All-electrical detection of spin dynamics in magnetic antidot lattices by the inverse spin Hall effect

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by  Matthias B. Jungfleisch

All-electrical detection of spin dynamics in magnetic antidot lattices by the
inverse spin Hall effect
Matthias B. Jungfleisch,1,a) Wei Zhang,1 Junjia Ding,1 Wanjun Jiang,1 Joseph Sklenar,1,2 John E. Pearson,1
John B. Ketterson,2 and Axel Hoffmann1
1)Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439,
USA
2)Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208,
USA
6
(Dated: 7 January 2016)
1
0 The understanding of spin dynamics in laterally confined structures on sub-micron length scales has become
2 asignificantaspectofthedevelopmentofnovelmagneticstoragetechnologies. Numerousferromagneticreso-
n nance measurements,opticalcharacterizationby KerrmicroscopyandBrillouinlight scatteringspectroscopy
a andx-raystudieswerecarriedouttodetectthedynamicsinpatternedmagneticantidotlattices. Here,wein-
J vestigate Oersted-fielddrivenspin dynamics in rectangularNi Fe /Ptantidot lattices with different lattice
80 20
6 parametersby electricalmeans and comparethem to micromagnetic simulations. When the system is driven
toresonance,adc voltageacrossthelengthofthe sampleisdetectedthatchangesitssignuponfieldreversal,

] which is in agreement with a rectification mechanism based on the inverse spin Hall effect. Furthermore, we
l
l show that the voltage output scales linearly with the applied microwave drive in the investigated range of
a
h powers. Our findings have direct implications on the development of engineered magnonics applications and
- devices.
s
e
m
Magnonic crystals, a new class of metamateri- magnon characteristics of the antidot lattice for real ap-
t. als with periodically modulated magnetic properties, plications.
ma have emerged as key building blocks in magnonics1. Ononehand,spinHalleffects16,17 havebeenprovento
The realization of spin-wave filters2, phase shifters3,
be excellent candidates for the interconversion between
d- interferometers4, spin-wave logic devices5 and grating electronic charge and spin currents15, and on the other
n couplers6 has been demonstrated and it is possible to hand,spinpumping18–22 andspin-transfertorque23,24 ef-
o tune the magnonic properties as desired by engineering fects are important methods to transform magnonic sig-
c the magnetic properties of the magnonic crystal. In this nals to spin currents and vice vesa25–28. We will focus
[ regard, ferromagnetic antidot lattices are prototypes of
hereonthedetectionsideoftheconversionpathway: the
1 magnonic crystals. The periodicity, dimensions, shape transformationofamagnonicsignal(spindynamics)into
v and material of an antidot lattice dictate the spin-wave
1 frequencies and their spatial distribution. These char-
2 acteristics are influenced by inhomogeneities of internal
2 magneticfieldsinlatticeswithlargerperiods,whereasat
1 smaller periods exchange fields play an important role7.
0
Since those parameters can easily be tuned by designing
.
1 the pattern and choosing the proper material, antidot
0
lattices are of fundamental importance in magnonics.
6
Spin dynamics in antidot lattices were investi-
1
: gated by numerous resonance measurements8–10, by
v x-ray spectroscopy11 and by optical techniques such
i
X as Kerr microscopy and Brillouin light scattering
spectroscopy8,9,12,13. In order to utilize antidot lattices
r
a in real magnonic applications, however, it is desirable to
integrate them in conventional dc electronic devices and
circuitries. The optimal signal processing pathway is14:
input electronic charge signal → spin current signal →
magnonic signal → spin current signal → output elec-
tronic charge signal,see Fig. 1(a). Signalprocessing and FIG.1. (Coloronline)(a)Pathwayforsignalprocessingfrom
transfer can be realized by magnons and ultimately it an electronic to a magnonic signal and back. Signal process-
wouldbe possible to harnessthe unique andcontrollable ing can be achieved by magnons. (b) Example of a scan-
ning electron microscopy image; here: antidot lattice B with
a=845nmandb=585nm. (c)Schematicoftheexperimen-
◦
tal setup. The antidot lattice is oriented at 45 with respect
to the signal line S. The dimensions a and b are given in
a)Electronicmail: jungfl[email protected] Tab. I.
