U.S. patent application number 17/584918 was filed with the patent office on 2022-09-22 for acoustic excitation and detection of spin waves.
The applicant listed for this patent is Oxford University Innovation Limited. Invention is credited to Tsz Cheong FUNG, John Francis GREGG, Burkard HILLEBRANDS.
Application Number | 20220299583 17/584918 |
Document ID | / |
Family ID | 1000006366168 |
Filed Date | 2022-09-22 |
United States Patent
Application |
20220299583 |
Kind Code |
A1 |
GREGG; John Francis ; et
al. |
September 22, 2022 |
ACOUSTIC EXCITATION AND DETECTION OF SPIN WAVES
Abstract
Apparatus for generating spin waves comprising a body (102) of
magnetic material and an elastic wave generator (120), wherein the
body (102) has a surface (108) and the elastic wave generator (120)
is arranged to transmit elastic waves so that they propagate
through the body (102) towards the surface (108) and are reflected
at the surface to form a standing elastic wave in the body (102),
thereby generating spin waves.
Inventors: |
GREGG; John Francis; (County
Wicklow, IE) ; FUNG; Tsz Cheong; (Hong Kong, CN)
; HILLEBRANDS; Burkard; (Kaiserslautern, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Oxford University Innovation Limited |
Oxford |
|
GB |
|
|
Family ID: |
1000006366168 |
Appl. No.: |
17/584918 |
Filed: |
January 26, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16461393 |
May 16, 2019 |
11249153 |
|
|
PCT/GB2017/053424 |
Nov 14, 2017 |
|
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17584918 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 33/1284 20130101;
H03B 15/006 20130101; H01F 10/24 20130101; H01L 41/04 20130101 |
International
Class: |
G01R 33/12 20060101
G01R033/12; H01L 41/04 20060101 H01L041/04; H03B 15/00 20060101
H03B015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 2016 |
GB |
1619559.6 |
Claims
1-29. (canceled)
30. Apparatus for generating elastic waves from spin waves, the
apparatus comprising a waveguide along which spin waves can
propagate, a piezoelectric element, two electrodes located on
opposite sides of the piezoelectric element, and an electrical
connection between the two electrodes, wherein the piezoelectric
element and the electrodes are mounted on the waveguide whereby
propagation of spin waves along the waveguide will generate an
oscillating electrical voltage across the piezoelectric element to
generate an elastic wave.
31. Apparatus according to claim 30 further comprising an elastic
wave detector arranged to detect the elastic wave thereby to
generate a detection signal wherein the waveguide has first and
second opposite sides, the piezoelectric element is located on a
first one of the opposite sides, and the elastic wave detector is
located on a second one of the opposite sides whereby the elastic
waves will propagate through the waveguide between the
piezoelectric element and the elastic wave detector.
32. Apparatus according to claim 31 wherein the piezoelectric
element and the elastic wave detector are located on opposite sides
of the waveguide so that the elastic wave will propagate through
the waveguide between the piezoelectric element and the elastic
wave detector.
33. A method of detecting spin waves in a waveguide, the method
comprising providing a piezoelectric element having first and
second opposite sides, and a pair of electrodes each located on a
respective one of the opposite sides, the electrodes having an
electrical potential therebetween, and detecting variations in the
electric potential thereby to detect the spin waves.
34. Apparatus for generating elastic waves from spin waves, the
apparatus comprising a waveguide in which spin waves can propagate,
the waveguide having a surface, and a magnetostrictive element
formed on the surface of the waveguide, whereby elastic deformation
resulting from a spin wave propagating along the waveguide in the
vicinity of the magnetostrictive element will generate an elastic
wave propagating through the waveguide.
35. Apparatus according to claim 34 further comprising an elastic
wave detector arranged to detect the elastic wave.
36. Apparatus according to claim 35 wherein the elastic wave
detector comprises a piezoelectric device comprising a
piezoelectric element and two electrodes arranged to detect a
varying electric field in the piezoelectric element.
37. Apparatus according to claim 35 wherein the elastic wave
detector is arranged opposite the magnetostrictive element on an
opposing surface of the waveguide.
38. Apparatus according to claim 36 wherein the elastic wave
detector is arranged opposite the magnetostrictive element on an
opposing surface of the waveguide.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. patent
application Ser. No. 16/461,393, filed May 16, 2019, which is the
National Stage of International Application No. PCT/GB2017/053424,
filed Nov. 14, 2017, which claims the priority to GB 1619559.6,
filed Nov. 18, 2016, which are entirely incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the generation and
detection of spin-waves, and in particular to acoustic systems for
such generation and detection.
BACKGROUND TO THE INVENTION
[0003] Spin waves correspond to the phase-coherent precession of
coupled magnetic spins of electrons in magnetically ordered
materials. FIG. 1 illustrates the precessing spins of a spin-wave
in a 1-D chain. The spins are precess coherently and angular
momentum is transferred as the wave propagates. The quanta of spin
waves are called magnons. Magnonics, a fusion of the words "magnon"
and "electronics", refers to the use of spin waves to transmit,
store, and process information. By contrast with electronic signal
processing in semiconductors which requires use of charge currents,
magnonics has potential advantages. Magnons, as data carriers for
computing devices, offer Joule-heat-free spin information transfer
and operate at high frequency, typically in the GHz regime, where
their micron sized wavelengths are conducive to progressive device
miniaturization.
