Strain-induced modulation of perpendicular magnetic anisotropy in Ta/CoFeB/MgOstructures investigated by ferromagnetic resonanceGuoqiang Yu, Zhenxing Wang, Maryam Abolfath-Beygi, Congli He, Xiang Li, Kin L. Wong, Paul Nordeen, HaoWu, Gregory P. Carman, Xiufeng Han, Ibrahim A. Alhomoudi, Pedram Khalili Amiri, and Kang L. Wang Citation: Applied Physics Letters 106, 072402 (2015); doi: 10.1063/1.4907677 View online: http://dx.doi.org/10.1063/1.4907677 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of boron diffusion on the perpendicular magnetic anisotropy in Ta|CoFeB|MgO ultrathin films J. Appl. Phys. 117, 043913 (2015); 10.1063/1.4906096 Opposite signs of voltage-induced perpendicular magnetic anisotropy change in CoFeB|MgO junctions withdifferent underlayers Appl. Phys. Lett. 103, 082410 (2013); 10.1063/1.4819199 Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers Appl. Phys. Lett. 102, 242405 (2013); 10.1063/1.4811269 Perpendicular-anisotropy CoFeB-MgO magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recordingstructure Appl. Phys. Lett. 101, 022414 (2012); 10.1063/1.4736727 Current-induced effective field in perpendicularly magnetized Ta/CoFeB/MgO wire Appl. Phys. Lett. 98, 142505 (2011); 10.1063/1.3579155

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Strain-induced modulation of perpendicular magnetic anisotropyin Ta/CoFeB/MgO structures investigated by ferromagnetic resonance

Guoqiang Yu,1 Zhenxing Wang,1,2 Maryam Abolfath-Beygi,1 Congli He,1 Xiang Li,1

Kin L. Wong,1 Paul Nordeen,3 Hao Wu,4 Gregory P. Carman,3 Xiufeng Han,4

Ibrahim A. Alhomoudi,6 Pedram Khalili Amiri,1,5 and Kang L. Wang1

1Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA2Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology,Wuhan 430074, China3Department of Mechanical and Aerospace Engineering, University of California, Los Angeles,California 90095, USA4Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,Beijing 100190, China5Inston Inc., Los Angeles, California 90095, USA6National Nanotechnology Research Center, King Abdulaziz City for Science and Technology, Riyadh 11442,Saudi Arabia

(Received 7 December 2014; accepted 27 January 2015; published online 17 February 2015)

We demonstrate strain-induced modulation of perpendicular magnetic anisotropy (PMA) in (001)-

oriented [Pb(Mg1/3Nb2/3)O3](1�x)-[PbTiO3]x (PMN-PT) substrate/Ta/CoFeB/MgO/Ta structures

using ferromagnetic resonance (FMR). An in-plane biaxial strain is produced by applying voltage

between the two surfaces of the PMN-PT substrate, and is transferred to the ferromagnetic CoFeB

layer, which results in tuning of the PMA of the CoFeB layer. The strain-induced change in PMA

is quantitatively extracted from the experimental FMR spectra. It is shown that both first and

second-order anisotropy terms are affected by the electric field, and that they have opposite voltage

dependencies. A very large value of the voltage-induced perpendicular magnetic anisotropy modu-

lation of �7000 fJ/V�m is obtained through this strain-mediated coupling. Using this FMR tech-

nique, the magnetostriction coefficient k is extracted for the ultrathin 1.1 nm Co20Fe60B20 layer,

and is found to be 3.7� 10�5, which is approximately 4 times larger than the previously reported

values for CoFeB films thicker than 5 nm. In addition, the effect of strain on the effective damping

constant (aeff) is also studied and no obvious modulation of the aeff is observed. The results are rel-

evant to the development of CoFeB-MgO magnetic tunnel junctions for memory applications.VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907677]

