In this material, a converse magnetoelectric effect and especially, an electric field-induced magnetic anisotropy and magnetization switching process have been observed at the changing s
Trang 1Electrical Field-Induced Magnetization Switching
in CoFe/NiFe/PZT Multiferroics
Nguyen Thi Minh Hong1, Pham Thai Ha1, Le Viet Cuong1, P T Long2, and Pham Duc Thang1,3,4
1Faculty of Engineering Physics and Nanotechnology and Laboratory for Micro and Nanotechnology,
University of Engineering and Technology, Vietnam National University, Hanoi 10000, Vietnam
2Department of Physics, Chungbuk National University, Cheongju 361-763, Korea
3UNESCO UNISA Africa Chair in Nanosciences and Nanotechnology, College of Graduate Studies, University of South Africa, Pretoria 0002, South Africa
4Nanosciences African Network, iThemba LABS-National Research Foundation of South Africa,
Pretoria 0001, South Africa
In this paper, we have investigated the change in magnetization of multiferroic material, based on magnetic nanostructured CoFe/NiFe film grown on the piezoelectric lead zirconate titanate (PZT), under the effect of the strain originated from PZT layer.
In this material, a converse magnetoelectric effect and especially, an electric field-induced magnetic anisotropy and magnetization switching process have been observed at the changing stages of applied electric voltage In addition, a significant relative change in magnetization, above 100%, is obtained, which facilitates practical applications of the materials This opens possibilities in achieving new types of memory devices, the low energy consumption devices, as well as other functionalities, such as voltage-tunable field sensing A simple theory based on strain-mediated magnetic-electric coupling is also presented to understand the origin of the change in magnetic properties of the materials.
Index Terms— Ferroelectrics, ferromagnetics, magnetization switching, multiferroics.
I INTRODUCTION
MULTIFERROIC materials in which magnetic and
ferroelectric orders coexist have emerged as one of
the promising materials for multifunctional applications in
data storage, field sensors, and next generation spintronic
devices, owing to the advantageous possibility of controlling
the magnetic state via the electric fields and vice versa
[1]–[3] Recently, the research interests have been focused
on the converse magnetoelectric (CME) effect and
magnetiza-tion (M) switching by electric field (E) in multiferroics thanks
to its potential application for new types of memory devices
and low energy consumption devices [4], [5] The physical
nature of electric-magnetic couplings is also of great interest
but has not been thoroughly explored
There are various ways to change magnetic anisotropy
of magnetic films One of the conventional methods is by
changing deposition conditions, e.g., substrate temperature,
gas pressure, and so on A new approach recently applied
for multilayered films is to use a multiferroic structure, in
which the magnetic properties can be tuned by an applied
electric voltage In the previous works, we reported the
finding of the CME effect and the electric-voltage-controlled
magnetic anisotropy in new CoFe/NiFe/piezoelectric lead
zir-conate titanate (PZT) heterostructured nanocomposites [6]–[8]
Interestingly, we observed the switching of the magnetization
at a suitably applied electric voltage
Manuscript received November 22, 2013; revised January 20, 2014;
accepted January 28, 2014 Date of current version June 6, 2014
Corre-sponding author: N T M Hong (e-mail: hongntm@vnu.edu.vn).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2014.2304518
This paper aims to investigate the change in magnetization
of nanostructured CoFe/NiFe/PZT multiferroics, induced by the presence of a stress originated from transverse polarized PZT piezoelectric layer In addition, we present a theoretical study on the CME effect and electric field-induced magnetiza-tion switching, the role of ferromagnetic layer thickness, and bias magnetic field on the magnetization switching process in this material
II EXPERIMENT NiFe and CoFe magnetic layers were magnetron sputtered
in sequence on commercial 500-μm-thick PZT piezoelectric
substrate having transverse polarization (APC-855, American Piezoceramics Inc.) A base pressure of 2× 10−7 torr was achieved prior to the deposition The deposition process was performed at room temperature in an argon gas environment
with a pressure ( pAr) of 2.2 mtorr and at a power of 50 W.
