the layer is not limited, allowing for numerous
applications, including light-emitting structures. The invention
demonstrates that the spin coherence of the organic material is
sufficiently long that the carriers do not lose their spin memory
even in traversing a thicker passive barrier. At least three
methods to fabricate the organic spin-valve devices are disclosed,
in which the difficulties associated with depositing the
ferromagnetic (FM) and organic layers are addressed.
[0006] The Advantages of organic spin-valves over existing
inorganic spin-valves are many. First, they are less expensive and
easier to fabricate than their inorganic counterparts. There are
also many more choices for the materials that make up the organic
spacer. As examples, an intermediate layer may be chosen that emits
light, changes its electrical properties upon illumination, can be
doped in situ, and is sensitive to environmental physical and
chemical properties such as humidity, oxygen level, and other
environmental factors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 shows a spin valve constructed in accordance with
this invention; and
[0008] FIG. 2 is a graph that demonstrates a substantial
magnetoresistance effect based upon a device constructed in
accordance with this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0009] FIG. 1 shows a spin valve constructed in accordance with
this invention. There are three important layers in this device,
namely the two ferromagnetic electrodes (FM1 and FM2), and an
organic layer as a spacer. The device may be built on any suitable
substrate material. The ferromagnetic electrodes can be metallic
(e.g. Co, Ni, Fe or their alloys), half-metallic (e.g. ReMnO.sub.3,
Re being a rare earth element, or CrO.sub.2), or semiconducting
(e.g. GaMnAs). The organic layer can incorporate .pi.-conjugated
semiconductor polymers or small molecules (e.g. Alq.sub.3). In the
vertical spin-valve devices that we have demonstrated so far, FM1
was a ferromagnetic oxide, La.sub.0.7Sr.sub.0.3MnO.sub.3, (LSMO);
FM2 was a composite layer consisting of Co and Al; whereas the
organic semiconductor was Alq.sub.3. We have fabricated and
realized a large MR in this device (FIG. 2).
[0010] Similar spin-valve devices can be realized in planar
geometry, in which FM1 and FM2 electrodes may be the same material
but need to have different widths in order to control the
magnetization switching in each electrode independently. According
to this embodiment, we fabricated a vertical device based on two
different FM electrodes. In addition, we have also fabricated the
spin-valve device and demonstrated its switching capability upon
the application of an external magnetic field.
[0011] In the vertical organic spin-valve devices, the
ferromagnetic layers are typically high-melting temperature
material, whereas the organic semiconducting layer has typically
low melting temperature. Accordingly, during the FM electrode
deposition process, the deposition temperature needs to be much
lower than the melting point of the organic materials if the
organic materials have been already deposited. Higher temperatures
may evaporate the organic film away or cause intermixing between
the organic and FM materials that would deteriorate their internal
magnetization. As a result the intermixing at the FM/organic
interfaces may destroy the magnetoresistance.
[0012] In addition, the metallic ferromagnetic electrodes typically
oxidize very fast in air. The oxidized interfaces are detrimental
to magnetoresistance in the final devices. So it is advantageous to
fabricate the metallic electrodes together with the organic
semiconductors in vacuum. Sputtering (a common deposition
technique) is not preferred for the metallic electrode deposition
if the organic layer is already deposited because the plasma is
detrimental to the organic semiconductors. Thus, the film
deposition is preferably carried out in vacuum at low temperatures.
For some spin-injecting electrodes such as the ferromagnetic oxides
(e.g. LSMO), in-situ deposition is not required in fabricating the
organic spin-valve since they do not react with O.sub.2 in air.
They can be predeposited, cleaned and then introduced into the
vacuum chamber prior to the organic and the second electrode
deposition.
FABRICATION METHODS
[0013] In the following we describe various alternative fabrication
methods for the organic spin-valve.
Method 1
[0014] In this method the first layer, FM1 is a predeposited
ferromagnetic electrode that is not air sensitive, the organic
layer is deposited on FM1 by thermal evaporation at a relatively
low temperature, whereas the deposition of the second ferromagnetic
layer, FM2 is done by thermal evaporation with cooled substrates
and/or with a cooled region near the evaporation source so that the
excess heat can be taken away. This ensures that the vacuum chamber
is at a sufficiently low temperature that the deposited organic
layer will not evaporate away or intermix with FM2 at the
interface. The thermal evaporation of FM2 can be replaced with
electron-beam evaporation, which typically produces less heat if
the evaporation is from a focused spot.