Manipulation of magnetization by electric currents and

voltages has attracted enormous research interest in the spin-

tronics community, due to its applications in magnetic logic

devices and magnetic random access memory (MRAM).1 In

particular, the current-induced spin transfer torque (STT)2–4

has been extensively studied for magnetization switching in

spintronic devices. However, the high energy dissipation and

density limitation due to the large critical current are still chal-

lenging for practical applications. Recently, manipulation of

magnetization by electric field in ferromagnetic/ferroelectric

(FM/FE) heterostructures has attracted considerable atten-

tion,5–18 as a pathway towards lower dynamic energy dissipa-

tion. A particular ferroelectric crystal material [Pb(Mg1/3Nb2/

3)O3]1�x–[PbTiO3]x (PMN-PT), which exhibits a large piezo-

electric effect, is often used in FM/FE heterostructures. In this

type of heterostructure, strain or charge induced by an electric

field applied on the PMN-PT layer is capable of manipulating

the magnetic anisotropy of the adjacent magnetic layer, result-

ing in a magnetoelectric effect. Experiments have demonstrated

that strain can produce reversible and permanent magnetic ani-

sotropy reorientation, as well as generate and control magneto-

static surface spin waves.14,16,19–21 The study of FM/FE

heterostructures has also resulted in the recently proposed appli-

cation in strain-transfer torque random access memory.22

In terms of the memory density, scaling behavior, and

thermal stability, perpendicularly magnetized ferromagnets

are more promising for applications at scaled technology

nodes than their in-plane magnetized counterparts.23,24 As a

result, techniques enabling the manipulation of perpendicular

magnetic anisotropy (PMA) in magnetic materials are highly

desired for memory applications. There have been a number

of experiments studying the manipulation of PMA through

voltage-induced strain in different material systems, such as

CoxPd1�x alloys,10 L10-FePt,8,9 [Co/Pd]n/CoFeB multi-

layers,11 CoPt,13 and FePd.12 However, the strain-mediated

modulation of PMA in typical MgO-based magnetic tunnel

junction (MTJ) materials (i.e., Ta/CoFeB/MgO structures)

has not yet been studied in detail, while it is needed to allow

for data readout using tunnel magnetoresistance (TMR) in

MTJs in practical applications. On the other hand, while

charge-mediated modulation of anisotropy has been widely

studied and used for switching in MTJs,25–34 the sensitivity

of PMA to changes in electric field reported in those cases is

significantly smaller than what can be expected in a strain-

mediated mechanism. In addition, recent studies have shown

that strain modulation may play a role in the observed volt-

age control of magnetic anisotropy (VCMA) effect in these

MTJ material stacks as well.35

Here, we have studied the influence of voltage-induced

strain on the PMA in ultrathin 1.1 nm CoFeB layer through fer-

romagnetic resonance (FMR) measurements. The experimental

results show that the PMA of an ultrathin 1.1 nm CoFeB layer

0003-6951/2015/106(7)/072402/5/$30.00 VC 2015 AIP Publishing LLC106, 072402-1

APPLIED PHYSICS LETTERS 106, 072402 (2015)

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can be modulated by voltage-induced strain and results in a

very large PMA modulation of up to �7000 fJ/V�m. In addi-

tion, the magnetostriction coefficient of the ultrathin 1.1 nm

CoFeB layer is extracted from these measurements, and is

observed to be larger than reported values in 5 nm thick

CoFeB film. The dependence of the effective damping con-

stant as a function of strain is also studied, showing no signifi-

cant strain dependence in the range of applied voltages in our

experiment.