[0004] The GHz frequency spin-wave spectrum is traditionally
divided into three regions: the magnetostatic or dipole region, the
dipole-exchange region, and the exchange region. The wavelengths of
the excitations get progressively shorter moving from the
magnetostatic region to the exchange region. The dynamics of
magnetostatic spin waves (MSW) are dominated by long-range
dipole-dipole interaction between the ordered spins. In a magnon
waveguide with lateral dimensions comparable with the wavelength of
the spin-waves, the magnetic boundary conditions give rise to a
diversity of dispersion relations. The ease with which these
dispersion relations may be controlled and modified by tuning the
amplitude and/or direction of the applied magnetic field underpins
numerous technological applications.
[0005] The most common means by which to excite MSWs in magnetic
waveguides employs narrow strip-line antennae. In this technique, a
high-frequency current is passed through the antennae, giving rise
to a magnetic field which couples to the magnetic structure of the
waveguide material, launching a wave. This technique is simple and
practical to implement but suffers from three limitations: owing to
the linear geometry, punctual MSW excitation is not possible, the
requirement for significant charge current makes the process
power-hungry and heat generating, and the antenna geometry is not
easily scalable to the nanometre scale on which information
processing will eventually need to function.
SUMMARY OF THE INVENTION
[0006] The present invention provides apparatus for generating spin
waves comprising a body of magnetic material and an elastic wave
generator, wherein the body has a surface and the elastic wave
generator is arranged to transmit elastic waves so that they
propagate through the body towards the surface and are reflected at
the surface, thereby generating spin waves. The reflection of the
elastic waves may be arranged to form a standing elastic wave in
the body.
[0007] The spin waves may be magnetostatic (dipole) spin waves,
dipole-exchange spin waves, or exchange spin waves.
[0008] The body may comprise a film having two opposite surfaces.
The elastic wave generator may be arranged to transmit elastic
waves in a direction perpendicular to the surfaces. The wavelength
of the elastic wave may be in the order of the film thickness to
enable efficient magnon pulse excitation. For example the thickness
of the film may be in the range from 0.25 to 6.0 times the elastic
wavelength. In some cases it may be preferable that the elastic
wave has a wavelength in the range from 0.5 to 2 times the
thickness of the film.
[0009] The elastic wave propagation direction may be perpendicular
to the surfaces of the film.
[0010] The film may be narrow, for example in the form of a strip,
so as to form a 1-dimensional waveguide in which spin waves can
propagate in one dimension along the length of the waveguide.
However the film may extend in two dimensions so as to form a
two-dimensional waveguide, in which spin waves can propagate in two
dimensions, for example in at least two orthogonal directions.
[0011] The film may be formed on a substrate. The elastic wave
generator may be formed on the opposite side of the substrate to
the film.
[0012] The surface may have a surface feature therein. The elastic
wave generator may be arranged to transmit the elastic waves so
that they propagate in a propagation direction towards the surface
feature. The elastic waves may be reflected at the surface. The
elastic waves may interact with the surface feature to generate
spin waves.
[0013] The apparatus may further comprise a further material
extending over the surface. The further material may have a
different acoustic refractive index than the body.
[0014] The apparatus may further comprise a further material
extending over the surface feature. The further material extending
over the surface and the further material extending over the
surface feature may be the same material.
[0015] The further material may have a pronounced spin-orbit
interaction, for example it may be gold.
[0016] The surface feature may be a recess in the surface. The
surface feature may have a maximum dimension of no more than 100
nm, or no more than 10 nm.
[0017] The elastic wave generator may be a piezoelectric device.
The device may comprise a piezoelectric element and two electrodes
arranged to apply an electric field across the piezoelectric
element to generate the elastic waves.
[0018] The two electrodes may be on opposite sides, e.g. on
opposing surfaces, of the piezoelectric element. Alternatively the
two electrodes may be on the same side of the piezoelectric element
and arranged to have an oscillating voltage applied between them,
and a further electrode may be provided on the opposite side of the
piezo electric element so that capacitive coupling between the
further electrode and one of said two electrodes produced an
electric field across the piezoelectric element.
[0019] The apparatus may further comprise a DC voltage source
arranged to apply a variable DC electric field to the piezoelectric
element thereby to shift the phase of the elastic waves.
[0020] The apparatus may comprise a phase shifting device located
between the elastic wave generator and the body of magnetic
material. The phase shifting device may be arranged to move the
elastic wave generator, for example to vary the distance between
the elastic wave generator and the body of magnetic material. This
may shift the phase of elastic waves, for example at one or more
surfaces of the waveguide. The phase shifting device may comprise a
piezoelectric element and electrodes to which a voltage can be
applied to vary a dimension of the piezoelectric element.
[0021] The invention further provides a method of generating spin
waves comprising providing a body of magnetic material having a
surface, applying a magnetic field to the magnetic material, and
transmitting elastic waves so that they propagate through the body
towards the surface and are reflected at the surface to form a
standing elastic wave in the body, thereby generating spin
waves.
[0022] The elastic waves may be generated using a piezoelectric
device comprising a piezoelectric element and two electrodes
arranged to apply an electric field across the piezoelectric
element to generate the elastic waves. The method may further
comprise applying a varying DC electric field to the piezoelectric
element thereby to shift the phase of the elastic waves and the
MSWs.
[0023] The present invention further provides apparatus for
generating elastic waves from spin waves, the apparatus comprising
a waveguide along which spin waves can propagate, a piezoelectric
element, two electrodes located on opposite sides of the
piezoelectric element, and an electrical connection between the two
electrodes, wherein the piezoelectric element and the electrodes
are mounted on the waveguide so that propagation of spin waves
along the waveguide will generate an oscillating electrical voltage
across the piezoelectric element to generate an elastic wave.
[0024] The apparatus may further comprise an elastic wave detector
arranged to detect the elastic wave thereby to generate a detection
signal.
[0025] The piezoelectric element and the elastic wave detector may
be located on opposite sides of the waveguide so that the elastic
wave will propagate through the waveguide between the piezoelectric
element and the elastic wave detector.
