Integrated Magnetics and Magnetoelectrics for Sensing, Power, RF, and Microwave Electronics

Abstract

As the rapid development of integrated magnetic and magnetoelectric, numerous novel devices including high performance on-chip transformers, inductors, filters, antennas, and sensors with unique advantages in power efficiency, size and tunability, etc. have been demonstrated. In this review, an overview of the development of magnetism and magnetoelectric will be firstly given. The conceptual illustration and materials used in integrated magnetoelectric will then be presented. Selections of on-chip devices from literatures will be shown to exemplify the integrated magnetic and magnetoelectric applications. Finally, the prospect and the direction of the future research will be discussed in the conclusion.

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Received 1 May 2021; revised 1 August 2021; accepted 23 August 2021.
Digital Object Identifier 10.1109/JMW.2021.3109277
Integrated Magnetics and Magnetoelectrics for
Sensing, Power, RF, and Microwave Electronics
YIFAN HE ,BINLUO , AND NIAN-XIANG SUN (Fellow, IEEE)
(Invited Paper)
Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115 USA
CORRESPONDING AUTHOR: Nian-Xiang Sun (e-mail: n.sun@northeastern.edu).
(Yifan He and Bin Luo contributed equally to this work.)
ABSTRACT As the rapid development of integrated magnetic and magnetoelectric, numerous novel devices
including high performance on-chip transformers, inductors, filters, antennas, and sensors with unique
advantages in power efficiency, size and tunability, etc. have been demonstrated. In this review, an overview
of the development of magnetism and magnetoelectric will be firstly given. The conceptual illustration and
materials used in integrated magnetoelectric will then be presented. Selections of on-chip devices from
literatures will be shown to exemplify the integrated magnetic and magnetoelectric applications. Finally,
the prospect and the direction of the future research will be discussed in the conclusion.
INDEX TERMS Magnetic, magnetoelectric, integrated devices, high frequency, sensing.
I. INTRODUCTION
Magnetic materials have a long history of usage since 4500
years ago when Chinese made the compass and also play an
important role in modern technologies like motor, electrical
generator, and transformer, etc [1]. With the development of
solid state physics, the magnetic properties and theories be-
yond the static states have been discovered and utilized includ-
ing the magnetostatic wave propagation, magnetization oscil-
lation and spintronics, which enable a wide variety of new ap-
plications especially for high frequency integrated electronics
and magnetic storage including waveguide [2], inductor [3],
filter [4], phase shifter [5], Magnetoresistive random-access
memory (MRAM) [6], spin-based transistor [7], oscillator [8]
and sensor [9] etc.
Magnetoelectrics (ME) refers to the coupling between the
magnetism and electricity and is one of the aspects of multifer-
roics which indicates the material presents two or more ferroic
properties (ferroelectricity, ferroelasticity, ferromagnetism). It
has been shown that the two-phase ME heterostructure that
couples through the mechanical strain can generate much
larger ME coupling compared with its single phase counter-
part [10]. The ME coupling effect provides a new methodol-
ogy to mediate and monitor the magnetism in the magnetic
material, and hence brings numbers of new potential features
to the devices mentioned above. The ME coupling can be
divided into two categories, namely direct ME effect (DME)
where the electric polarization is controlled by the magnetic
field and converse ME effect (CME) where the magnetization
is controlled by the electric field.
Since the emerging and prosperity of complementary
metal-oxide-semiconductor (CMOS) and thin-film technol-
ogy, both magnetic and magnetoelectric materials have been
integrated into the batch fabrication and incorporated in num-
bers of compact, power efficient on-chip devices. Among
them, the integrated magnetic material serves versatile func-
tions and will be elaborated later, DME is majorly used
for sensing applications [11] and energy harvesting [12]
where the external magnetic field is coupled to magnetic
phase and generate an electric filed at output, whereas the
CME is utilized to achieve numbers of integrated elec-
tric field tunable devices including tunable inductor [13],
filter [14], etc. The newly demonstrated nanoelectromechani-
cal system (NEMS) transmitting ME antennas [15] also rely
on the CME. A selection of on-chip radio frequency (RF)
/microwave, power and sensing devices will be shown in
this paper to elaborate the typical application structures, fea-
tures and benefits brought by the integrated magnetics and
magnetoelectrics.