The sample structure studied in this experiment was

PMN-PT (001 crystal orientation)/Ta (8)/Co20Fe60B20

(CoFeB) (1.1)/MgO (2)/Ta (2), where thickness values are

given in nanometers. The (001) orientated PMN-PT crystal

substrate of 0.5 mm thickness is used to generate in-plane

biaxial strain when voltage is applied between its top and

down surfaces due to its piezoelectric property. The other

layers were deposited at room temperature in an AJA

International Physical Vapor Deposition System with base

pressure of less than 2� 10�8 Torr. The metallic layers were

grown by DC-sputtering. The MgO layer was grown by

RF-sputtering from an MgO target. The sample was subse-

quently annealed at 250 �C for 30 min to improve crystallin-

ity and enhance PMA. The sample was characterized by a

vibrating sample magnetometer (VSM) to determine the sat-

uration magnetization (Ms) and obtain magnetic hysteresis

loops. The FMR measurements were carried out on a Bruker

EPR spectrometer using a microwave cavity with frequency

of 9.5 GHz. The sample was mounted on a locally made

probe with electrical wires for applying voltage. The sample

holder also allows for changing the angle between the film

normal direction and the direction of the applied magnetic

field. The FMR spectrum, i.e., the field derivatives of the

absorbed power, was measured by a lock-in technique, where

the applied modulated ac magnetic field is 10 Oe in the FMR

measurement. The measured sample had a 2 mm� 2 mm

square shape. The electric field was applied across the PMN-

PT as shown in Fig. 1(a). The applied maximum voltage in

this experiment is 200 V, corresponding to an electric-field

strength E of 0.4 MV/m. All the measurements were carried

out at room temperature.

Figure 1(b) shows the normalized magnetization as a

function of in-plane and out-of-plane magnetic fields. The

two loops exhibit a similar saturation magnetic field, which

indicates a perpendicular magnetic anisotropy nearly cancel-

ling the shape-induced demagnetization field. The value of

Ms is determined to be 1250 emu/cm3. The field derivatives

of the absorbed power are shown in Fig. 1(c) for different

applied magnetic field angles (hH), defined as the angle

between the magnetic field and the normal direction of the

sample plain. Based on the FMR spectra, the resonance field

HR (approximated as the center field between the positive and

negative peak fields) and the peak-to-peak line-width DH can

be extracted for different hH values, as will be discussed later.

To study the strain-induced change of PMA in the

CoFeB layer, the FMR spectra were measured with different

voltages applied to the PMN-PT substrate, as shown in Figs.

2(a)–2(c). Figure 2(a) shows the HR dependence on the elec-

tric field for hH¼ 10�. The FMR spectra are shown in Fig.

2(b), which correspond to the black curve in Fig. 2(a). A

similar modulation of HR has also been observed in other

material systems.18,36–38 The curve for a full sweep of the

electric field is a typical butterfly-shaped loop, corresponding

to the electric-field-induced strain in the PMN-PT substrate.

For hH¼ 10�, the value of HR is shifted to a lower field by

FIG. 1. (a) Schematic of the sample structure and the measurement configu-

ration. (b) In-plane (red solid circles) and out-of-plane (blue open squares)

magnetic hysteresis loops of the samples for zero electric field. (c) FMR

spectrum as a function of external magnetic field for different magnetic field

angles (hH).

FIG. 2. (a) Strain-induced modulation of the FMR field (HR) by an electric

field applied across the PMN-PT substrate for hH¼ 10�. Measured FMR

spectra for electric field values of E¼ 0, 0.2, and 0.4 MV/m at hH¼ 10� (b)

and 70� (c), which correspond to black curve in Fig. 2(a), i.e., sweeping

electric field from the positive maximum.

072402-2 Yu et al. Appl. Phys. Lett. 106, 072402 (2015)

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128.97.89.222 On: Tue, 24 Feb 2015 21:35:52

positive electric fields. However, for hH¼ 70�, HR is shifted

to a higher magnetic field.

To further show the electric field effect, the hH depend-

ence of HR at different electric fields of E¼ 0, 0.2, and 0.4

MV/m is shown in Fig. 3(a). The curves can be analyzed by

using Kittel’s formula including the first- and second-order

uniaxial PMA fields,39,40 i.e., f ¼ ðcglB=2p�hÞffiffiffiffiffiffiffiffiffiffiffi

H1H2

p, with

H1 ¼ HR cos ðhH � hMÞ þ Hef fK1 cos2hM � Hk2 cos4hM and H2

¼HR cos hH�hMð ÞþHef fK1 cos 2hM�1

2Hk2ðcos 2hM þcos 4hMÞ.