[0026] The invention further provides a method of detecting spin
waves in a waveguide, the method comprising providing a
piezoelectric element and a pair of electrodes located on opposite
sides of the piezoelectric element, and detecting variations in the
electric potential between the electrodes thereby to detect the
spin waves.
[0027] The invention further provides apparatus for generating
elastic waves from spin waves, the apparatus comprising a waveguide
in which spin waves can propagate, and a magnetostrictive element
formed on the surface of the waveguide, whereby elastic deformation
resulting from a spin wave propagating along the waveguide in the
vicinity of the magnetostrictive element will generate an elastic
wave propagating through the waveguide.
[0028] The apparatus may further comprise an elastic wave detector
arranged to detect the elastic wave. The elastic wave detector may
comprise a piezoelectric device comprising a piezoelectric element
and two electrodes arranged to detect a varying electric field in
the piezoelectric element.
[0029] The elastic wave detector may be arranged opposite the
magnetostrictive element on the opposing surface of the
waveguide.
[0030] The apparatus may further comprise any one or more features
of the preferred embodiments of the invention which are shown in
the accompanying drawings, as will now be described in more detail
by way of example only.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic representation of spin waves;
[0032] FIG. 2 is schematic view of an apparatus according to an
embodiment of the invention for generating MSWs;
[0033] FIG. 3a is a cross section through the apparatus of FIG.
2;
[0034] FIG. 3b shows the formation of an elastic standing wave in
the apparatus of FIG. 2;
[0035] FIG. 3c shows possible modes of the standing waves of FIG.
3b;
[0036] FIG. 3d shows magnetoelastic coupling of the modes of FIG.
3c;
[0037] FIG. 3e shows the effect of waveguide thickness on
magnetoelastic coupling;
[0038] FIG. 4a is a cross sectional view through a piezoelectric
elastic wave generator according to an embodiment of the
invention;
[0039] FIG. 4b is a cross sectional view of a piezoelectric elastic
wave generator according to a further embodiment of the
invention;
[0040] FIG. 5 shows schematically a sputtering plant suitable for
producing the device of FIG. 4;
[0041] FIG. 6 is a vertical section through a magnetron forming
part of the plant of FIG. 5;
[0042] FIG. 7 is a transverse section through the magnetron of FIG.
6;
[0043] FIG. 8a shows a first event in the operation of the
apparatus of FIG. 2;
[0044] FIG. 8b shows a second event in the operation of the
apparatus of FIG. 2;
[0045] FIG. 8c shows a third event in the operation of the
apparatus of FIG. 2;
[0046] FIG. 8d shows a fourth event in the operation of the
apparatus of FIG. 2;
[0047] FIG. 9a show signals from the piezoelectric device and
detector aerial in the apparatus of FIG. 2 during operation with an
antenna-probe distance of 0.86 mm;
[0048] FIG. 9b show signals from the piezoelectric device and
detector aerial in the apparatus of FIG. 2 during operation with an
antenna-probe distance of 2.36 mm;
[0049] FIG. 9c show signals from the piezoelectric device and
detector aerial in the apparatus of FIG. 2 during operation with an
antenna-probe distance of 3.36 mm;
[0050] FIG. 9d show signals from the piezoelectric device and
detector aerial in the apparatus of FIG. 2 during operation with an
antenna-probe distance of 4.36 mm;
[0051] FIG. 10 is a graph of MSW transit time as a function of
distance travelled as measured using the apparatus of FIG. 2;
[0052] FIG. 11 is a graph of calculated and measured speeds of MSWs
as a function of external magnetic field angle;
[0053] FIG. 12a shows the cross sectional view of a first designs
of MSW detector;
[0054] FIG. 12b shows the cross sectional view of a second designs
of MSW detector; and
[0055] FIG. 13 is a schematic view of a MSW generator according to
a further embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] Referring to FIG. 2, a magnon launcher according to the
invention comprises a magnetic material 100, such as yttrium iron
garnet (YIG), with an external magnetic field B.sub.ext applied to
it so that magnetostatic spin waves (MSWs) can propagate through
it. Other spin waves, such as exchange spin waves can also
propagate through the magnetic material 100, and where MSWs are
referred to specifically herein this should be taken to refer to
spin waves generally where the context allows. The magnetic
material 100 may be in the form of a layer or film 102 which may be
formed on a substrate 104 so that it has an inner surface 106 which
is in contact with the substrate 104 and an outer surface 108 which
is parallel to the inner surface 106. The external magnetic field
B.sub.ext may be at some angle .theta..sub.ext to the normal to
surfaces 106, 108. The layer 102 of magnetic material 100 may form
a waveguide along which MSWs can propagate. For example, the
magnetic material 100 may be a high quality monocrystalline YIG
thin film grown by liquid phase epitaxy. The substrate 104 may be
formed of Gadolinium Gallium Garnet (GGG). The YIG film in the
embodiment of FIG. 2 is 7.8 .mu.m thick and the GGG substrate 104
is 0.45 mm thick. The external magnetic field B.sub.ext will
produce an internal magnetic field H.sub.in within the magnetic
material 100 which will be at some angle .theta. to the surfaces
106, 108. The waveguide 102 may be in the form of a strip. The
waveguide 102 may form a 1-dimensional waveguide, in which the MSW
propagates along the length of the waveguide 102. Alternatively the
waveguide 102 may extend significantly in two dimensions forming a
2-dimensional waveguide, in which case MSWs can in general
propagate in a range of directions within the two-dimensional plane
of the waveguide, although there are some directions in which
propagation is not possible. The magnetic material may have a cubic
crystal lattice, for example if it is YIG. The <111> crystal
axis of the magnetic material may be perpendicular to the outer
surface 108 of the waveguide 102. The <110> axis may be
parallel to the direction 110 in which MSW propagates.