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
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HE ET AL.: INTEGRATED MAGNETICS AND MAGNETOELECTRICS FOR SENSING, POWER, RF, AND MICROWAVE ELECTRONICS
II. MAGNETOELECTRIC EFFECT, MATERIALS AND
RELEVANT PHYSICAL MECHANISMS
As one category in multiferroic material, ME material shows
ferroelectricity and ferromagnetism simultaneously and hence
provides a potential methodology for achieving multiple func-
tions in one device and subsequently device miniaturization
[10]. The first experimental demonstration of ME effect was
achieved in single phase Cr2O3material by Dzyaloshinski
in 1960 [16], numerous efforts have been put into discov-
ering new material for larger ME coupling effect. However,
researchers found in 2000 that in respect to the transition
metal atomic orbital, the conditions forming the ferroelectric-
ity and ferromagnetism excludes each other, which explains
the difficulty of synthesize highly efficient material with large
ME effect under various temperatures [17]. On the other hand,
strain mediated two-phase ME composite, which typically
consists of an electrostricive/piezoelectric phase and a mag-
netostrictive phase, is capable to overcome the physical limi-
tation of its single-phase counterpart and present stronger ME
coupling. The two-phase ME composite utilizes the material
strain to convert the magnetic field to electric polarization in
DME case and to convert electric field to magnetization in
CME case. To quantify the strength of ME coupling, the direct
and converse ME coefficient are named and defined as [18]:
αDirect =∂P
∂H(1)
αConverse =∂M
∂E(2)
In the above equations, Pis the electric polarization, His
the applied magnetic field, Mis the magnetization and Eis the
applied electric field.
To better describe the ME coupling condition in designing
practical applications or analyzing the experimental results,
magnetically induced voltage ME coefficient is defined and
widely used [19]:
αME =∂E
∂H(3)
where His the applied magnetic field and Eis the induced
electric field.
It is clear that for two-phase ME composite material, the
intrinsic material properties and the interface condition are
of significance to ensure a decent coupling condition. As an
example, the theoretical upper limit of ME coefficient of a
L-L (longitudinally magnetized longitudinally poled) config-
uration ME composite can be given as [20]:
αME=nd33,md33,p
nsE
33 1−k332+(1−n)sH
33
(4)
where d33,mand d33,p are the longitudinal piezomagnetic and
piezoelectric coefficient, k33 is the electromechanical cou-
pling coefficient of the piezoelectric layer, sE
33 and sH
33 are the
elastic compliance for the piezoelectric and magnetostrictive
layers, and nis a thickness fraction of the magnetostrictive
layers.
The strong ME coupling effect can boost the performance
of DME based sensing and energy harvesting in terms of
the sensitivity or energy conversion efficiency, as well as the
function tunning range of CME based devices. In the rest
part of the section, we will present various material choices
in both piezoelectric/electrostrictive phase and magnetostric-
tive phase that are widely adopted for achieving strong ME
coupling. For the CME-based devices, since device features
are typically enabled by different magnetic film properties and
CME is adopted to alter these properties for reconfigurability,
we will discuss the magnetic properties tuning mechanisms
induced by the strain brought by CME in this section and elab-
orate the material choices and configurations in the following
sections that focus on the devices.
A. CME INDUCED MAGNETIC PROPETY TUNING
MECHANISMS
As previously mentioned, unlike DME whose applications
typically requires a strong ME coupling, CME-based appli-
cations focus more on the reconfigurability brought by the
strain-mediated magnetic properties. For a simple multiferroic
composite with a magnetic thin film deposited or glued on
a piezoelectric layer, a strain generated in piezoelectric layer
by applied electric field is transferred to magnetic thin film.
Due to the induced magnetoelastic energy and the tendency of
energy minimization, the influence of the strain upon the static
and dynamic behavior of magnetization can be quantitively
denoted by the voltage induced effective in-plane magnetic
field [21, 22], which is given as:
Hef f =3λs·Y·def f ·E
μ0Ms
(5)
where λsis the saturation magnetostriction of the magnetic
film, Yis the Young’s modulus of the magnetic film, Msis
the saturation magnetization of the magnetic film; deff is the
effective piezoelectric coefficient of the piezoelectric layer,
and Eis the electric field applied on the piezoelectric layer.
In this part, we will discuss two magnetic property tuning
mechanisms that are induced by the strain, including E-field
tuning ferromagnetic resonance (FMR) and E-field tuning
permeability. The abovementioned mechanisms serve as the
basis for the reconfigurability of multiple integrated high fre-
quency, power and sensing devices. There are some more tun-
ing mechanisms including E-field tuning magnetoresistance
and E-field tuning M-switching, the former one is utilized to
adjust the sensitivity and detection range of magnetoresistance
sensors [23], the later one is majorly adopted for magnetic
storage devices like ME random access memory (MERAM)
to control the magnetization rotation [24]. These two mech-
anisms will not be elaborated in this work considering their
rather minor applications.
