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:
[email protected]; [email protected]; and [email protected]
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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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 MgO and PZT MTJs.
112402-4 Chien et al. Appl. Phys. Lett. 108, 112402 (2016)
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