Electric-field-induced spin wave generation using multiferroicmagnetoelectric cells

Sergiy Cherepov,1 Pedram Khalili Amiri,1 Juan G. Alzate,1 Kin Wong,1 Mark Lewis,1

Pramey Upadhyaya,1 Jayshankar Nath,1 Mingqiang Bao,1 Alexandre Bur,2 Tao Wu,2

Gregory P. Carman,2 Alexander Khitun,3 and Kang L. Wang1

1Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA2Department of Mechanical and Aerospace Engineering, University of California, Los Angeles,California 90095, USA3Department of Electrical Engineering, University of California, Riverside, California 92521, USA

(Received 27 August 2013; accepted 10 January 2014; published online 25 February 2014)

In this work, we report on the demonstration of voltage-driven spin wave excitation, where spin

waves are generated by multiferroic magnetoelectric (ME) cell transducers driven by an alternating

voltage, rather than an electric current. A multiferroic element consisting of a magnetostrictive Ni

film and a piezoelectric [Pb(Mg1/3Nb2/3)O3](1�x)–[PbTiO3]x substrate was used for this purpose.

By applying an AC voltage to the piezoelectric, an oscillating electric field is created within the

piezoelectric material, which results in an alternating strain-induced magnetic anisotropy in the

magnetostrictive Ni layer. The resulting anisotropy-driven magnetization oscillations propagate in

the form of spin waves along a 5 lm wide Ni/NiFe waveguide. Control experiments confirm the

strain-mediated origin of the spin wave excitation. The voltage-driven spin wave excitation,

demonstrated in this work, can potentially be used for low-dissipation spin wave-based logic and

memory elements. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865916]

The scaling of semiconductor-based complementary

metal-oxide-silicon (CMOS) logic devices is facing signifi-

cant difficulties, due to the increased static power dissipation

of CMOS transistors at small length scales.1–3 Hence, there

is an increasing interest in developing technologies that can

enable beyond-CMOS computing.4–6 Spintronic logic tech-

nologies4,5,7,8 have emerged as leading candidates for this

purpose, given their combination of high speed, endurance,

potentially improved variability issues, and zero standby

power due to nonvolatile operation.6 One of the most impor-

tant requirements for any beyond-CMOS logic device is a

low dynamic power consumption per operation.6,7,9 Spin

waves (magnons) have been proposed as a potential means

to enable beyond-CMOS computation devices, where propa-

gation and interference of waves in a magnonic

waveguide7,10–13 are utilized for information transfer and

processing. The potential energy efficiency of spin wave

based devices is associated with the wave nature of the infor-

mation carrier which, unlike charge-based devices, does not

necessarily suffer from ohmic losses.6,7,14 To maintain low

energy consumption of magnonic devices, however, the spin

wave generation element needs to be energy-efficient as

well, so as to minimize the dynamic power dissipation.

However, the commonly used current-driven transducers

such as inductive antennas15 and spin transfer torque16 ele-

ments require large currents to operate, resulting in high

power dissipation and scaling issues due to intrinsic ohmic

losses.

To reduce the power dissipation associated with magnet-

ization excitation in spin wave logic, in this work, we propose

and demonstrate spin wave transducers where spin waves are

generated by alternating voltages (i.e., electric fields) rather

than currents. We refer to such devices as magnetoelectric

(ME) cells. Instead of single phase multiferroics,17–19 which

often provide rather small magnetoelectric coupling, in this

work a multiferroic heterostructure, i.e., a magnetostrictive/

piezoelectric bi-layer is used for this purpose, having a layer

structure similar to those previously reported in Refs. 20–22.