1) E-FIELD TUNING FERROMAGNETIC RESONANCE
FMR refers to the resonance happens when the spontaneous
Larmor precession frequency of magnetization coincides with
the external electromagnetic wave excitation. In this case,
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the external excitation signal is absorbed for magnetization
precession and attenuated due to the presence of the mag-
netic damping. FMR was first experimentally demonstrated
by Griffiths in 1946 [25]. In 1951, Kittel proposed Kittel
equation [26] that determined the FMR frequency:
fFMR
=γ(Hdc+Ha+(Ny−Nz)4πMs)(Hdc+Ha+(Nx−Nz)4πMs)
(6)
In equation 6, γis the gyromagnetic ratio; Hdc is the applied
DC magnetic field; Hais the spontaneous magnetic anisotropy
field, Nx,N
y, and Nzare the demagnetizing factors deter-
mined by the dimension of the magnetic material.
It Is Clearly Shown in Equation 6 With the Effective Mag-
netic Field Induced By the strain, Hdc Changes and Hence
Shift the FMR frequency.
For the magnetic film exhibiting in-plane magnetization
parallel to E-field induced effective magnetic field, the tun-
ability of the FMR frequency can be approximated as [27]:
f
f≈1
2
Hef f
Ha
(7)
For the magnetic film exhibiting out-of-plane magnetiza-
tion parallel to E-field induced effective magnetic field, the
tunability of the FMR frequency can be approximated as:
f
f≈Hef f
Ha−Ms
(8)
Fig. 1(a) presents a typical test setup for FMR absorp-
tion of magnetic material thin films. The FeGaB/PZN-PT
ME composite was placed on top of a transmission line.
The electromagnetic wave propagated in the transmission line
was coupled with the magnetic material and a notch in the
transmission coefficient S21 spectrum would appear at the
corresponding FMR frequency due to the energy absorption.
The S21 spectrum of the FeGaB/PZN-PT ME composite in the
self-biased condition (without the excitation of bias magnetic
field) is shown in Fig. 1(b) [21]. It can be noted that the
FMR where the maximum absorption peak resides can be
tuned by electric fields in the self-bias condition over a wide
range of frequency from 1.75 GHz at zero electric field to
7.57 GHz at 6 or 8 kV/cm, with a tunable frequency range of
5.82 GHz. The wide electrostatically tunable frequency range
corresponding a mean tunable frequency per unit electric field
of 15 GHz·cm·kV-1 and a tuning ratio of fmax/fmin =4.3 have
been achieved.
2) E-FIELD TUNING PERMEABILITY
Permeability is another crucial parameter of magnetic mate-
rials. High permeability materials are widely used in trans-
former and inductor applications to concentrate the magnetic
flux and hence increase the inductance and magnetic coupling
factor. With the need of the reconfigurability, the ME compos-
ite can serve as the magnetic core for these devices.
FIGURE 1. (a) Microwave absorption test setup for FeGaB/PZN-PT ME
composite. (b) Electric field dependence of the transmission coefficient
(S21) spectra of FeGaB/PZN-PT ME composite. (Reproduced from [21]).
FIGURE 2. Magnetic hysteresis loops of the FeGaB/PZN-PT multiferroic
heterostructure under different external electric fields. (Reproduced
from [21]).
The effective field generated by the strain in ME composite
changes the magnetic anisotropy field and thereafter the easy
axis direction of the magnetic material. Therefore, the effec-
tive permeability describing the magnetization response under
the applied magnetic field in a certain direction change with
the applied electric field applied on the ME composite.
To exemplify the tunning mechanism, the magnetic hystere-
sis of FeGaB/PZN-PT ME composite under different electric
fields is shown in Fig. 2 [21]. The tunable hysteresis loop
shows an in-plane anisotropy field ranging from 20 Oe at
0 kV/cm and 700 Oe at 6 kV/cm. The different slopes of
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HE ET AL.: INTEGRATED MAGNETICS AND MAGNETOELECTRICS FOR SENSING, POWER, RF, AND MICROWAVE ELECTRONICS
TAB L E 1. Comparison of Commonly Used Ferro/Piezoelectric/Electrostrictive Materials for ME Applications
the hysteresis loop indicate the applied electric field have
changed the magnetization response and hence the effective
permeability in in-plane direction.
To quantify the effective permeability μeff with the presence
of strain, the following equation is given in [13]:
μef f =4πMs
Hef f
+1(9)
where the effective magnetic anisotropy field can be given as:
Hef f =Ha+Heff (10)
where Hais defined in equation 6 and Heff is defined in (5)
as the electric field induced effective anisotropy magnetic field
in the magnetic films.
B. PIEZOELECTRIC/ELECTROSTRICTIVE MATERIALS FOR
MAGNETOELECTRICS
Among piezoelectric/electrostrictive materials, lead zirconate
titanate (PZT), lead magnesium niobate–lead titanate (PMN-
PT), lead zinc niobate–lead titanate (PZN-PT) and aluminum
nitride (AlN) have been widely used in integrated ME de-
vices to convert electric excitation to strain. Table 1 shows
a cross comparison of the most commonly used piezoelec-
tric/electrostrictive materials for integrated ME applications.