Here, c is the gyromagnetic ratio defined as c ¼ glB=�h, where

g is the Lande factor, lB is the Bohr magneton, �h is the

reduced Planck constant, and hM is the angle between magnet-

ization and the normal direction. Hef fK1 and Hk2 are introduced

for simplification,39 with 12

MsHef fK1 ¼ K1 þ 2K2 �M2

s =2l0

and 14

MsHk2 ¼ K2, where K1 and K2 are the first- and second-

order PMA energy densities. In addition, the value of hM is

given by sin 2hM ¼ 2HR=Hef fK1 sinðhH � hMÞ þ HK2=Hef f

K1

sin 2hM cos2hM. The g factors, Hef fK1 and Hk2, are considered

as fitting parameters. The experimental data for all the three

electric field values can be well fitted by the above equations,

as shown in Fig. 3(a). The fitted g factors for E¼ 0, 0.2, and

0.4 MV/m are 2.07, 2.07, and 2.06, respectively. After getting

the Hef fK1 and HK2 values, the values for K1 �M2

s =2l0 and K2

as a function of electric field can be obtained, as shown in Fig.

3(b). Since the M2s =2l0 term is independent of the electric

field, the first term reflects the dependence of K1 on the elec-

tric field. Interestingly, it is observed that both first and

second-order anisotropy terms are affected by the electric

field, and that they have opposite voltage dependencies. In

particular, K1 increases with electric field, while K2 decreases

with electric field. In addition, it is shown that the dependence

of K1 on electric field is more pronounced than that of K2.

The electric field dependences of K1 and K2 are approximately

7540 fJ/V�m and �570 fJ/V�m, respectively. A similar de-

pendence of both first and second order anisotropy terms on

electric field, exhibiting opposite slopes, has recently also

been observed in measurements of the charge-mediated

VCMA in CoFeB/MgO samples fabricated on a regular

Silicon wafer, where the strain-mediated coupling mechanism

described in this work is absent.41

The effective perpendicular anisotropy energy density

Kef f ¼ K1 þ K2 �M2s =2l0 is then calculated, which is

shown in Fig. 3(c). Kef f increases linearly with electric field,

with a slope of �7000 fJ/V�m. It is noted that this large sen-

sitivity of the perpendicular anisotropy energy to electric

field fully originates from the strain coupling. The charge-

mediated VCMA effect observed at interfaces of thin CoFeB

films does not play a role here, since a thick Ta buffer is

inserted between the PMN-PT and CoFeB layers, and the

voltage is not applied across the MgO film. Note also that

the strain-mediated VCMA is �200� larger than that

reported in systems where the effect is charge mediated, with

typical values of �30–40 fJ/V�m.28,31,33,42

The strain-induced anisotropy change can be written as11

Dkef f ¼ � 32ezEf=ð1� �Þ, where k is the magnetostriction

coefficient, ez is the strain along the z axis, � is the Poisson ra-

tio, and Ef is the Young’s modulus for magnetic layer.

Among these parameters, k has not been measured for ultra-

thin CoFeB layers in previous reports (1.1 nm in our case).

The value of k can be estimated by quantifying the electric

field-induced strain. In the present experiment, the electric

field-induced strain was measured by a typical biaxial strain

sensor,14 and was determined to be ex ¼ ey ¼ 0:0251% for

E¼ 0.4 MV/m. Assuming that the in-plane compressive strain

is perfectly transferred to the CoFeB layer, ez can be

expressed as ez ¼ �2ex�ð1� �Þ. The value of � is assumed to

be 0.3, and Ef is 1:5� 1012 dyn/cm2 for CoFeB layers.11 The

strain-induced change of PMA is Dkef f ¼ kef f ð0:4 MV=mÞ�kef f ð0 MV=mÞ ¼ 2:7� 104 erg/cm3. Based on these values,

the magnetostriction coefficient for ultrathin 1.1 nm CoFeB

layer can be determined to be ¼ ð3:760:2Þ � 10�5.

Interestingly, this value is considerably larger than that

reported in thicker CoFeB films with thickness larger than

5 nm (�1 �10�5).43 This suggests that the strain effect will

be more pronounced in ultrathin 1.1 nm CoFeB layer, which

may benefit the design and development of strain-assisted

memory elements with small write voltage.