In this structure, the electric-field-induced strain in a piezo-

electric [Pb(Mg1/3Nb2/3)O3](1�x)–[PbTiO3]x (PMN-PT) sub-

strate converts to magnetization excitations using the

magnetostrictive properties of a Ni layer. It has been shown

that quasi-static electric fields on the order of 60.8 MV/m

applied across a PMN-PT ferroelectric layer can modify the

magnetization configuration in an adjacent magnetostrictive

layer, inducing effective anisotropy fields more than 300

Oe.21–23 Moreover, it was recently shown that the magnet-

ization in a magnetostrictive ferromagnetic layer can be

dynamically excited using Surface Acoustic Waves

(SAW).24,28 Here, we propose that a similar effect can be

used for magnetization excitation by alternating strain-in-

duced-anisotropy, created by RF voltages applied to the

ME cell. In this work, we show that two voltage-driven

piezoelectric transducers placed in proximity to each other

on a common ferromagnetic bus can be used for genera-

tion and detection of the propagating spin wave. The

capacitive ME cells, considered here, do not suffer from

Ohmic losses intrinsic to current-driven spin wave excita-

tion mechanisms. Hence, the power dissipation of these

transducers is determined only by the negligible AC cur-

rent flow during operation, caused by RF driving of the

device capacitance. The ME cell capacitance, i.e., power

dissipation due to AC current is dramatically reduced with

further scaling down of the device.

Figure 1(a) shows a schematic of the performed experi-

ment as well as a microscopic picture of the studied devices.

0003-6951/2014/104(8)/082403/5/$30.00 VC 2014 AIP Publishing LLC104, 082403-1

APPLIED PHYSICS LETTERS 104, 082403 (2014)

A 5 lm wide ferromagnetic stripe (spin wave bus) was litho-

graphically defined from the ferromagnetic bilayer consisting

of Ni (20 nm) and Ni80-Fe20 (20 nm) layers, sputtered onto a

single crystal piezoelectric PMN-PT, (x� 32%), cut in the

(110) orientation. The Ni layer provides magnetostrictive

properties while the soft Ni80Fe20 layer provides a low

damping for spin wave propagation. The 200 nm thick alumi-

num loop antennas and ME cells (see Fig. 1(a)), used for

spin wave excitation and detection were designed as sym-

metric shorted and open coplanar transmission lines, respec-

tively. They were built using 2 lm wide Al stripes and

patterned on top of the 200 nm SiO2 insulation layer using

standard lift off techniques, as shown in Figure 1 (inset).

Antennas and ME cell structures were separated by 2 lm

edge-to-edge distance. The ME cell electrodes provide direct

electrical contact to the piezoelectric PMN-PT substrate via

dedicated openings in the insulating SiO2 layer, as schemati-

cally shown in Figure 1(a). The microwave loop antennas,

driven by an applied RF current, were designed to induc-

tively excite magnetostatic spin waves,25,26 with a calculated

wavelength of �8 lm. The spin wave generation by the volt-

age driven ME cell can be understood in the following way:

an RF voltage is applied to the piezoelectric PMN-PT sub-

strate through the opening in the insulating SiO2 layer. The

alternating electric field, localized between the electrodes,

generates oscillating strain in the piezoelectric substrate

underneath the spin wave bus, which, in turn, leads to mag-

netoelastic spin wave excitation in the magnetostrictive Ni

layer due to alternating strain induced anisotropy. The gener-

ated spin wave propagates within the Ni/NiFe spin wave bus

to the other terminal of the device. Spin wave propagation

up to 40 lm was observed in our devices.

We used a vector network analyzer (VNA) to perform

propagating spin wave spectroscopy (PSWS) measure-

ments27 as shown in Figure 1(a). The two-port measurements

were performed with each port of the VNA connected to the

loop antenna and ME cell, respectively, using broadband sur-

face probes. The recorded scattering parameters carry infor-

mation about magnetization excitation (S11 and S22) and

bi-directional spin wave propagation (S21 and S12) in the

studied device, as schematically shown in Figure 1(b). The

two-port measurements were performed with external mag-

netic fields in a range of 6500 Oe, applied along the x axis.

At each magnetic field step, the magnetization response was

probed in a frequency range of 1–10 GHz using the VNA

operating at �5dBm power. The excitation amplitude

applied to the ME cell contacts was kept below the de-poling

field of the PMN-PT, which was found to be around

1.6 kV/cm. The saturation magnetization of the ferromag-

netic bilayer, used as spin wave bus in the studied devices,

was found to be �0.72 T as measured using superconducting

quantum interference device (SQUID) magnetometry.

