Excitation and detection of short-waved spin waves in ultrathin Ta/CoFeB/MgO-layer system suitable for spin-orbit-torque magnonics

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by  T. Brächer

Excitation and detection of short-waved spin waves in ultrathin
Ta/CoFeB/MgO-layer system suitable for spin-orbit-torque magnonics
T. Br¨acher,1,a) M. Fabre,1 T. Meyer,2 T. Fischer,2 O. Boulle,1 U. Ebels,1 P. Pirro,2 and G. Gaudin1
1)SPINTEC, UMR-8191, CEA-INAC/CNRS/UJF-Grenoble/Grenoble-INP, 17 Rue des Martyrs, 38054 Grenoble,
France
2)Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universita¨t Kaiserslautern,
67663 Kaiserslautern, Germany
(Dated: 6 January 2017)
We report on the excitation and detection of short-waved spin waves with wave vectors up to about
40radµm−1 in spin-wave waveguides made from ultrathin, in-plane magnetized Co Fe B (CoFeB). The
7 8 72 20
CoFeBisincorporatedinalayerstackofTa/CoFeB/Mgo,alayersystemfeaturinglargespinorbittorquesand
1
0 a large perpendicular magnetic anisotropy constant. The short-waved spin waves are excited by nanometric
2 coplanar waveguides and are detected via spin rectification and microfocussed Brillouin light scattering spec-
troscopy. We show that the large perpendicular magnetic anisotropy benefits the spin-wave lifetime greatly,
n
resulting in a lifetime comparable to bulk systems without interfacial damping. The presented results pave
a
J the way for the successful extension of magnonics to ultrathin asymmetric layer stacks featuring large spin

5 orbit torques.

]
ll Magnonics and magnon spintronics1–6 explore the Ta/CoFeB/MgO stack, a layer system which has been
a
transport of information in the form of spin waves and previously reported to feature large damping- and field-
h
magnons,theirquanta,forpost-CMOSdevicesaswellas like SOTs18,19. In addition, this layer system features
-
s their interconnection with conventional electronics. The a large perpendicular magnetic anisotropy (PMA) con-
e
wave-nature of spin waves and their wavelength in the stant. At the investigated CoFeB thickness, the mag-
m
sub-micronrangeatmicrowavefrequenciesrendersthem netization lies still within the film plane. Nevertheless,
t. an excellent candidate for a new generation of wave- the large PMA contribution opposes the dipolar stray
a
based computing devices. The recently discovered ex- field and, this way, greatly reduces the dipolar effects
m
citing new ways to manipulate magnetization dynamics on the spin-wave dispersion. Consequently, it lowers the
- in ultrathin magnetic layers sandwiched in an asymmet- spin-wavefrequencyandincreasesthespin-wavelifetime.
d
ric layer stack promise the creation of new magnonic de- We demonstrate that the spin-wave lifetime in the stud-
n
o vices and the replacement of Oersted fields by effective ied layer systems is comparable to the spin-wave life-
c fields created by spin orbit torques (SOTs)7–10. These timeinthickferromagneticfilms, despitethepresenceof
[ torques and the consequent fields can be created by a strong interfacial damping in the ultrathin films. Conse-

1 current flowing through the layer systems via spin-orbit quently,theselayersystemsareaninterestingalternative
v coupling and the broken structural inversion symmetry. to thicker metallic layers with the additional benefit of
9 Unlike Oersted fields, they are localized to the current large SOTs.
9 carrying interfaces. They allow to control the direction
The studied Ta/CoFeB/MgO trilayer has been de-
3 of the static magnetization11–13, the excitation of mag-
1 positedbysputter-depositiononanoxidizedSisubstrate.
netizationdynamics14,15 andthecontrolofthespin-wave
0 The CoFeB has been deposited in a wedge ranging from
damping in microscopic structures16. However, most
. athicknessof0.8nmuptoathicknessof1.6nmoverthe
1 research14–16regardingspinwavesandSOTshasbeenre-
0 rangeofa4inchwaferandthetrilayeriscappedwithan
strictedtocomparablythickfilmswiththicknessesabove
7 additional layer of Al O and Ta O . The Ta thickness
4−5nm. In contrast, SOTs are significantly more pro- 2 3 2 5
1 is 5nm, the thickness of the capping oxides is 1.5nm
nounced in ultrathin films with thicknesses well below
: (MgO), 2nm (Al O ) and 1nm (Ta O ), respectively.