Method 2
[0015] A second method can be independently used or used together
with the first method. The main idea is to deposit a very thin FM2
layer (thickness of the order of few nm) onto the organic layer so
that the high deposition temperature will be needed for a
relatively short time. We note that a very thin layer (.about.1 nm
or so) of ferromagnetic material is already adequate to establish
its ferromagnetism at the interface in order to produce the
magnetoreistance. Since a very thin FM2 layer is deposited then if
one starts with relatively thick organic layer, some of it would
evaporate away during the FM2 layer deposition, but some would
remain deposited on the first predeposited FM1 layer. To ensure the
device electrical connection and to protect the relatively thin FM2
layer, a low melting temperature metal (e.g. Al, Au) is evaporated
on top of FM2.
[0016] For demonstrating the organic spin valve we have used the
second method. The predeposited FM1 layer was a LSMO ferromagnetic
film. We deposited 120 nm thick film of the .pi.-conjugated organic
molecule AlQ.sub.3 (purchased from Aldrich), and FM2 layer was a
3.5 nm thick of cobalt. A protective layer of aluminum was then
deposited onto FM2. The magnetization properties of the
ferromagnetic layers FM1 and FM2 were separately measured by
magneto-optical technique (MOKE) and the temperature dependence of
the magnetization and coercive magnetic field was recorded.
Method 3
[0017] We refer to the third method as a flip/bond method. It can
be used independently or together with the above two methods. This
method works with either metallic or other ferromagnetic electrodes
as FM1 and FM2 layers. Both FM1 and FM2 electrodes are deposited
first following by an organic layer deposition in vacuum. Then the
electrodes that are already covered with the organic can be taken
out of the vacuum chamber. One electrode can be flipped with its
organic overlayer facing the other electrode with its own organic
overlayer. Then the two organic layers are brought together. The
electrodes can be aligned and then bonded by heating up to a
relatively low temperature to promote adhesion. This methods
ensures low temperature deposition and no intermixing at the
metal/organic interfaces
MATERIALS
[0018] The organic vertical spin valve is composed of at least
three layers; two ferromagnetic layers and an organic semiconductor
layer. Here we mention various possible materials that can be used
for this device.
[0019] 1. Ferromagnetic Layers
[0020] These may be metallic, half metallic or magnetic
semiconductors. Metallic ferromagnetic may be iron, cobalt, nickel
and their composites. The half metallic can be manganites and other
magnetic oxides.
[0021] 2. Organic Semiconductors (.pi.-conjugated) These can be
polymers such as polythiophenes, polyparaphenylenes,
polyparaphenylenevynylenes, and polyfluorenes and their block
co-polymers. Also they can be small oligomers of the above such as
4-thiophene, 6-thiophen, etc, or 3-PPV, such as distyryl benzene,
etc. Also they can be small molecules such as porphyrines,
AlQ.sub.3, PBD, dendrimers, etc.
[0022] In summary, we have fabricated and demonstrated an organic
spin-valve device. In addition, we have also successfully shown
that carriers (electron and/or holes) with aligned spins can be
injected into and transported coherently through .pi.-conjugated
organic semiconductor films. This opens up a new field with
opportunities to add new functionalities to the existing
spin-devices or develop entirely new devices.
OTHER APPLICATIONS
[0023] (a) The resistance of the organic spin-valves can be tuned.
This may be carried out by engineering the HOMO-LUMO levels of the
organic semiconductors relative to the ferromagnetic electrode
materials. This can have a great impact in magnetic recording and
magnetoresistive random access memory technologies.
[0024] (b) Since it has been discovered that the organic
semiconductors generally have a long spin diffusion length, the
organic spacer in the spin-valves can be made relatively thick.
This can make the fabrication process much more reproducible and
reliable in magnetic read heads and magnetoresistive random-access
memory.
[0025] (c) The organic layer is a semiconductor; therefore, the
conventional spin-valves can be made active with very interesting
possibilities. [0026] (i) The spin-valve can be fabricated to show
a characteristic I-V response curve of a diode. This can be
achieved if the work-functions of the two ferromagnetic electrodes
are chosen to be very different from each other. This would
eliminate or greatly simplify the complicated and expensive CMOS
process used for isolation transistors in the present
magnetoresistive random access memory. [0027] (ii) The organic
layer may be chosen to emit light. Then using ferromagnetic
electrodes, FM1 and FM2 to respectively inject electrons and holes
then the organic spin-valve actually is transformed into an organic
light emitting diode (OLED), with electroluminescence emission upon
application of an external bias voltage. We note that the
efficiency of such OLED is greatly enhanced if the spins of the
injected electrons and holes are controlled by an external magnetic
field. In addition the electroluminiescence emission intensity may
be controlled by an external magnetic field. [0028] (iii) The
electrical characteristic properties, as well as the MR value might
be changed upon light illumination. [0029] (iv) Again the
electrical properties may be changed upon in situ doping in the gas
phase.
* * * * *