Figure 2 shows the measured experimental reflection

and transmission signals as a function of applied magnetic

field H and frequency of excitation. The distinctive mag-

netic-field-dependent signal, visible in Figures 2(a) and 2(d),

corresponds to magnetization excitations created by the cur-

rent driven antenna (S11) and by the voltage-driven ME cell

(S22), respectively. The magnetic field dependent resonant

feature, visible on S21 and S12 transmission measurements,

represents a spin wave signal propagating along the spin

wave bus, generated by the antenna and detected by the ME

cell and vice versa, in Figures 2(b) and 2(c), respectively.

Here, the transmission and reflection data are plotted after

subtraction of the magnetic-field-independent background

signal, which had an amplitude of about �55 dBm, at

2500 Oe bias magnetic field. The observed difference in the

shape of the resonance curves obtained by antennas and ME

cells (S11 and S22, respectively) is determined by the spin

wave excitation mechanism, i.e., by the different (frequen-

cy-dependent) coupling efficiencies in the case of the two

transducer geometries. We also performed experiments on

simultaneous spin wave generation and detection between

two identical ME cells, as shown in Figure 2(e). The meas-

urements indicate that spin waves indeed can be generated

and detected only by ME cells. Although the magnetization

excitation by strain has been reported earlier,21 and a number

of experimental and theoretical works on generation and

propagation of magnetoelastic waves in ferromagnets were

performed in the past,29–31 the observed results present a

demonstration of voltage-driven generation of propagating

spin waves in confined ferromagnetic structures which are

more suitable for spin wave logic applications, and confirm

that multiferroic ME cells can be used for generation of spin

waves as well as for their detection.

FIG. 1. (a) Schematic of the studied device: Spin wave generation and prop-

agation measurements using a vector network analyzer were performed on

the 5 lm wide Ni/NiFe bus lithographically defined on a PMN-PT piezoelec-

tric substrate. Inset shows cross-section view of the ME cell. (b) The sche-

matic of two-port measurements of transmission (S21 and S12) and reflection

(S11 and S22) measurements between conventional loop antennas and

voltage-driven magnetoelectric cells.

082403-2 Cherepov et al. Appl. Phys. Lett. 104, 082403 (2014)

We next analyze the ferromagnetic resonance (FMR) fre-

quencies from S22 reflection parameter measurements on the

voltage driven ME cell shown in Figure 2(d). The data show

an excellent match to the analytically calculated dynamic

response of the magnetization, described by the Kittel equa-

tion.32 The ferromagnetic resonance frequency is given by

f 2 ¼ c2p

� �2

H2 þ 4pMSH� �

; (1)

where H is the external magnetic field, MS is the average sat-

uration magnetization of the bilayer, and c is the gyromag-

netic ratio. The position of the resonance peak extracted

from the experimental S22 response is shown as black circles

in Figure 2(f), while the solid back line represents the analyt-

ical fit obtained by solving Eq. (1). The fitting of the experi-

mentally obtained resonance peak dispersion to the

analytical data reveals the effective saturation magnetization

of the Ni/NiFe bilayer to be MS� 0.73 T, which is in excel-

lent agreement with independent SQUID measurements per-

formed on the same multilayer.

In order to confirm the magnetoelastic nature of spin

wave generation, control PSWS experiments between current-

driven loop antennas and voltage-driven (strain-mediated)