v 2nm due to their interfacial origin17. In addition, many 2 3 2 5
i The used growth conditions ensure that the bottom Ta-
X studies have been restricted to spin waves with wave-
layer grows in the β-phase, resulting in a large spin Hall
lengths longer than 500nm, since these are comparably
r anglewithintheTalayer13,23. Afteranannealingstepof
a easy to excite and detect.
the wafer at 250◦C for 1.5h, the PMA is enhanced and
In this Letter, we report on the excitation and detec-
leads to a transition to an out-of-plane magnetization of
tion of short-waved spin waves with wavelengths down
the CoFeB at thicknesses below approximately 1.2nm.
to about λ ≈ 150nm in spin-wave conduits made from
AroundaCoFeBthicknessofabout1.4nm,whichisstill
ultrathin Co Fe B (CoFeB). The CoFeB features a
8 72 20 well in-plane, the trilayer is structured into spin-wave
thickness of about d = 1.4nm and is sandwiched in a
waveguides with a length of 10µm and widths ranging
from 500nm to 5µm by electron-beam lithography and
ion-beametching. Consequently,leadsmadefromTi/Au
are defined at the short edges of the waveguides. These
a)Electronicmail: [email protected] are used for the measurement of the rectified DC volt-
2
age created in the trilayer due to spin rectification if
the magnetization is excited into precession20–22. Sub-
sequently, the waveguides are capped by a 30nm thick
layer of Al O by means of atomic layer deposition. In a
2 3
last series of steps, nanometric, shorted coplanar waveg-
uides (CPWs) with different sizes are patterned on top
ofthewaveguidesbyacombinationofanelectronbeam-
lithography and an optical lithograhpy step and electron
beam evaporation. The CPWs are made from a dou-
ble layer of Ti/Au with thicknesses of 5nm and 30nm,
respectively. They feature three different sizes and the
correspondingstructuresarepatternedinrowsalongthe
gradientintheCoFeBthickness. TypeAfeatures100nm
wide wires with a s = 500nm center-to-center spacing,
type B 50nm wires with a s = 300nm center-to-center
spacingandtypeC50nmwireswithas=150nmcenter-
to-center spacing. A schematic of the fabricated struc-
tures is shown in Fig. 1 (a).
The reported electrical measurements have been per-
formedon5µmwidewaveguides, eachfeaturingadiffer-
ent CPW of type A, B and C. For a first characteriza-
tion of the properties of the waveguides, their dynamic
propertiesarestudiedbymeansofspintorqueferromag-
netic resonance spectroscopy (STFMR)20–22. For these
measurements, the CoFeB waveguides are positioned be-
tween a pair of coils under an angle of 45◦ between the
waveguides and the magnetic field within the film-plane.
Usinghigh-frequencyprobes,theyareconnectedtoami-
crowave generator via the capacitive part of a Bias-T.
The generator provides an RF current leading to the ex-
citation of magnetization dynamics in the waveguide as
the magnetic field is swept across resonance. A lock-in FIG. 1. (Color online) a) Schematic of the investigated
sample and the geometry of the SWR measurements: A
amplifier (LIA) is connected to the inductive part of the
Ta/CoFeB/MgOtrilayerispatternedintoaspin-wavewaveg-
Bias-T, which allows for a detection of the rectified DC
uidewithleadstomeasurethevoltagedropalongthewaveg-
voltage created by these magnetization dynamics within
uide. On top of an insulating layer of Al O , a nanometric,
the waveguide. The LIA is used to modulate the ampli- 2 3
shortedCPWwithwirespacingsactsasspin-waveexcitation
tudeofthemicrowavegeneratoratafrequencyof10kHz
source. b) Exemplary excitation spectra at an excitation fre-
to enhance the signal-to-noise ratio in the experiment. quency of 4.8GHz (solid lines) and analytical calculations of
Exemplary STFMR spectra are shown by the shaded the expected excitation spectra of the CPWs (dashed lines).
peaks in Fig. 1 (b). From measurements of this kind, we Black: CPW type A, red: CPW type B, green: CPW type
C.TheshadedpeaksshowexemplarySTFMRspectraofthe
determined the effective magnetization M =M −H
eff s ⊥ waveguides (see text for additional information).
in the waveguide by an analysis of the dependence of
the resonance frequency on the applied magnetic field.