2
(a)
TABLE I. Overview of the investigated antidot lattices. Py 9 GHz
4 10 GHz
thickness: 15 nm,Pt thickness: 5 nm. V) 11 GHz
Antidotlattice lattice constant a (nm) hole width b (nm) µV (DC 2 111234 GGGHHHzzz
A 755 519 age 0
B 845 585 c volt -2
d
C 942 713 -4
-3000 -2000 -1000 0 1000 2000 3000
Magnetic field H (Oe)
an electronic dc charge signal, see Fig. 1(a). (b) Experiment (c) Simulation
In this Letter, we investigate spin dynamics in rect- 14 14
angular antidot lattices made of bilayers consisting of Hz) 12 Hz) 12
G G
a ferromagnetic Ni80Fe20 layer (permalloy, Py) and a y f ( 10 y f ( 10
c c
Pt capping layer with varying dimensions and period- en 8 en 8
u 1st u
icity. In contrastto conventionalresonanceexperiments, eq 6 2nd eq 6
Fr 3rd Fr
where dynamics is driven by a microwave signal and the 4 4
1000 2000 3000 1000 2000 3000
response of the magnetic systems is quantified by an ab-
Magnetic field H (Oe) Magnetic field H (Oe)
sorptionmethod,weusearectificationmechanismbased
on pure dc techniques. Spin dynamics in the Py layer
FIG.2. (Coloronline)(a)Typicaldcvoltagespectrum(here:
are induced by a microwavedriven Oerstedfield and de-
antidotlatticeB)measuredatanappliedpowerof+15dBm.
tected by dc voltage that is explained by a rectification
The low frequency spectra are omitted to provide better
based on the inverse spin Hall effect (ISHE) in the Pt.
readability. The resonance signals show a mostly symmet-
Itis shownthatvariousmodes determined bythe design ric Lorentzian lineshape and change their polarity upon field
of the pattern contribute to the dc voltage proving the reversal. (b) Frequency vs. field relation extracted from the
possibility to utilize engineeredantidotlattices for infor- spectrum shown in Fig. 2(a) for thethreemodes that can be
mation processing/transportintegrated in dc circuitries. identified clearly. (c) Simulated frequency vs. field relation,
Themodespectrumisconfirmedbymicromagneticsimu- here: antidot lattice B.
lations. Powerdependentmeasurementsconfirmalinear
response of the antidot lattice, which is desirable for the ◦
field are oriented at an angle of 45 ], whereas the square
development of electronic devices.
antidot lattice is oriented parallel to it. Magnetization
The samples were fabricated in the following fash-
dynamics is excited by the Oersted field generated by a
ion: In a first step, dc leads were fabricated by mag-
microwave current in the CPW. We use an amplitude
netron sputtering and photolithography on intrinsic Si
modulationofthe signalgeneratorandlock-intechnique
substrates with a 300 nm thick thermally grown SiO
2 to detect the dc voltage output. For a particular mea-
layer. The antidot lattices of various dimensions were
surement, the rf power and frequency are kept constant
then written by electron beam lithography (see Tab. I).
(power range +10 to +18 dBm, frequency range: 4 to
A double layer positive resist of PMMA was spin-coated
14 GHz) while the external magnetic field is swept and
prior to electron beam exposure. After exposure and
the dc output is recorded. When the system is driven to
development, 15 nm-thick permalloy and 5 nm-thick Pt
resonance, the magnetization precession in the Py layer
layersweredepositedusingelectronbeamevaporationat
generatesaspinaccumulationatthePy/Ptinterfacethat
rates of<0.3 ˚A/s without breakingthe vacuum. The re-
diffuses into the Pt layer, a phenomenon that is known
sistwaslifted-off inacetone. Figure 1(b) showsa typical
as spin pumping effect18. This spin current gives rise to
scanning electron microscopy image (SEM), for lattice
anelectronicchargeimbalanceinthe Ptlayerdue to the
B, see Tab. I. The antidot lattices cover an area of ap-
inversespinHalleffect. The conversionfroma spin-into
proximately800×20µm2 intotalandthelateraldimen-
charge current is described by:
sions of each investigated antidot lattice is summarized
in Tab. I. In a subsequent step a 50 Ω-matched coplanar J~ ∝θ J~ ×~σ, (1)
C SH S
waveguide (CPW) made of Ti/Au (3 nm/150 nm) was
fabricated by magnetron sputtering and photolithogra- whereJ~C isthe charge-currentdensity, θSH the spinHall
phy. The antidot lattice and the CPW were separated angle that describes the efficiency of the conversion, J~
S
by a 80 nm-thick MgO layer to avoid any electrical con- is the spin-current density and~σ is the spin polarization
tact and the dc leads are kept between the central line vector.
and the ground plate within the CPW in order to mini- The investigation of different mode spectra in antidot
mize inductively coupled currents in the sample. lattices and the examination of the underlying effects is
Figure 1(c) illustrates the experimental setup and the very intriguing and subject of many studies in magnon-
measurement configuration. The microwave driven Oer- ics. However, we focus here on the fact that these well
sted field (in y-direction) is aligned at 45◦ to the exter- knownresonancescanindeedbedetectedbypuredcelec-
nalmagnetic field[in(1,1)-direction;CPWandexternal tricalmeans. Atypicalbi-polarspectrumrecordedatan
3
expected and experimentally observed spectra is found.
A As in experiment three modes are found in the investi-
Lattice B
Lattice C gated magnetic field range, see Fig. 2(c).