In this CoFe/NiFe/PZT multiferroic structure, the thickness
of CoFe layer is fixed at 190 nm The total thickness of NiFe/CoFe layers was changed from 200 to 280 nm These
samples are noted as S1, S2, S3, and S4, respectively Finally,
a Ta thin layer was sputtered on the top of the samples to prevent oxidization for ferromagnetic layers
Magnetic hysteresis loops, M (H ), were measured using
a vibrating sample magnetometer (VSM) 7400 (Lakeshore)
with the magnetic field (H ) applied in the film plane and
normal to the film plane For the CME effect measurement, the electrodes, having the size of 5× 0.5 mm2, are placed on two sides of transverse polarized PZT substrates by using silver adhesive glue (Fig 1) The CME measurement was performed using the VSM on 5× 5 mm2 sample, placed in a varied 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
Trang 2Fig 1 Experimental setup of CoFe/NiFe/PZT multiferroics for CME
measurement.
external bias magnetic field (Hbias) and an applied electric
voltage (V ) ranged from −400 to 400 V
III RESULTS ANDDISCUSSION
For the samples S1, S2, S3, and S4, magnetic hysteresis
loops have been measured at room temperature and at
various anglesα = 0° and 90° in which α is the angle between
the film plane direction and the external bias magnetic field
direction The results, not shown here, indicate that in-plane
magnetic anisotropy dominates for all multiferroic samples
due to the magnetic anisotropic contribution of NiFe and
CoFe ferromagnetic layers The magnetization change as a
function of voltage applied along the PZT substrates, M (V ),
measured at various bias magnetic field Hbias= 0, ±50, ±100,
±1000 Oe aligned parallel to sample plane (α = 0°), are
shown in Fig 2
The results point out an almost linear dependence of
mag-netization M on voltage V for all samples Under applied
electric voltage, PZT substrate is elongated in the film plane
and the strain in PZT substrate leads to a strain in NiFe/CoFe
layers Consequently, this alters the magnetic anisotropy of
the magnetic layers via magnetoelastic coupling It indicates
that the elastic stress transfers from PZT piezoelectric phase to
NiFe/CoFe magnetic phase In this effect, the magnetoelastic
energy associated with coupling between the stress and the
magnetization and stress anisotropy field H σ can be formed
in the NiFe/CoFe In this case, the elastic stress associates
with magnetization of magnetostrictive materials and thus, the
stress anisotropy can be induced [9] The change in
magne-tization, M/M, measured at α = 0° and Hbias = −50 Oe
under a voltage ranged from 0 to 400 V is larger than 100%
We observe a maximum valueM/M of 138% for the sample
S2 (corresponding to M = 550 μemu).
The strength of the stress anisotropy is dependent on the
product of the saturation magnetostriction of ferromagnetic
materials λ S and the applied stress σ, namely λ S σ [10].