[0057] An ultrasonic transducer 120, which may be a piezoelectric
transducer, for example formed of ZnO, is arranged to transmit
elastic waves in the form of bulk acoustic waves (BAWs) into the
magnetic material 100. The transducer 120 may be grown at the back
side 112 of the substrate 104, i.e. on the opposite side of the
substrate 104 to the waveguide 102. The transducer is arranged to
transmit the BAWs in a BAW propagation direction, which is the
direction in which they will propagate into the waveguide, as shown
by the arrow 114 in FIG. 2. The BAW propagation direction may be
perpendicular to the outer surface 108, and the inner surface 106,
of the waveguide 102. The BAW propagation direction may therefore
be perpendicular to the propagation direction (or directions) 110
of the MSWs in the waveguide. In this configuration, volume MSWs,
i.e. spin-wave modes that are active (non-decaying) throughout the
whole ferrite region can be excited. When the internal magnetic
field H.sub.in is perpendicular to the inner and outer surfaces
106, 108 of the waveguide 102, the spin-wave has positive group
velocity meaning that it is in the same direction as that of the
phase velocity. Such spin-wave modes are known as the forward
volume magnetostatic spin-waves (FVMSW). In contrast, when the
magnetic field H.sub.in is pointing parallel to the inner and outer
surfaces 106, 108 of the waveguide 102, backward volume
magnetostatic spin-waves (BVMSW) with their phase velocities
opposite to the group velocities can exist. If the external
magnetic field is pointing at an arbitrary angle .theta..sub.ext to
the normal of the plane, both FVMSW and BVMSW types can propagate
simultaneously in the waveguide 102.
[0058] In order for the elastic waves generated in the waveguide
102 to generate MSWs there has to be coupling between the elastic
waves and the MSWs. The requirements for effective coupling will
now be described with reference to FIG. 3. In FIG. 3a, the
waveguide 102, substrate 104 and transducer 120 of FIG. 2 are
shown. Assuming that the acoustic impedances of the substrate 104
and the waveguide 102 are similar, and different to that if air (or
any coating that may be applied to the waveguide), then elastic
waves generated by the transducer 120 will be reflected at the
outer surface 108 of the waveguide and a standing wave can be set
up in the waveguide 102 as shown in FIG. 3b. The oscillating
magnetic field generated by the acoustic standing wave due to
magnetoelastic coupling is then mode coupled to the MSWs. FIG. 3c
shows the various modes of possible MSWs in the waveguide 102,
assuming the unpinned condition in which the amplitude of the MSWs
is at a maximum at both the inner and outer surfaces 106, 106 of
the waveguide. The n=0 mode, in which the MSW has a constant
amplitude across the thickness of the waveguide, is the most easily
detected using an antenna. The higher mode is when the MSW has
varying amplitude across the thickness of the waveguide. The odd
mode is where the MSW amplitude is equal and opposite at the two
surfaces 106, 108 of the waveguide. The even mode is where the MSW
amplitude is equal at the two surfaces 106, 108 of the waveguide.
Referring to FIG. 3d, in principle, the oscillating magnetic field
caused by acoustic disturbance can couple to infinite family of
modes, however, the higher modes have lower excitation efficiencies
owing to their low MSW speeds, hence only the lowest order mode
(n=0) mode is excited. Referring to FIG. 3e, in theory, the maximum
coupling to the n=0 mode MSWs is where the thickness L of the
waveguide 102 is an odd number of quarter wavelengths of the
elastic waves, i.e. L=.lamda./4, L=3.lamda./4, L=5.lamda./4 where
.lamda. the elastic wavelength in the waveguide. Therefore the
waveguide should preferably have a thickness which is close to an
odd multiple of a quarter of the wavelength of the elastic waves in
the body, or which differs from an odd multiple of a quarter of the
wavelength of the elastic waves in the body by no more than one
eighth of that wavelength. This puts the thickness closer to the
optimum thickness than to the least optimum thickness. In practice,
the propagation of spin waves does not reduce to zero at the
theoretical minima, and provided the thickness of the waveguide is
of the order of the wavelength of the acoustic wave, spin waves can
be successfully generated. For example the thickness of the
waveguide may in the range from 0.25 to 4 times the acoustic
wavelength, or in the range from 0.5 to 2 times the acoustic
wavelength.
[0059] It will be appreciated that, if the size of the transducer
is large relative to the wavelength of the MSWs then there will be
destructive interference between the MSWs generated at different
points across that volume in the MSW propagation direction.
Therefore, the transducer 120 can only be used to generate MSWs
that have a wavelength greater than the diameter of the
transducer.
[0060] Referring to FIG. 4 the transducer 120 may comprise a layer
300 of piezoelectric material, such as zinc oxide, between two
electrodes 302, 304. One of the electrodes 302 may be formed on the
substrate 104, with the piezoelectric material 300 formed on that
electrode 302, and the other electrode 304 formed on the
piezoelectric layer 300. One of the electrodes 302 may therefore be
between the substrate 104 and the piezoelectric layer 300, and the
other electrode 304 may be on the opposite side of the
piezoelectric layer to the substrate 104. Each of the electrodes
302, 304 may comprise a main layer 306, 308 of metal, preferably
highly conductive such as gold, and may comprise a further layer
310, 312 of a further metal, such as chromium, which may act as an
adhesive agent preventing the main layer 306, 308 from peeling away
due to intrinsic stress. The electrodes should be made as thin as
possible so as to avoid mass loading, but in reality thin
electrodes tend to be porous and less conductive. Therefore, the
gold layer 306, 308 may be, for example, 25 nm thick, and the
chromium layer may be, for example 4 nm thick. The piezoelectric
layer may be of the order of 1000 nm thick, and the diameter of the
transducer may be around 300-500 .mu.m, but can be miniaturised to
nm scale using lithography.