Lattice Figure3comparesthedcvoltagespectrumoftheinves-
C tigatedantidot lattices,excited at 6 GHz and +15dBm.
D
e V The different lattices show distinct features in the mode
ag structure. The modes of lattice A and C lie closer
volt together than those of B, leading to the conclusion
c
d that dipolar interactions emerging for narrower stadium
widths (a−b) are the dominant tuning parameter here
2 µV (see Tab. I). On the other hand, for the larger width b,
a larger resonance field is observed. This can be under-
stood as a decrease of demagnetization with increasing
-2000 -1000 0 1000 2000
b and, thus, an increase of the resonance field at a par-
Magnetic field H (Oe) ticular excitationfrequency10. The magnitude of the de-
tected voltage is basically independent of the lattice pa-
FIG. 3. (Color online) Comparison of the voltage spectra of rameters,seeFig.3. Thisdemonstratesthepossibilityto
the different antidot lattices at a fixed excitation frequency tune the magnonic frequency characteristics as desired.
of 6 GHz and microwave power of +15 dBm. The lattices
Next, we will focus on power-dependent studies. Lin-
featurethefollowing stadiumwidths(a−b)andwholewidth
b. A: (a−b) = 236 nm, b = 519 nm, B: (a−b) = 260 nm, ear response is a necessary requirement for the utiliza-
b=585 nm, C: (a−b)=229 nm, b=713 nm. tion of potential magnonic devices in electrical circuits.
Therefore, we carried out microwave power dependent
measurements. As is apparent from the power depen-
dence shown in Fig. 4 that the dc signals of all three
applied power of +15 dBm is shown in Fig. 2(a) (here modesincreasewithpower. Themagnitudeoftheoutput
shown: antidot lattice B). We clearly observe distinct signalofeachmodeasafunctionofpowerisshowninthe
modesinthedcspectraatparticularfrequency–fieldval- insetinFig.4(pleasenotethelinearscale;P inmW).We
ues. Thelow-frequencymodesarenotshowninFig.2(a) can draw two important conclusions from this measure-
to increase the readability of the viewgraph. As is ap- ment. Firstly,weobservealinearresponseofthesystem:
parent from the figure, the modes exhibit a mostly sym- the output signal increases linearly with the drive in the
metric Lorentzian lineshape. The most likely source for investigatedrangeofappliedpowers. Thisshowsthatwe
this behavior is a spin-to-charge current conversion due operateinthelinearregimeandnononlineardynamicsis
to the ISHE. For anunstructured sample orientedin the excited. Secondly,thefirstandthirdmodeshowapprox-
samewayashereoriftherewasasubstantialphaseshift
betweentherf currentandthemagnetizationoscillation
present,wewouldexpecttoobservealargerantisymmet-
5
riccontributionifthesignalwasdominantlygeneratedby V)
+10 dBm µ
a rectification due to the anisotropic magnetoresistance +12 dBm (C4 2nd
(AMR)29. Besidesthat,thepolarityofthevoltagesignal 4 +14 dBm VD3
changessignwhenthemagnetizationdirectionischanged V) +16 dBm ge 2
[Fig. 2(a)], which also suggests that the observed volt- µ (DC 3 +18 dBm 2nd volta 1 1st3rd
age is most likely due to the ISHE29, Eq. (1). However, e V dc
othereffectssuchasspinrectification29,magnoniccharge ag 2 1st 10 20 30 40 50 60
pInudmeppeinngd3e0n,3t1oofrtAhMe Rre2c9ticfiacnantiootnbme ceochmapnliestmelythreuleedxpoeurti-. dc volt 1 u Power P3 (rmdW)v
mental data unambiguously demonstrates an easy way
to detect spin dynamics in magnonic crystals by pure dc 0
electrical means. Furthermore, it is interesting to note
thatthe magnitude ofthe detecteddc signaliscompara- 0 500 1000 1500 2000 2500 3000
ble to that of unpatterned Py/Pt stripes32. In the spec- Magnetic field H (Oe)
trum shown in Fig. 2(a) we can clearly distinguish three
different modes. We analyze their frequency–magnetic
FIG. 4. (Color online) Voltage spectrum as function of the
fielddependenceasshowninFig.2(b). Asexpectedfrom applied microwave power at an fixed excitation frequency of
the Kittel equation, the resonance frequency increases 10 GHz (antidot lattice A). Three modes denoted as 1st,
with the externally applied magnetic field. In order to 2nd and 3rd, are detectable at all microwave powers. At
corroborate the experimentally observed spectra, micro- higherpowers additional modes (labeled as uand v) emerge.
magneticsimulationswereperformedusingmumax333,34. The inset illustrates the linearity of the generated dc output
Anexceptionalgoodagreementbetweenthetheoretically voltagewith power; please notethelinearscale of thepower.
4
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