Enhanced voltage-controlled magnetic anisotropy in magnetic tunnel junctions with anMgO/PZT/MgO tunnel barrierDiana Chien, Xiang Li, Kin Wong, Mark A. Zurbuchen, Shauna Robbennolt, Guoqiang Yu, Sarah Tolbert,Nicholas Kioussis, Pedram Khalili Amiri, Kang L. Wang, and Jane P. Chang Citation: Applied Physics Letters 108, 112402 (2016); doi: 10.1063/1.4943023 View online: http://dx.doi.org/10.1063/1.4943023 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/108/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in 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 Very low 1/f barrier noise in sputtered MgO magnetic tunnel junctions with high tunneling magnetoresistance J. Appl. Phys. 112, 123907 (2012); 10.1063/1.4769805 Boron diffusion in magnetic tunnel junctions with MgO (001) barriers and CoFeB electrodes Appl. Phys. Lett. 96, 262501 (2010); 10.1063/1.3457475 Perpendicular magnetic tunnel junction with tunneling magnetoresistance ratio of 64% using MgO (100) barrierlayer prepared at room temperature J. Appl. Phys. 103, 07A911 (2008); 10.1063/1.2840016 Temperature dependence of the resistance of magnetic tunnel junctions with MgO barrier Appl. Phys. Lett. 88, 212115 (2006); 10.1063/1.2206680

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Enhanced voltage-controlled magnetic anisotropy in magnetic tunneljunctions with an MgO/PZT/MgO tunnel barrier

Diana Chien,1,a) Xiang Li,2,a) Kin Wong,2 Mark A. Zurbuchen,2,3 Shauna Robbennolt,4

Guoqiang Yu,2 Sarah Tolbert,4 Nicholas Kioussis,5 Pedram Khalili Amiri,2,6,b)

Kang L. Wang,2,b) and Jane P. Chang1,b)

1Department of Chemical and Biomolecular Engineering, University of California, Los Angeles,California 90095, USA2Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA3Center for Excellence in Green Nanotechnologies (CEGN), University of California, Los Angeles,California 90095, USA4Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA5Department of Physics, California State University, Northridge, California 91330, USA6Inston, Inc., Los Angeles, California 90095, USA

(Received 30 September 2015; accepted 18 February 2016; published online 15 March 2016)

Compared with current-controlled magnetization switching in a perpendicular magnetic tunnel

junction (MTJ), electric field- or voltage-induced magnetization switching reduces the writing

energy of the memory cell, which also results in increased memory density. In this work, an ultra-

thin PZT film with high dielectric constant was integrated into the tunneling oxide layer to enhance

the voltage-controlled magnetic anisotropy (VCMA) effect. The growth of MTJ stacks with an

MgO/PZT/MgO tunnel barrier was performed using a combination of sputtering and atomic layer

deposition techniques. The fabricated MTJs with the MgO/PZT/MgO barrier demonstrate a VCMA

coefficient, which is �40% higher (19.8 6 1.3 fJ/V m) than the control sample MTJs with an MgO

barrier (14.3 6 2.7 fJ/V m). The MTJs with the MgO/PZT/MgO barrier also possess a sizeable tun-

neling magnetoresistance (TMR) of more than 50% at room temperature, comparable to the control

MTJs with an MgO barrier. The TMR and enhanced VCMA effect demonstrated simultaneously in

this work make the MgO/PZT/MgO barrier-based MTJs potential candidates for future voltage-

controlled, ultralow-power, and high-density magnetic random access memory devices. VC 2016AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4943023]

There is a fast-growing need in the semiconductor indus-

try for alternative memory technologies which can combine

nonvolatile operation, high speed, high endurance, and high

density in a single silicon-compatible device. Magnetic ran-

dom access memory (MRAM) is an emerging candidate pro-

viding potential advantages in a range of standalone and

embedded memory applications. Present MRAM devices typi-

cally utilize current-controlled switching of magnetization via

the spin transfer torque (STT)1,2 or spin-orbit torque (SOT)3,4

effects to write information into magnetic bits. However, the

use of currents results in a memory cell size (i.e., bit density)

limitation due to the large size of the required access transis-

tors,5,6 and large dynamic switching energy due to Ohmic

power dissipation. Therefore, there has been a great interest in

using an applied voltage (instead of current) to manipulate the

magnetization of nanoscale magnetic tunnel junctions (MTJs).