[0061] A variable voltage source may be connected between the top
and bottom electrodes 302, 304 and the voltage applied between them
oscillated to generate elastic waves. However, as shown in FIG. 3,
the top electrode 304 may be grounded and connected via the voltage
source 314 to a further top electrode 316, having the same layer
structure, which is isolated from ground and forms a signal
electrode, to which the varying voltage (relative to the grounded
electrode) is applied. The signal electrode 316 may be
significantly smaller than the grounded top electrode 304, which
may be of a similar size to the bottom electrode 302. The bottom
electrode, which may have no direct connection to ground, may then
be coupled to the grounded top electrode 304 by a large
capacitance, making it effectively grounded also, so that an
oscillating voltage applied between the two top electrodes 306, 316
produces an oscillating potential difference between the signal
electrode 316 and the bottom electrode 302 which generates elastic
waves from the piezoelectric layer 300.
[0062] This structure with two top electrodes 304, 316 means that
the bottom electrode 302 does not need to be connected to the
electrical supply, which avoids the need for post-deposition
lithography to expose the bottom electrode for connection. This
saves time and cost and avoids the possibility of such lithography
damaging the piezoelectric film 300.
[0063] A further advantage of piezoelectric transducers is that
they can be used to modulate the phase of the elastic waves that
they generate, and hence also of the MSWs generate. This may be
achieved by applying a variable DC power supply 320 to apply a
variable DC voltage across the piezoelectric element 300, either
using the electrodes 302, 304 which are also used for the elastic
wave generation, as shown in FIG. 4, or using separate electrodes.
Varying this DC voltage changes the thickness of the piezoelectric
element 300, and therefore varies the distance of the effective
point of generation of the elastic waves from the waveguide 102.
This therefore varies the phase of MSWs at any point along the
waveguide relative to the phase of the oscillating voltage used to
generate the elastic waves. A similar effect may be obtained by
providing a further piezoelectric layer between that 300 used to
generate the elastic waves and the waveguide, for example on the
surface of the substrate 104, with electrodes on either side of it
so that its thickness can be varied by applying the variable DC
voltage across it. Indeed, in such a structure which is a modified
version of that of FIG. 4, the inner electrode 302 of the device of
FIG. 4 could serve also as one of the electrodes for the variable
DC voltage.
[0064] Referring to FIG. 4(b) to achieve a similar phase modulation
effect, a second piezoelectric element 318 such as quartz may be
deposited underneath the first piezoelectric element 120 which is
used to generate the elastic wave. This second piezoelectric
element 318 may have electrodes 324, 322 on its top and bottom
surfaces. One of the electrodes 322 may therefore be between the
substrate 104 and the second piezoelectric element 318, and the
other of the electrodes 324 may be between the second piezoelectric
element 318 and first piezoelectric element 120. The variable DC
supply 320 may then be connected between the electrodes 322, 324 so
that variation in the DC voltage from the supply 320 causes a
variation in the thickness of the second piezoelectric element 318,
which shifts the phase of the elastic waves generated in the
substrate 104 and the waveguide 102 by the first piezoelectric
element 120.
[0065] The transducer 120 may be arranged to operate in the GHz
range of frequencies, for example from 1 to 10 GHz. For the
transducer 120 to function effectively at these frequencies and to
be made on length scales suitable for magnonics systems, it is
important for the piezoelectric material to be of high quality. If
ZnO is used, then a suitable method of forming the ZnO layer is
magnetron sputtering. Magnetron sputtering can be performed using
either a DC or an RF electrical supply. As ZnO is a semiconductor
an RF sputtering system may be used, operating for example at 13.56
MHz which applies an oscillating voltage to the target relative to
the metal walls of the vacuum chamber.
[0066] Referring to FIG. 5, a sputtering plant suitable for
producing the transducer 120 comprises a vacuum chamber 400, which
is cylindrical and of stainless steel with a glass cover 401 in
which is situated a target 402 and a magnetron 404. The chamber 400
has an evacuation port 406 that is connected to a diffusion pump
408, for example an E06 oil diffusion pump, via a liquid nitrogen
trap 410. A butterfly valve 412 is provided in the evacuation port
406 to close it when the chamber 400 has been evacuated. Sputtering
gas, such as an argon/oxygen mixture, can be introduced into the
chamber 400 via a leak valve, and a Peltier cooler is provided to
reduce the moisture content of the gas mixture before it enters the
chamber 400. A Meissner trap 414 is provided in the chamber 400,
and cooled by liquid nitrogen, to selectively remove any residual
water vapour from the sputtering gas mixture in the chamber. The
chamber 400 is mounted on a mechanical support 416 with the
nitrogen trap and diffusion pump 408 located below the support
410.
[0067] A substrate holder 420 is supported in the chamber 400, on a
manoeuvrable feedthrough 422 above the target 402. An RF frequency
power source 424, in this case operating at 13.56 MHz, is connected
to the magnetron 404, and therefore to the target 402, via a
matching circuit 426. The power source 424 is connectable to the
substrate holder 420 for sputter cleaning the substrate, but the
substrate holder is otherwise electrically floating.
[0068] Referring to FIGS. 6 and 7, the magnetron may be designed in
various ways, but in one embodiment magnets 500 of opposite
polarities are arranged on a mild steel back plate 502.