Here, M denotes the saturation magnetization of the
s
CoFeB and H⊥ the anisotropy field due to the PMA. saturation magnetization25 of Ms = 1250kAm−1, the
The resonance frequencies are fitted using the Kittel obtainedvaluesofMeff correspondstoaneffectivesurface
equation in the absence of any in-plane anisotropies, ne- anisotropyconstantofK⊥ ≈0.60mJm−2,avalueinthe
glecting the small shape-anisotropy of the wire. This expectedrangeforTa/CoFeB/MgOlayersystemsfeatur-
is justified due to the low thickness and the compara- ing the investigated CoFeB thickness25,26. In all waveg-
bly large width of the wire. From this analysis, we uides we find field-linewidths on the order of a few mT,
find M ≈ 176kAm−1 for structures with CPWs of featuring a linear dependence on the FMR-frequency.
eff,C
type C, M ≈ 154kAm−1 for type B and M ≈ From the measured values, we extract an upper limit
eff,B eff,A
134kAm−1 for type A (the corresponding Kittel fits are of the Gilbert-damping parameter of about α ≈ 0.012,
shown by the white lines in Fig. 2). These different assuming the entire damping is Gilbert-type.
values are due to the different positions of the CPWs The relevant material parameters being determined,
along the thickness gradient: From type C to type A, we now address the excitation of propagating spin waves
the thickness of the CoFeB reduces and, consequently, with finite wave vectors in the waveguides. For an elec-
the anisotropy field H slightly increases. Assuming a trical characterization of the spin-wave excitation by the
⊥
3
CPWs, these are connected to the microwave generator experimentduetothefinitelinewidthoftheexcitedspin
using high-frequency probes. The RF current provided waves. Thisis,thus,mediatedbythespin-wavedamping,
by the generator creates a dynamic Oersted field around which is not included in the analytical formalism, and
the CPWs. The leads at the waveguide edges are again not due to the wave-vector dependent excitation proper-
connectedto the LIA.Inthiskindof spin-wave rectifica- tiesoftheCPWs. Theenvelopeoftheexpectedintensity
tionexperiment(SWR),therectifiedvoltagearisingfrom dropsbelow2%beyondthethirdminimumoftheCPWs
the spin-wave excitation in the spin-wave waveguide by of type A, beyond the second minimum for type B and
theCPWisdetectedasafunctionoftheappliedfieldfor beyond the first minimum for type C. This wave vector
agivenexcitationfrequency. Thewaveguidesaremagne- corresponds to about 40radµm−1 for all three types of
tizedalongtheirshort-axis,tomaximizethetorquefrom CPWs and is equivalent to a wavelength of λ = 166nm
the in-plane field created by the CPWs. In this geome- for type A and of λ = 150nm for type B and C. For
try,thecontributionoftheinverseSpinHalleffecttothe larger wave vectors, the relative intensity of any excited
rectified voltage is maximal, while the contribution due spin waves in the maximum of the excitation efficiency
to the anomalous magnetoresistance is minimum27. Un- is below the noise of the used experimental setup. Up
like STFMR, SWR gives access to spin waves with finite to this limit, the detection efficiency via the spin rectifi-
wave vectors, the excitation spectrum being defined by cation seems independent of the spin-wave wave vector,
the geometry of the CPW. since the measured spectra are solely determined by the
features of the excitation source. This finding is in ex-
The solid lines in Fig. 1 (b) show exemplary SWR
cellent agreement with a study of parametrically excited
measurements performed on the three different waveg-
exchange magnons in macroscopic samples made from
uides. The SWR voltage is proportional to the square of
yttrium iron garnet30 and proves the versatility of this
the dynamic magnetization and, thus, the spin-wave in-
approach for the detection of traveling waves31, even in
tensity. The measurements have been performed on the
micro- and nanostructures.