ME cells were performed. The test samples have the same ge-

ometry, layout, and material parameters as the samples

described above; however, their material structures were

modified in a way to suppress strain-induced magnetization

excitation. In a first control sample, the piezoelectric PMN-

PT substrate was replaced by a Si substrate. In this case, the

S11 magnetic-field-dependent FMR signal was detected only

from the loop antennas inductively coupled to the spin wave

bus, as shown in Figure 3(a). The S22 parameter measured on

the magnetoelectric cell, shown in Figure 3(b), did not have

any magnetic-field-dependent signal, due to the negligible

strain response from the non-piezoelectric Si substrate. A sim-

ilar result was obtained on a reference sample where the spin

wave bus, deposited on the piezoelectric PMN-PT substrate,

had only a single non-magnetostrictive NiFe layer33,34 (no Ni

layer). Here, the FMR response S11 is again detected only by

the inductive loop antenna, which is inductively coupled to

the ferromagnetic material, as seen in Figure 3(c). In this test

sample, the saturation magnetization Ms of the NiFe bus was

estimated from the Kittel fit of the experimental data, shown

in Figure 2(f) as red triangles, and was found to be 0.78 T,

larger than the Ms of the Ni/NiFe bi layer as expected. The

negligible magnetostriction of the NiFe layer in this sample

eliminates the strain-induced FMR response of the ME cell

(S22), as shown in Figure 3(d). In both control samples, the

transmission parameters S21 and S12 do not show any mag-

netic field dependent features. The difference in the S11 dis-

persion curves shown in Figure 2 and control data shown in

Figure 3 can be explained by a small variation in antenna ge-

ometry and different thickness and roughness of the magnetic

films, as well as by the presence of intrinsic stress during sam-

ple fabrication on the piezoelectric substrate. The performed

test measurements provide a qualitative confirmation of the

strain-induced nature of spin wave generation.

This work shows a proof of concept of spin wave gener-

ation and detection by voltage-driven magnetoelectric cells;

FIG. 2. Two-port measurements between

antenna and ME cell. Magnetic field de-

pendence of the FMR signal generated

by antenna and ME cell are shown on

(a) and (d), respectively. Figure (b) rep-

resents the spin wave signal detected by

the ME cell, while being generated by

the loop antenna. Figure (c) shows the

transmission signal generated by the

ME cell and detected by the loop

antenna, while (e) shows the transmis-

sion signal recorded between two ME

cells. Figure (f) shows the fitting to the

Kittel equation for the experimental

FMR data detected on the samples con-

taining Ni/NiFe bilayer (black circles)

and only NiFe (red triangles) spin wave

bus films.

082403-3 Cherepov et al. Appl. Phys. Lett. 104, 082403 (2014)

however, further optimization is required for potential appli-

cations. The spin wave generation efficiency, essential for

real applications, can be increased by ME cell geometry opti-

mization, reducing mechanical clamping by the bulk sub-

strate, and proper impedance matching on the excitation and

detection elements. Full optimization also requires consider-

ation of the magnetoelastic nature of the excited waves,29–31

which is beyond the scope of this work. Additionally, the

spin wave propagation length can be enhanced by using low-

loss materials such as magnetostrictive Heusler alloys35,36 or

dielectric materials such as yttrium iron garnet (YIG) films15

instead of the Ni/NiFe spin wave bus, which contains a high-

damping Ni37,38 layer necessary for strain-induced magnet-

ization excitation in our experiment.

In conclusion, we report on the voltage-induced strain-

mediated generation and detection of propagating spin waves

using multiferroic magnetoelectric cells. We demonstrated

that current-driven antennas, commonly used for spin wave

generation in magnonic devices, can be replaced by voltage-

driven ME cells based on ferroelectric-magnetostrictive

bilayers. It should be noted that the amplitude of the spin

waves generated in this demonstration is still rather small,

and may not be suitable for direct implementation in a spin

wave logic gate. However, this problem can be addressed by

using materials with lower damping, improving the genera-

tion efficiency of the ME cells by geometrical and materials

optimization, as well as through the use of spin wave ampli-

fiers.39,40 The voltage-driven spin wave excitation mechanism

can then be utilized in spin wave logic circuits, allowing for

compact and energy-efficient logic elements which have a

potential for beyond-CMOS logic and memory device

realization.

The authors gratefully acknowledge financial support

from the DARPA Non-Volatile Logic program, by the NSF

Nanosystems Engineering Research Center for Translational

Applications of Nanoscale Multiferroic Systems (TANMS),

and by NRI through the Western Institute of Nanoelectronics.

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FIG. 3. (a) and (b): Antenna (a) and

ME cell (b) absorption spectra meas-

ured on the sample, where Ni/NiFe spin

wave bus is fabricated on a regular Si

wafer. The voltage applied to the ME

cell fabricated on the non-piezoelectric

Si wafer does not create strain and

hence does not lead to spin wave gener-

ation. (c) and (d): Reflection signal as a

function of bias magnetic field meas-

ured using antenna (c) and ME cell (d)

on the sample where only a NiFe spin

wave bus was fabricated on the piezo-

electric PMN-PT substrate. The absence

of the magnetostrictive Ni layer elimi-

nates spin wave generation by the

voltage-driven ME cell. These experi-

ments confirm the strain-induced nature

of spin wave generation in our ME

cells.

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