Specifically, a central magnet 504 is located at the centre of the
back plate 502, and peripheral magnets 506 are arranged in a circle
around the periphery of the back plate, with the polarity of the
peripheral magnets 506 all being the same as each other, and
opposite to that of the central magnet 504. The target 402, which
in this case is of ZnO, is supported over the magnets 500 on the
opposite side to the back plate 502 and parallel to it. At a
certain radial distance r from the centre of the back plate 502,
the B-field produced is parallel to the target surface 508. Since
the electrons drift in the ExB direction, and the electric field is
substantially normal to the target surface, or vertical as shown in
FIG. 6, these electrons are confined in the ring of radius r near
the target surface 508 where the plasma is at its densest. This
region of the magnetron is known as the racetrack and it
corresponds to maximum target erosion. The magnets 500 are strong
N42 neodymium iron boron (NdFeB) permanent magnets which provide
confinement of a dense plasma.
[0069] The ratio of the target diameter to thickness is kept as
high as possible. This geometry tends to enhance the field near the
racetrack, thus giving stronger plasma confinement. It has the
disadvantage that the thin targets necessarily employed have a
limited lifetime. A large diameter target is therefore desirable to
allow a target thickness of at least 5 mm and hence a reasonable
working life. The ZnO target in the embodiment shown has a diameter
of 89 mm and a thickness of 6.6 mm.
[0070] A thermally and electrically conducting silver-loaded epoxy
glue may be used to attach the back of the target 402 to a copper
disk 510 which may be soldered onto a brass annulus 512. The
annulus 512 may be mounted on the back plate 502 between the
central and peripheral magnets 504, 506. This whole target unit may
be fitted to the magnetron assembly using two screws, which allows
the targets to be conveniently interchangeable.
[0071] The magnetron is water cooled using a cooling circuit 514 in
the back plate 502 to avoid overheating of either the target
material (which causes cracking) or the magnet (residual
magnetization degrades at high temperature). A vacuum gap is
engineered between the annulus and the NdFeB to avoid heat transfer
from the hot target.
[0072] A grounded shield 516 is arranged close to the target 402,
extending behind the back plate 502 and around the sides of the
magnetron 404, to avoid plasma from striking at the back of the
magnetron. This prevents unwanted sputtering and thus minimizes the
impurities introduced to the fabricated film.
[0073] In operation, the electric field generated by the power
source 424 acts as a trap for any free electrons in the chamber 400
and imparts an oscillating kinetic energy to them in one dimension.
Quasi-elastic collisions with gas atoms feed this energy
progressively into the other two dimensions where it accumulates
until the electron acquires enough energy to ionise a gas atom,
thereby triggering another identical process and hence a chain
reaction which then maintains the sputtering plasma. The
positive-going excursions of the target suck in bursts of highly
mobile electrons in the plasma. The target is thus set at a
negative DC voltage so the positive ions are strongly attracted
towards the target surface. They thus bombard the target and
transfer their momentum to the ejected surface atoms. These ejected
atoms migrate to the adjacent substrate and a thin film is produced
as these atoms reform on the substrate surface.
[0074] Sputtering is performed in a vacuum chamber at a working
pressure that is typically in the range from 1 mtorr to 50 mtorr.
The low pressure allows the sputtered particles to travel directly
to the substrate without hitting other gas molecules on the way.
During the sputtering process, ions and molecules in the sputtering
plasma eventually de-excite emitting photons that give rise to a
glow that enables the plasma location and characteristics to be
visually monitored. A satisfactory ZnO sputter discharge in an
Ar/O2 gas mixture has an easily recognised lilac glow.
[0075] In order to grow high quality ZnO films, the sputtering
conditions need to be controlled. During growth of ZnO films in an
Ar/O.sub.2 sputtering environment, the substrate is bombarded by
negative oxygen (O.sup.-) ions and positive ions from the ionised
gas. The positive ions are accelerated towards the substrate with
an energy e(V.sub.s,DC-V.sub.f,DC) (of the order of 10 eV) which is
the DC component of the difference in potential between the plasma
(which can be measured for any system using a Langmuir probe) and
the substrate. The power of the ion bombardment of the substrate
per unit area is hence proportional to J (V.sub.s,DC-V.sub.f,DC)
where J is the ion flux rate in the plasma. Tests using the system
of FIGS. 4 and 5 to grow c-axis oriented ZnO films, showed that,
for any deposition rate of ZnO, reducing the energy
e(V.sub.s,DC-V.sub.f,DC) and the ion flux J, and therefore reducing
the ion bombardment power, tends to improve the crystal quality, as
measured using X-ray diffraction. Therefore, in general terms we
can define a parameter:
.XI.=J(V.sub.s,DC-V.sub.f,DC)/D,
where D is the deposition rate of ZnO onto the substrate, and as
shown in FIG. 6, minimizing the parameter .XI. will tend to
maximise the quality of the ZnO transducer element. It was shown
that good quality transducers can be made by keeping .XI. below
1.3*10.sup.26 Vm.sup.-3.
[0076] Hence, an unbalanced ZnO magnetron which promotes ion
bombardment of the substrate is therefore not conducive to growing
c-axis oriented ZnO films. It is believed that the best growth
conditions for c-axis oriented ZnO films could be more easily
achieved by using a balanced magnetron in a sputtering condition
with high substrate target distance, high power and low pressure. A
balanced magnetron (which can be achieved by making the area of the
ring pieces and the central magnet roughly equal) with strong
magnetic field confinement prevents charge species from escaping
the target region so that a nearby substrate can simultaneously
enjoy the benefit of high growth rate and low ion bombardment that
are the key to making high quality c-axis oriented piezoelectric
films.