three different waveguides with different CPWs, using a
Figure 2 compares the measured excitation spectra
microwavefrequencyof4.8GHzwithanappliedpowerof
of the three different CPW types to the correspond-
P =800µW=−1dBm. This corresponds to the regime
ing expected excitation spectra. The measured volt-
of linear excitation, whereas an increase of the power by
age/expectedspin-waveintensityisdisplayedcolor-coded
about 2 − 3dB leads to a deviation of the linear scal-
asafunctionoftheappliedfieldandfrequency. Thespec-
ing of the measured voltage with the applied microwave
trahavebeennormalizedindividuallytotheirmaximum
power. The voltages have been normalized to their indi-
at each frequency to account for the changes of the in-
vidual maximum, which is on the order of 2µV for the
put impedance of the CPW and of the used microwave
applied power. As can be seen from the figure, the spac-
equipment. All color maps use an identical, logarithmic
ingbetweentheFMR(shadedpeak)andthepeakofthe
scale. Ascanbeseenfromthefigure,themeasuredspec-
CPW excitation gets larger and the width of the SWR
tra are in good qualitative and also quantitative agree-
peaks grows as the size of the CPW is reduced. This is
mentintheprobedfield-andfrequency-range. Intheen-
duetothefactthatthemaximumspin-wavewavevector
tire range, the noise-limited maximum detectable wave-
the CPW can excite increases. This is governed by the
vector is about 40radµm−1 and is determined by the
Fourier spectrum of the Oersted-field distribution cre-
ated by the CPW24,28. To describe the spin-wave spec- Fourier spectrum of the excitation source. In addition
to the color-coded spectra, the white lines correspond to
trum in the waveguides, we use the analytical formalism
theKittelfitsobtainedfromtheSTFMRmeasurements.
presented in Ref. 29 with the incorporation of the PMA
discussedintheSupplementalmaterialofthisLetter. As-
suminganeffectivewidthofthewaveguideofw =5µm To prove the propagating character of the excited
eff
and an exchange constant of A =10pJm−1 as well as waves, we study the excitation of a CPW of type B on a
ex
the other material parameters stated above, we calcu- 2µmwidewaveguidebymeansofmicrofocusedBrillouin
late the expected spin-wave intensity spectrum excited light scattering (BLS)? . This technique gives access
by the CPWs which are represented by the dashed lines to the spin-wave dynamics for waves with wavelengths
in Fig. 1 (b). Hereby, we average over the two emission larger than approximately 300nm with a spatial resolu-
directions along the wire, which are not equal due to the tion of the same order of magnitude. In the BLS mea-
interplay of the in-plane and out-of-plane component of surements, the field is fixed at µ |H | = 55mT along
0 ext
the Oersted field (cf., e.g., Ref. 29 and the comparison the short axis of the waveguide. A microwave source is
to the spectra obtained by Brillouin light scattering fur- again connected to the CPW and the frequency is swept
ther down in this Letter). Furthermore, we only take at an applied power of 1.26mW = +1dBm. This larger
into account the first waveguide mode in the calcula- powerhasbeenchosentoensureasufficientlylargespin-
tions. The envelope of the analytical calculations is in wave excitation for a good signal-to-noise ration in the
good agreement with the experimentally obtained spec- BLS experiment. Figure 3 (a) shows the BLS spectra
tra. The main difference arises in the minima of the measured directly next to the CPW for the two different
CPW excitation, which are situated at integer multiples field polarities. The measured spectrum in the vicinity
of2π·s−1andwhichcannotbecompletelyresolvedinthe of the CPW is compared to the analytically expected
4
FIG.2. (Coloronline)Color-codedmeasuredSWRvoltage/expectedspin-waveintensitiesasafunctionoftheappliedfrequency
andappliedmagneticfield. Theupperpanelshowsthemeasurementandthelowerpaneltheanalyticalcalculations. a)CPW
type A: Wire width 100nm, wire spacing s = 500nm. b) CPW type B: Wire width 50nm, wire spacing s = 300nm. c)
CPWtypeC:Wirewidth50nm,wirespacings=150nm. ThewhitelinesrepresenttheKittelfitsobtainedfromtheSTFMR
measurements. Thecalculationsdonotcontainthebroadeningoftheexcitationpeaksduetothefinitedampinginthematerial.
spin-wave excitation spectrum (dashed lines) and the which corresponds to a spin-wave wavelength of 300nm,
spin-wave dispersion relation of the fundamental mode corresponds to the maximum wave vector which can be
(dotted line). As can be seen from the figure, the two detected by the microfocussed BLS setup. Hence, for
field polarities exhibit a strong asymmetry in terms of larger wave vectors, the BLS-detection becomes insensi-
the overall intensity. Both, measurement and calcula- tive and, thus, the measured spin-wave spectrum does
tions, have been normalized to the maximum intensity not reflect the wave-vector dependence of the excitation
in the efficient emission direction (in this case for posi- source anymore.