The demonstration of strain-induced change of PMA in

ultrathin 1.1 nm CoFeB layer is also relevant to the under-

standing of VCMA in MTJs. In particular, in MTJs, the

voltage-induced modulation of PMA is usually believed to

be due to accumulated charge effects.25,26 However, a recent

FIG. 3. (a) The hH dependence of the resonance field HR for different elec-

tric field values of E¼ 0, 0.2, and 0.4 MV/m. (b) The electric field depend-

ence of first and second order magnetic anisotropy parameters Hef fK1 and HK2.

(c) The electric field dependence of the effective magnetic anisotropy energy

density Kef f (multiplied by film thickness).

072402-3 Yu et al. Appl. Phys. Lett. 106, 072402 (2015)

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128.97.89.222 On: Tue, 24 Feb 2015 21:35:52

experiment has indicated that a voltage applied across the

MgO barrier can also generate strain in MgO due to its pie-

zoelectric effect.35 This strain can be transferred to the adja-

cent CoFeB layer, hence supplementing the charge-mediated

change of anisotropy. In particular, it is interesting to com-

pare the voltage-induced strain in the PMN-PT and MgO

cases. The strain (ez) response to the electric field in MgO is

calculated to be �3.7� 10�5 ppm per V/m, based on the data

in Ref. 35. By comparison, the ez value is determined to be

�1.25� 10�3 ppm per V/m in PMN-PT, i.e., �33� larger

than in MgO. Hence, the voltage-induced strain on the ultra-

thin CoFeB layer in MgO-based MTJs, although �33�smaller in the case of MgO compared to PMN-PT, may also

contribute to the modulation of PMA.

Figure 4(a) shows the electric field effect on the line-

width DH as a function of hH. The value of DH varies non-

monotonically with hH, which is consistent with previously

reported results.40,44 This change is believed to be due to ex-

trinsic effects, such as two-magnon scattering and magnetic

inhomogeneities.40,44 Using an approach similar to a previ-

ous work,40,44 we estimated the effective damping constant

as aeff ¼ffiffiffi

3p

DHppdð2pf=cÞ=dHR=ðH1 þ H2Þ. The calculated

aeff as a function of hH is shown in Fig. 4(b). aeff varies

between 0.04 and 0.09, which indicates that the intrinsic

damping constant a is most likely smaller than 0.04, which is

expected for 1.1 nm CoFeB film. It is noted that we cannot

see an obvious modulation of aeff by the electric field. These

results suggest that the strain may not effectively affect the

damping constant.

In summary, we experimentally demonstrated strain-

induced modulation of PMA in (001)-PMN-PT/Ta/CoFeB/

MgO/Ta structures by FMR. It has been observed that the

strain is able to shift the FMR spectra by changing the PMA

of the CoFeB layer. The first- and second-order anisotropy

energy densities were both obtained by fitting the experimen-

tal data, and show opposite dependencies on the applied

electric field. The change of the effective PMA by strain was

further quantitatively analyzed. A very large value of the

PMA modulation by electric field of �7000 fJ/V�m is

obtained. Based on the extracted change of anisotropy, the

magnetostriction coefficient k for 1.1 nm-thick CoFeB was

obtained to be 3.7� 10�5, which is significantly larger than

reported values in 5 nm thick CoFeB films. In addition, the

effect of strain on the effective damping constant was incon-

spicuous in the present samples within the accuracy limits of

our current experiment.

This work was partially supported by the National

Science Foundation Nanosystems Engineering Research

Center for Translational Applications of Nanoscale

Multiferroic Systems (TANMS), and in part by the DARPA

program on Nonvolatile Logic (NVL). This work was also

supported in part by the FAME Center, one of six centers of

the Semiconductor Technology Advanced Research network

(STARnet), a Semiconductor Research Corporation (SRC)

program sponsored by the Microelectronics Advanced

Research Corporation (MARCO) and the Defense Advanced

Research Projects Agency (DARPA). We would like to

acknowledge the collaboration of this research with King

Abdul-Aziz City for Science and Technology (KACST) via

Center of Excellence for Green Nanotechnologies (CEGN).

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