[0077] Referring now back to FIG. 2, to demonstrate that magnons
can be excited by acoustics, a magnon launcher as shown in FIG. 2
was produced in which a row of identical transducers 120 were
formed on the back surface 112 of the substrate 104, spaced apart
along the <110> direction of the GGG crystal, i.e. along the
direction of propagation of magnons and along the length of the
waveguide. An antenna 116 was formed across the waveguide on the
YIG film 102. The antenna comprised a thin wire and was connected
to an amplifier and a rectifier and a digital oscilloscope used to
measure the resultant signal. This allowed MSWs, which generate
oscillating electric signals in the antenna, to be detected. The
temporal response of the waveguide when it was subject to an
acoustic pulse excitation was then monitored. The sequence of
events that occur in such a system after the acoustic wave is
launched is shown in FIG. 8, in which only one of the transducers
is shown. Firstly, at time t=-T/2, where T is the round trip time
of the acoustic wave from the transducer 120 back to the transducer
120 after reflection from the outer surface of the waveguide, a BAW
pulse is launched by applying a 15 ns pulse of 3 GHz carrier to the
top electrode of a ZnO transducer 120. After time T/2 (t=0), the
acoustic pulse, having passed through the YIG inner surface 106 and
propagated through the YIG, is reflected from the YIG outer surface
108, setting up a standing wave in the YIG waveguide and thereby
creates a lateral propagating spin-wave. This wave propagates along
the waveguide, and after a time T.sub.m (at t=T.sub.m), the wave is
picked up by the antenna 116 and is converted into an
electromagnetic signal. This signal is subsequently processed with
appropriate amplification and rectification and is monitored using
a fast digital oscilloscope. As the reflected acoustic wave returns
into the transducer 120 at t=T, the acoustic echo signal is
detected using the transducer 120, and the acoustic wave is
reflected back towards the YIG waveguide. This cycle repeats until
the acoustic wave fades out due to round-trip attenuation.
[0078] The acoustic pulse response of the waveguide at a top
dot-antenna distance of 0.86 mm (by choosing to contact the
transducer that is closest to the antenna) was first measured and
the magnetic field was tuned such that a 3 GHz magnon signal can
transmit through the YIG waveguide. FIG. 9(a) shows in its top half
the plot of the output antenna data at B=90 mT and
.theta..sub.ext=30.degree. at the antenna-transducer distance of
0.86 mm; the echo signals of the phonons are also plotted in the
bottom half of the same graph for comparison. It can be clearly
seen that for every acoustic echo, there is a corresponding magnon
signal. The acoustic wave decays exponentially as expected due to
attenuation and the generated magnons also decays proportionally.
The magnon transit time was measured by finding the difference in
time between the middle point of two consecutive acoustic echoes
(in the time at which the phonons hit the YIG) and the time of
arrival of the magnon signal. This measurement gives 27.8 ns which
agrees with the predicted result according to our calculation of
the magnon velocity for FVMSW as shown in FIG. 11.
[0079] It will be noted in FIG. 9a that there are some spurious
signals near the point when the electromagnetic signal is injected
initially. This is caused by two effects. Firstly, there is a
direct electromagnetic coupling between the contact and the antenna
and thus a signal is measured at almost exactly the instant at
which the input acoustic signal is launched. Secondly, the stray
electromagnetic field of the probe extends far away spatially, so
spin-waves can be excited near the antenna; this accounts for the
spin-wave signals that are measured shortly after the activation of
the input signal.
[0080] To confirm that what we observed is unambiguously the magnon
signal is generated by the phonons, we repeated the experiment at
various top dot-antenna distances while keeping .theta..sub.ext
fixed at 30.degree.. We grew the transducers such that the top dots
are 0.5 mm apart so that the magnons are excited from fixed and
known top dot-antenna distances (0.86 mm+(p-1).times.0.5 mm) where
p=1, 2 . . . is the number of dots counted from the antenna 116. As
we excited the magnons further and further away from the receiving
output antenna, it can be seen that the corresponding magnon peak
moved to a later time, also shown in FIG. 9(b), (c), (d). FIG. 10
shows the dependence of the magnon transit time on the top
dot-antenna distance. It is clear from the FIG. 10 that the transit
time scales linearly with the distance the magnon travels, and the
most important observation lies in the fact that the y-intercept of
the plot is very close to zero (only around -2.5 ns), which
suggests that the magnons are indeed generated as the phonons hit
the YIG waveguide 102.
[0081] Considerable acoustically excited magnons can be measured
when the bias external field is held at an intermediate angle to
the YIG waveguide, i.e. when .theta..sub.ext is neither 0.degree.
or 90.degree.. For example in the system of FIG. 2, the optimum
angle .theta..sub.ext may be between 5.degree. and 30.degree..
[0082] Referring back to FIG. 9 we experimentally measured the
magnon transit time by finding the difference in time between the
middle point of two consecutive acoustic echoes (i.e. the time at
which the phonons hit the YIG) and the time of arrival of the
magnon signal. We found that this measurement gives 27.8 ns which
agrees with predicted result according to earlier measurement and
calculation of the magnon velocity as shown in FIG. 11.
[0083] As indicated above, the acoustic-magnon excitation mechanism
is not only applicable for MSW, and it is also possible to excite
short wavelength, exchange magnons using the same approach. This
can be achieved by, for example using a miniaturised transducer,
corresponding to that of FIG. 4a but on a smaller scale, or by
patterning nanostructures with dimensions comparable to the spin
wave wavelength on to the opposing side of the transducer (i.e. on
the YIG side of the waveguide for example).
[0084] Referring to FIG. 12(a), a piezoelectric detector for MSWs
can be designed using similar principles to the launcher of FIG. 2.