The electric-field effect, or the voltage-controlled magnetic

anisotropy (VCMA) effect, is utilized to temporarily lower the

interfacial perpendicular magnetic anisotropy (PMA) of the

free layer during the writing operation, thus reducing the writ-

ing energy required to overcome the energy barrier between

the two stable magnetization states.7,8

A promising type of electric-field-controlled memory

device has been realized in perpendicular magnetic tunnel

junctions using the Ta/CoFeB/MgO material system, where

both high tunneling magnetoresistance (TMR)9,10 and

VCMA-induced magnetization switching11–14 have been

demonstrated. For large memory array (>1 Gb) with scaled

CMOS below 14 nm, VCMA coefficients larger than 200 fJ/

V m may be needed.15,16 However, the traditional Ta/

CoFeB/MgO system offers limited VCMA in the range of

10–60 fJ/V m.17–23

To achieve a larger VCMA effect, multiple approaches

have been explored, such as using different seed and cap

layers adjacent to the ferromagnetic layer.17,24–27 Ab initioelectronic structure calculations have revealed that epitaxial

strain has a dramatic effect on increasing the VCMA.28

Another promising method is by utilizing different dielec-

trics. As the VCMA effect originates from the charge accu-

mulated at the CoFeB/oxide interface when voltage is

applied,29 it has been demonstrated theoretically that using a

single oxide or multiple layers of oxides with higher dielec-

tric constant(s) (�) can induce a higher VCMA coefficient,

thus a reduction in voltage for magnetization switching.30 In

past experimental works, enhanced VCMA effect was meas-

ured in CoFeB/oxide structures using MgO/Al2O3 and MgO/

HfO2/Al2O3 as the gate oxide,23 but lacked an electrical

readout because full MTJ was not fabricated. Moreover,

there has been intensive research on MTJs using barrier

materials other than MgO. However, MTJs using SrTiO3

with CoFe electrodes had a rather low TMR around 10%;31

likewise, multiferroic tunnel junctions with ferroelectric

a)D. Chien and X. Li contributed equally to this work.b)Authors to whom correspondence should be addressed. Electronic addresses:

pedramk@ucla.edu; wang@ee.ucla.edu; and jpchang@ucla.edu

0003-6951/2016/108(11)/112402/5/$30.00 VC 2016 AIP Publishing LLC108, 112402-1

APPLIED PHYSICS LETTERS 108, 112402 (2016)

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17:43:14

barriers such as PbZr0.2Ti0.8O3 and BaTiO3 with Co/Fe and

La0.7Sr0.3MnO3 electrodes only demonstrated a reasonable

TMR below room temperature.32,33 Therefore, to achieve

better writing efficiency with reliable readout for voltage-

controlled MRAM, it is critical to have a sizeable room

temperature TMR in addition to VCMA enhancement after

integration of high-� oxide(s) into the stack.

In this work, an ultra-thin layer of high-� lead zirconate

titanate (PZT or Pb(ZrxTi1-x)O3) was integrated into the

MgO tunnel barrier in order to enhance the VCMA effect

while maintaining a sizeable TMR. A combination of sput-

tering and atomic layer deposition (ALD) techniques was

used to grow MTJ stacks with an MgO/PZT/MgO tunnel bar-

rier. Based on measurements on an ensemble of MTJ devices

with the MgO/PZT/MgO barrier, the VCMA coefficients

were improved by about 40%, and the room-temperature

TMR values were comparable—only slightly lower than in

those of MgO barrier MTJs.

PZT has been commonly used in Ferroelectric Random

Access Memory (FeRAM) devices34–36 and has been used in

multiferroic tunnel junctions.32 In this work, PZT thin film

was integrated into the tunnel barrier because it has one of

the largest dielectric constants (i.e., 300–1300 for 1–3 lm

PZT thin films37,38). Due to the fact that the PZT was inter-

faced with MgO on both sides, the interfacial dead layer that

is intrinsic to the electrode/dielectric boundary is expected to

be negligible in our film.39 PZT deposition was performed

via ALD,40,41 which has been previously shown to provide

conformal atomically smooth ultra-thin films with precise

control over composition and thickness.42

MTJs with a pure MgO tunnel barrier were used as the

reference sample and compared to the MTJs with the MgO/

PZT/MgO tunnel barrier (hereafter referred to as MgO MTJ

and PZT MTJ, respectively). Sample structures are schemati-

cally illustrated in Figure 1, with the following structures:

Ta(18 nm)/Co20Fe60B20(0.9 nm)/MgO(2.5 nm)/Co20Fe60B20

(2.0 nm)/Ta(4 nm)/Pt(2 nm) for the MgO MTJ, and Ta(18 nm)/

Co20Fe60B20(0.9 nm)/MgO(1.0 nm)/PZT(1.5 nm)/MgO(1.0 nm)/

Co20Fe60B20(2.0 nm)/Ta(4 nm)/Pt(2 nm) for the PZT MTJ.