tivemagneticfields). Thestrongasymmetryismediated
To analyze the spin-wave propagation within the
by the PMA (see Supplementary material for more in-
waveguide,thespin-waveintensityismeasuredalongthe
formation): It decreases the ellipticity of precession and,
waveguide at different positions across its width. The
this way, increases the influence of the out-of-plane com-
intensity averaged across the width is shown as a func-
ponent of the CPW fields on the spin-wave excitation.
tion of the position along the waveguide for some ex-
Consequently, its interference with the in-plane compo-
emplary frequencies within the detection range of BLS
nent becomes more pronounced. Thus, the use of CPWs
in Fig. 3 (b). Solid lines indicate exponential fits to
or similar excitation sources for the spin-wave excitation
the data. These have only been performed for distances
in ultrathin films with large PMA results intrinsically
which are sufficiently far from the CPW to ensure that
in a pronounced uni-directional spin-wave emission. It
the laser spot is not partially on the CPW. As can be
should be noted that on the other side of the CPW, the
seenfromthefits,thespin-wavedecaylengthinitiallyin-
opposite behavior is observed, i.e., the negative polarity
creases with increasing frequencies as the decay becomes
is favored. By comparing the calculation (parameters of
more shallow. However, for large frequencies, the de-
astructureoftypeB)totheexperiment, itbecomesevi-
cay becomes steeper. Around a frequency of 3.5GHz it
dent that they only agree up to the first minimum of the
reaches its largest value of an amplitude decay length
CPW excitation. This is expected since this minimum,
of about 600nm. This value is in excellent agreement
5
with the prediction by the adopted analytical formalism
presented in the Supplemental material, assuming the
aforementioned values of α and M . It corresponds
eff,B
to a spin-wave amplitude lifetime of about 3ns and a
group velocity of about 200ms−1. The decrease of the
decay length with increasing frequency is not expected
from the analytical calculations, which predicts an ap-
proximately constant decay length. This indicates the
presenceofawave-vectordependentdampingmechanism
in the measurements which is not incorporated into the
analyticalformalism. Itshouldbenotedthatthelifetime
ofabout3nsislargeincomparisontothevalueexpected
from a material system featuring these magnetic param-
eters in the absence of PMA. This can be comprehended
from the ellipticity contribution to the lifetime for the
ferromagnetic resonance. The FMR lifetime32 is given
by 1/τ =αγµ (H +M /2) and, thus, a reduction of
0 ext eff
M significantly increases the lifetime at low magnetic
eff
fields. The obtained value of 3ns is comparable to the
lifetime in thicker ferromagnetic metallic materials such
asNi Fe orthehalf-metallicHeuslercompoundCMFS
81 19
in the absence of pronounced interfacial damping33,34.
Toconclude,wehavedemonstratedtheexcitationand
detection of short-wave spin waves in spin-wave waveg-
uides made from a Ta/CoFeB/MgO layer stack incor-
porating an ultrathin layer of CoFeB. We have demon-
strated a wave-vector insensitive electrical detection by
spin rectification up to a spin-wave wave vector of at
least 40radµm−1, which could be verified by comparing
the measured voltage-spectra to the expected excitation
spectra from the used nanometric CPWs. By employing
microfocussedBLS,wehavefurthermoreverifiedthatthe FIG.3. (Coloronline)a)Measured(solidline)andcalculated
spin-waveemissionbytheCPWsexhibitsastronglypre- (dashed line) spin-wave spectra detected directly next to the
ferred emission direction due to the large PMA in the antenna for ±µ Hext = 55mT. The dotted line shows the
0
investigatedlayersystems. ThefactthatthislargePMA analyticallycalculateddispersionrelationofthefundamental
also results in a comparably large spin-wave lifetime and mode (right y-axis). The magenta line marks the BLS de-
the value of the reported current-induced SOT in this tection limit at k ≈ 19radµm−1. b) Spin-wave intensity as
a function of the position along the waveguide for different
material system renders it a highly interesting system
excitation frequencies. Solid lines are exponential fits.
for magnonic applications.
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