The detector may be formed on a waveguide 600, which may comprise a
magnetic material 602, again such as YIG, on a substrate 604 such
as GGG. The detector 601 may comprise a piezoelectric element 606
and an electrically conductive element 608. The conductive element
608 may have an inner part 610 which may be formed on the surface
of the waveguide 600, and which extends between the waveguide and
the piezoelectric element 606, and an outer part 612 which is
formed on the outer surface of the piezoelectric element 606
furthest from the waveguide 60, with the inner and outer parts 610,
612 being connected together by a connecting part 614 which extends
round one side of the piezoelectric element 606. The inner and
outer parts 610, 612 therefore act as electrodes which, if a
potential difference is set up between them, will apply an electric
field to the piezoelectric element.
[0085] The conductive element 608 may be formed of gold or another
highly conductive metal. The piezoelectric element 606 may be
formed of ZnO or another piezoelectric material. The method of
forming the detector may be similar to that described above for
forming the transducer 120, and the resulting size of the detector
may be of the order of hundreds of nanometres in length and width,
the piezoelectric element may be of the order of 1000 nm thick and
each part of the conductive element may be around 25 nm thick.
[0086] The waveguide is arranged so that MSWs will propagate along
its length in a propagation direction indicated by 616 (into the
page in the diagram) which is perpendicular to the loop defined by
the `C` shaped electrode of the detector.
[0087] The detector 601 works by generating elastic waves, i.e.
phonons, in response to the transmission of MSWs along the
waveguide 600. As MSWs propagate past the detector, the oscillating
magnetic field of the MSWs induces an oscillating electric current
in the conductive element 608, which generates an oscillating
electric potential difference between the inner and outer parts
610, 612 of the conductive element. This potential difference is
applied as a voltage across the piezoelectric element 606 which
therefore generates elastic waves 618 propagating perpendicular to
the inner and outer surfaces of the YIG waveguide and the surfaces
of the substrate 604.
[0088] A further piezoelectric detector 620 may be mounted on the
back surface of the substrate 604 which picks up the elastic waves
and generates an electric output signal which can then be processed
by a suitable device such as an oscilloscope to form an electrical
detector output. As the two detectors 601, 620 are located on
opposing surfaces of the waveguide 600, the elastic waves detected
by the detector 620 are those which are propagated across the
waveguide 620 in the direction perpendicular to the two opposing
surfaces of the waveguide.
[0089] Referring to FIG. 12b, an alternative way to convert magnons
into phonons is to use magnetostrictive elements. The magnon-phonon
convertor can be formed by depositing a magnetostrictive element
622 on top of the surface of a magnonic waveguide 102 (e.g. the
surface of the YIG thin film). The stray field of the spin-wave
propagating along the waveguide with then induce an oscillating
stress in the magnetostrictive element 622 which leads to phonon
generation. The convertor can then be used as an effective bulk
acoustic wave generator which generates acoustic waves
perpendicular to the surface of the waveguide substrate. A further
piezoelectric detector 620 mounted on the back side of the
waveguide can be used to detect the acoustic wave generated as
described above with reference to FIG. 12a.
[0090] The magnetostrictive 622 element may be produced very
simply. Suitable materials include iron, nickel, and terbium-iron
alloys. The magnetostrictive element 622 may be formed simply by
forming a small block of the material on the surface of the
waveguide.
[0091] Referring to FIG. 13 while many applications of the MSW
launcher will be in thin film waveguides, elastic waves can also be
used to generate MSWs in a bulk three-dimensional body 700 of
magnetic material. In this case, rather than whole thickness of the
magnetic material being used to set up a standing wave, the
magnetic material has structural features formed in it which are
sized so as to perform the same function as the thin film 102 of
FIG. 2. For example, a simple rectangular recess 702 may be formed
in one surface 708 of the magnetic material 700, and a coating of
material of a different acoustic refractive index formed on the
surface 708 so that it covers the surface and fills the recess 702.
The magnetic material may be YIG and the coating material may be
ZnO which is significantly softer than YIG. Alternatively the
coating material may be gold or another material having a
pronounced spin-orbit interaction. The coating material may in some
cases cover or fill the surface feature, such as the recess 702 and
not the extend over the rest of the surface 708 of the magnetic
material. Alternatively it may extend over the surface 708 of the
magnetic material, but not fill or extend over the surface feature
or recess 702.
[0092] Other shapes of surface feature, such as a tapered recess
720 or a rounded or hemispherical recess 722 can also be used in
place of, or as well as, rectangular recesses 720.
[0093] A piezoelectric transducer 704, which may be the same as
those described above with reference to FIG. 4, may be formed on
another surface 706 of the magnetic material 700, and arranged to
generate elastic waves in the magnetic material propagating towards
the recess 702. The two surfaces 706, 708 may be parallel to each
other, but in general this is not a requirement. When the
transducer 604 transmits elastic waves through the magnetic
material 600 so that they reach the recess 602, the elastic waves
are reflected from the surface 608 and also impinge on the recess
or recesses 702, 720, 722. These surface irregularities form
symmetry breakers which result in the generation of spin waves
where the elastic waves interact with the surface irregularities.
The resulting spin waves can propagate out from the surface
irregularity in a range of directions through the three dimensional
body of magnetic material.
[0094] The surface features 702, 720, 722 have a size which can be
characterized by the maximum distance they extend from plane of the
surface 708 into the body 700 in the direction perpendicular to the
surface 708. This can be referred to as the depth of the surface
feature. The size can also be characterized by a maximum width,
i.e. the maximum width of the feature in any direction parallel to
the pane of the surface 708. Either one or both of these dimensions
may be selected so as to be in the nanometer range so as to be
small enough to enable excitation of small wavelength exchange
dominated spin waves, but larger than the lattice parameter of the
magnetic material. For example either one or both of these
dimensions may be less than 1 .mu.m, or less than 500 nm, but will
generally be greater than 10 nm.
* * * * *