In the theoretical model, the orientations of magnetization
are defined by the competition between the stress anisotropy
field H σ and magnetocrystalline anisotropy field HK We
con-sider first the magnetoelastic energy which is given by
Eme = K σsin2θ where the stress anisotropy constant Kme
is expressed as K σ = (3/2)λ S σ Apparently, K σ > 0 favors
θ = 0° where the magnetization is parallel to the stress
axis Contrary, K σ < 0 favors perpendicular alignment of the
magnetization (θ = 90°) This means that when the absolute
values of the stress anisotropy constant K σ are larger than the
magnetocrystalline anisotropy constant K1, the magnetization will be aligned along the easy direction determined by the stress anisotropy For the studied samples, PZT substrate has transverse polarization and NiFe/CoFe layers have in-plane magnetic anisotropy, which means the parallel alignment of
magnetization is relative to the stress axis, thus K σ > 0
Mean-while NiFe/CoFe layers have the magnetostriction coefficient
λ S > 0, therefore σ > 0 indicates that the stress in those
layers is tensile This is in accordance with the above results
of increasing M with changing V
On further investigation on the dependence of M on var-ied V , one can observe the electric field-induced switching of
magnetization which occurs at certain applied voltage, called
the magnetization reversed voltage Vrev (as shown in Fig 3)
At this voltage, large strains produced in PZT can transfer
to NiFe/CoFe and induce a change in the orientations of the magnetization, i.e., in the magnetic anisotropy This allows us
to control magnetization switching by using an electric field instead of a traditional magnetic field At the same magnetic
field Hbias, Vrev decreases with increasing the thickness of NiFe
In this paper, the thickness of ferromagnetic layer CoFe has been reduced to 190 nm in comparison to that in the previous works [7] Consequently, the results show a decrease
of Vrev from −500 to −75 V at the same bias magnetic field of 5 Oe This finding is interesting since it reduces the consumption energy in the applications Hence, these multifer-roic structures CoFe/NiFe/PZT have a promising application
for future spintronic devices For sample S3 especially, the electric field-induced magnetization switching can be achieved
at Vrev = −70 V without Hbias The magnetization state can
be controlled by an external voltage, even without the use of a bias magnetic field This opens a possibility for application of CME devices with size reduction and exclusion of interference effect from electromagnet or permanent magnet [11]
In order to provide more explanations to the field-induced magnetization switching phenomenon, the voltage-induced magnetic susceptibilityχ was derived from M(V ) data, taken
at various magnetic field Hbias = −50, −100, −200, −500, and−2000 Oe and shown in Fig 3 for one typical sample S1
As can be seen from Fig 4, first, χ has a negative value
at high applied voltage When decreasing applied voltage,
χ decreases to a minimum, increases then cancels at fixed voltage (Vrev) and finally changes in sign One observes that the higher Hbiasis, the higher Vrevrequires At high magnetic field, the magnetization rotates progressively from easy axis
to the field direction In this state, a much higher energy is
necessary to switch magnetization and therefore, Vrevincreases
with increasing Hbias Once magnetization is aligned along the direction of the magnetic field, magnetization switching process no longer occurs andχ approaches to zero In general,
magnetization switching can be decided by the competition between the magnetic field energy and applied electric field energy
In addition to considering the magnetization changes of samples as a function of applied voltages at various magnetic
fields, we also observed the difference of M value measured
Trang 3Fig 2. Magnetization M as a function of applied voltage V measured at various magnetic fields at angle α = 0°.
Fig 3. Magnetization reversed voltage value (Vrev ) for samples measured
at various magnetic fields.
at various angles α (Table I) The results show that the
dependence M (V ) still has a virtually linear increase as we
increase the applied voltage on PZT substrate However, the
change in magnetization decreases gradually when magnetic
field direction deviates from the film plane direction Notably,
this change is very small atα = 90° and even Vrevcan achieve
a voltage as small as 0.5 V for sample S3 at magnetic field
Fig 4 Voltage dependence of magnetic susceptibilityχ for sample S1
These obtained results can be explained based on the magnetization process which is described by the model for different orientations of applied magnetic fields Atα = 0°,
magnetization and magnetic field direction are parallel together (having in-plane magnetic anisotropy) Thus, mag-netization switching process requires much more electric field energy to rotate magnetization in the reverse direction On the other hand, when α = 90°, smaller voltage is needed to
Trang 4TABLE I
V ALUE OFVrev FOR S AMPLES M EASURED AT V ARIOUS
A NGLEαAND ATHbias = −100 Oe
reverse magnetization from the initial state to the orientation
of applied magnetic field Thus, the rotation of magnetization
toward applied magnetic field direction leads to a vantage of
energy, causing the decrease of Vrev value
IV CONCLUSION The magnetic properties, including CME effect and electric
field-induced magnetization, of nanostructured multiferroic
CoFe/NiFe/PZT films have been studied Under the strain
originated from PZT piezoelectric layer in the presence of the
external electric voltage, induced switching of magnetization
and magnetic anisotropy have been observed This opens
possibilities in achieving new types of memory devices, low
energy consumption devices, as well as other functionalities,
such as voltage-tunable field sensing
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