The stacks were deposited on thermally oxidized Si sub-

strates using an AJA magnetron sputtering system and ther-

mal ALD. All metallic layers were DC sputtered. The

Co20Fe60B20 bottom free layer has a thickness of 0.9 nm

and the top fixed layer has a thickness of 2.0 nm; they were

out-of-plane and in-plane magnetically anisotropic, respec-

tively.9 For the MgO MTJ, a 2.5 nm thick MgO tunnel bar-

rier was grown by RF sputtering, while for the PZT MTJ, a

1.0 nm thick MgO layer was first sputtered, then a 1.5 nm

thick PZT film was deposited via ALD at a substrate temper-

ature of 250 �C, and finally, a 1.0 nm thick MgO was sput-

tered to form the MgO/PZT/MgO tunnel barrier. The

synthesis of PZT thin film has been outlined in previous

papers.40,41 The PZT MTJ film stack was annealed at 200 �Cunder vacuum both before the PZT deposition and after

depositing the whole film stack. Since the PZT MTJ film

stack was also in-situ annealed during the ALD process

under 250 �C, the MgO MTJs were annealed at 250 �C for a

fair comparison. MTJ devices with elliptical diameters of

4� 16 lm and 4� 12 lm were subsequently fabricated using

standard photolithography and dry etching techniques.

First, material properties of the MTJ stacks were charac-

terized using Kratos AXIS X-ray photoelectron spectroscopy

(XPS) and an FEI Titan scanning transmission electron

microscope (STEM). XPS confirmed the composition ratio

Zr:Ti¼ 52:48 of the PZT thin film deposited on the bottom

layers of a film stack, as shown in Figure 2(a). Note that it

has been shown that PZT exhibits enhanced properties (e.g.,

dielectric constant) at the morphotropic phase boundary

composition of Zr:Ti¼ 52:48.43 The XPS survey scan also

showed the Mg KLL, Co 2p, Fe 2p, and Ta 4d elemental

peaks. Note that the B 1s peak was not observed because the

estimated XPS penetration depth is limited to 10 nm and due

to the fact that most of the boron has diffused far into the Ta

layer due to the annealing process.44,45 Cross-sectional TEM

was performed on the fabricated MgO MTJ and PZT MTJ

devices, as shown in Figures 2(b) and 2(c), respectively, in

which the arrows indicate the general location of layer inter-

faces, spaced per Figure 1. Nano-diffraction patterns were

collected for both cross-sections, as shown in the insets of

Figures 2(b) and 2(c). A selected-area aperture was used for

the MgO MTJ, but in order to maximize diffracted intensity

from the �3 nm thick MgO/PZT/MgO layers-of-interest in

the PZT MTJ, a highly condensed probe was employed,

elongated along the in-plane direction of the film, which pro-

vided informative results due to the FEI Titan’s parallel

beam nearly all the way to the crossover point. The inset dif-

fraction patterns clearly showed that the MgO had crystal-

lized; however, indexing of the remaining spots to either

CoFeB or PZT was not possible due to resolution limita-

tions. Next, unpatterned MgO and PZT MTJ stacks were

characterized for their magnetic properties using supercon-

ducting quantum interference device (SQUID) magnetome-

try. The saturation magnetizations (Ms) were measured to

be 1017 6 22 emu/cm3 and 932 6 41 emu/cm3 for MgO and

PZT MTJ stacks, respectively, indicating that the ALD PZT

deposition had not significantly affected the magnetic prop-

erties of the CoFeB layers.

The MTJs were then measured electrically to investi-

gate the VCMA effect via the TMR readout at room

FIG. 1. Schematics of (a) MgO MTJ, and (b) PZT MTJ with the MgO/PZT/

MgO tunnel barrier measured in this work. Devices measured had elliptical

dimensions of 4� 16 lm and 4� 12 lm. Arrows show the magnetic aniso-

tropic directions of the CoFeB top fixed layer (in-plane) and the bottom free

layer (perpendicular).

112402-2 Chien et al. Appl. Phys. Lett. 108, 112402 (2016)

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17:43:14

temperature.17,46 The resistance was measured as the in-

plane magnetic field was swept while voltages were applied

between �300 to þ300 mV, as shown in Figure 3(a). At zero

magnetic field, the magnetic moment of the bottom CoFeB

free layer was perpendicular and that of the top fixed CoFeB

layer was in-plane, while at the maximum in-plane magnetic

field, the two CoFeB layers were both in-plane magnetized.

Hence, the resistance decreased as the magnetic field was

increased. The resistance-area (RA) products of the PZT and

MgO MTJ in Figure 3(a) were 98 kX lm2 and 14 kX lm2,

respectively, which are typical for voltage-controlled

MRAM.11,15 Using the equation G ¼ GSð1þ P2F cos hÞ, the

measured conductance G of the MTJ was related to the rela-

tive angle between the two CoFeB layers, where GS was the

mean surface conductance, h was the angle between two

CoFeB layers, and PF was the effective spin polarization.47

As the top 2.0 nm thick CoFeB layer was fixed at an in-plane

direction, the in-plane magnetization component Mx of the

bottom free layer CoFeB can be obtained by Mx

MS¼ cos h

¼ GðHÞ�Gð0ÞGðHmaxÞ�Gð0Þ, where G(H), G(Hmax), and G(0) are, respec-

tively, the MTJ conductances at in-plane magnetic field H, at

the maximum in-plane magnetic field measured, and at zero

external field.23,46 Note that here Hmax was determined by

saturation of the free layer magnetization to the in-plane ori-

entation, and the PZT MTJ demonstrated a higher saturation

field than the MgO MTJ. The perpendicular magnetic anisot-

ropy energy Eperp can then be calculated by conducting the

following integration for the free layer from the perpendicu-

lar easy axis (at zero external field), to the in-plane hard axis

(at Hmax): Eperp ¼ MS

Ð 1

0Hd Mx

MS

� �.17,18

Next, using the equation Ki ¼ ð2pMS2 þ EperpÞtCoFeB,

the value of interfacial PMA (Ki) was obtained,9 where

tCoFeB is the thickness of the CoFeB free layer. Finally, the

VCMA coefficient n was determined by n ¼ DKi=DEef f ,

where the effective electric field Eeff was calculated by divid-

ing the applied voltage V with the total thickness d of the

tunnel barrier. All measurements were performed at room

temperature.

The VCMA coefficients n (i.e., the slope of Ki versus

Eeff plot) are shown in Figure 3(b) for two representative

MgO and PZT MTJ devices. A total of six devices were

measured for each MTJ stack. The average VCMA coeffi-

cients were naverage¼ 14.3 6 2.7 fJ/V m for MgO MTJs, and

naverage¼ 19.8 6 1.3 fJ/V m for PZT MTJs, as shown in

Figure 4(a). Therefore, by incorporating the PZT film into

the MgO barrier, the VCMA effect was shown to be

enhanced by about 40%.

From the physics point of view, this enhanced VCMA

effect could be understood as follows. As indicated from abinitio calculations, the Ki stems from the hybridization of Fe/

FIG. 2. (a) XPS confirms PZT composition of Zr:Ti¼ 52:48 of a 1.5 nm

thick PZT film deposited on Ta (18 nm)/CoFeB (0.8 nm)/MgO (1.0 nm) fol-

lowed by annealing at 200 �C for 30 min. Schematic of stack is shown in the

inset, with the red arrow denoting the bottom free CoFeB layer to have a

perpendicular magnetic anisotropy. TEM of (b) MgO MTJ annealed at

250 �C for 30 min and (c) PZT MTJ annealed at 200 �C for 30 min. Both

images are scaled identically, with the substrate-side in the bottom of each

image. Nano-diffraction patterns are shown as inset in both (b) and (c),

which have the same scale.

FIG. 3. (a) Resistance vs. in-plane magnetic field of varying applied voltages

from �300 to 300 mV for the PZT MTJ device; inset showing that of the refer-

ence MgO MTJ device and (b) interfacial perpendicular magnetic anisotropy

(Ki) vs. applied electric field (Eeff) for an MgO and PZT MTJ device.

112402-3 Chien et al. Appl. Phys. Lett. 108, 112402 (2016)

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17:43:14

Co 3d orbitals and O 2p orbitals at the CoFeB/MgO inter-

face.48,49 The application of a positive electric field (i.e.,

top electrode of the MTJ at a higher electric potential)

across the MgO barrier induces accumulation of electrons

at the bottom CoFeB/MgO interface, which in turn affects

the hybridization of Fe/Co and O orbitals, thus decreasing

the value of Ki,29,30 which is consistent with the data shown

in Figure 3(b). Hence, if the interface charge density rq

increases for the same applied electric field Eef f , a larger

VCMA coefficient (n ¼ DKi=DEef f ) can be effectively

achieved. The interface charge density rq can be expressed

as rq ¼ �0�ef f V=d ¼ �0�ef f Eef f , where �0 is the permittivity

of free space, and �eff is the effective dielectric constant of

the tunnel barrier.23,30 Thereby, for the same tunnel barrier

thickness and applied voltage, the increase in the effective

dielectric constant �eff by incorporating PZT in the tunnel

barrier gives rise to a larger interface charge density change

at the CoFeB/MgO interface, thus resulting in a larger over-

all VCMA.

From the obtained VCMA ratio between the PZT MTJ

and the MgO MTJ, the dielectric constant for the PZT ultra-

thin film could also be calculated using a serial capacitor

assumption. For the PZT MTJ, the effective dielectric constant

was �PZT�MTJ ¼ ðdMgO þ dPZTÞ=ðdMgO=�MgO þ dPZT=�PZTÞ,while for the MgO MTJ, the dielectric constant was assumed

to be �MgO�MTJ ¼ �MgO ¼ 10.50 As the change of interfacial

PMA is proportional to the change of interface charge density,

i.e., DKi / Drq, it is deduced that n / �ef f .23 Thus, based on

the VCMA coefficients obtained for PZT and MgO MTJ, the

dielectric constant of the PZT ultra-thin film was estimated to

be 28.4, a plausible value taking into account the 1.5 nm PZT

thickness, as well as existing literature values for an ultra-thin

ALD PZT film.39,41

The VCMA coefficients were also plotted against Ki

(Figure 4(a)) and TMR ratio (Figure 4(b)) for all measured

MgO and PZT MTJ devices. The PZT MTJs were observed

to have a larger VCMA effect and a slightly smaller TMR ra-

tio compared to the MgO MTJs. The VCMAaverage was

14.3 6 2.7 fJ/V m for MgO MTJs, and 19.8 6 1.3 fJ/V m for

PZT MTJs. The TMRaverage was 61.4 6 11.5% for MgO

MTJs, and 53.1 6 1.7% for PZT MTJs. Note that the TMR

ratio here was defined by TMR ¼ ðRap � RpÞ=Rp, where the

anti-parallel resistance Rap was calculated according to equa-

tion 1=Rap ¼ 2=Rort � 1=Rp,47 where the parallel resistance

Rp was the resistance at the maximum magnetic field or

1=GðHmaxÞ, and the orthogonal CoFeB configuration resist-

ance Rort was the resistance at zero external magnetic field

or 1=Gð0Þ.Compared with other works on Ta/CoFeB/MgO in the

literature with the VCMA coefficients ranging from 10 to

60 fJ/V m,17–23 the VCMA coefficient values in our PZT and

MgO MTJs are at the lower bound, but the VCMA effect can

be improved by optimizing a number of parameters, includ-

ing annealing conditions,27 surface roughness,51 and intrinsic

strain28 of the layers. Nevertheless, a 40% enhancement in

the VCMA coefficient was achieved by using the MgO/PZT/

MgO tunnel barrier while a relatively high TMR was still

preserved.

In conclusion, by combining atomic layer deposition and

magnetron sputtering techniques, an ultrathin PZT layer was

incorporated into the MgO tunnel barrier of a magnetic tunnel

junction. The resulting magnetic tunnel junctions using a

high-� tunnel barrier were shown to have both large tunneling

magnetoresistance (>50%) and an enhanced VCMA effect

(by 40%) at room temperature. This high-� tunnel barrier MTJ

is a potential candidate for future voltage-controlled, ultralow-

power, high-density MRAM devices.

This work was supported by the NSF Nanosystems

Engineering Research Center for Translational Applications

of Nanoscale Multiferroic Systems (TANMS). The authors

are grateful to the UCLA Nanoelectronics Research Facility

(NRF), California NanoSystems Institute (CNSI), specifically

the Materials Lab at the Molecular Instrumentation Center

(MIC), Nano and Pico Characterization Lab (NPC), and

Electron Imaging Center for NanoMachines (EICN), for their

assistance and use of lab equipment. The authors would like

to acknowledge the collaboration of this research with King

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

The Center of Excellence for Green Nanotechnologies

(CEGN). The authors would also like to thank Professor Greg

Carman and Professor Chris Lynch for fruitful discussions.

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The circles are drawn to illustrate the distribution of VCMA coefficients for

the MgO and PZT MTJs.

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