PZT is a polycrystalline ceramic ferroelectric material with a
high electromechanical coupling coefficient and an effective
piezoelectric coefficient of −200 to −300 pC/N [28] at mor-
photropic phase boundary (MPB) composition corresponding
to Zr/Ti ratios of about 58/42. The PZT piezoelectric ceramic
also has the advantages of low-cost and easier shaping process
[29]. The main drawback of PZT materials is that the contam-
ination during the process is often unavoidable due to the lead
in it, which greatly degrades its application in magnetoelectric
devices [30]. Moreover, the relatively high acoustic loss of the
PZT film at high frequency range prevents it from achieving
an efficient ME coupling. In Table 1, PZT materials are clas-
sified into hard and soft PZTs, where the hard PZT materials
usually have smaller piezoelectric coefficient with higher en-
ergy efficiency. Depending on the circumstance of the specific
applications, various types of PZT materials are commercially
available. Different from the PZT film, aluminum nitride
(AlN) is a textured polycrystalline non-ferroelectric material
with an advantage of lower loss tangent and compatibility
with CMOS technology. Another unique advantage of AlN
is its high electric Curie temperature, which is desirable for
electronics applications working under high temperature of
inferior heat dissipation environment. However, the moderate
piezoelectric coefficient of about −2 pC/N makes it unsuitable
for ME devices requiring high ME coupling [31]–[33].
Compared with PZT and AlN, lead magnesium niobate–
lead titanate (PMN-PT) and lead zinc niobate–lead titanate
(PZN-PT), as single crystal ferroelectric materials, show
larger piezoelectric coefficients and lower loss tangents. The
piezoelectric performance of these materials highly depends
on their composition and crystal orientation. The composition
here means the element ratio of rhombohedral phase (PZN
or PMN) and tetragonal phase (PT). The PZN-PT composi-
tion that induce MPB with (001) crystal cut can give a d33
piezoelectric coefficient as high as 2800 pC/N [34]. For the
PMN-PT film, a 31% PT and optimized cut give a d31 of about
-1800 pC/N and a d33 of about 2000 pC/N [35]. Although
the forementioned two materials deliver superior piezoelectric
performance, the difficulties of composition control during the
deposition of these materials may induce the lack of perfor-
mance repeatability. The grain growth of these single crystal
materials is also challenging. Two major deposition methods
include flux method and Bridgman method [36].
C. PIEZOMAGNETIC/MAGNETOSTRCTIVE MATERIALS FOR
MAGNETOELECTRICS
In general, the desired properties of piezomag-
netic/magnetostrictive phase in ME composite for achieving
a strong ME coupling are high saturation magnetostriction λs
and high piezomagnetic coefficient dλ/dH. Depending on the
specific applications, other magnetic and elastic properties
of the materials may also be considered. The low magnetic
loss tangent, low FMR bandwidth and low coercive field
are essential for high frequency application. Some particular
properties like E effect where the Young’s modules of
materials vary with the applied magnetic field, are also
needed for specific applications including sensors.
A cross-comparison of commonly used piezomag-
netic/magnetostrictive materials for ME composite is given
in Table 2 including Terfenol-D (Tb0.7Dy0.3Fe2), Galfenol
(Fe81Ga19 ), FeGa, FeCoSiB, FeGaB, FeBSiC (Metglas) and
FeGaC.
Based on the Terfenol-D with the largest magnetostriction,
the Terfenol-D/PZN-PT composite with giant electric field
induces effective magnetic anisotropy field of 3500 Oe, cor-
responding to a magnetoelectric coefficient of 580 Oe·cm/
kV [44]. The magnetoelectric coefficient is almost 4 times
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TAB L E 2. Comparison of Commonly Used Piezomagnetic/Magnetostrictive Materials for ME Applications
FIGURE 3. (a) Boron content dependent magnetostriction property of FeGaB thin film material. (b) Boron content dependent FMR linewidth of FeGaB
thin film material. (Reproduced from [48]).
of that in Fe3O4/PZN-PT [22]. Even though Terfenol-D ex-
hibits giant saturation magnetostriction of 1600 ppm, it is
lossy and has a high saturation field of several kOe, which
induces a relatively small piezomagnetic coefficient. Com-
pared with Terfenol-D, binary FeGa alloys show a high satura-
tion magnetostriction constant of 400 ppm for single crystals
(Galfenol(Fe81Ga19 )) and 275 ppm for directional solidified
polycrystalline alloys (FeGa), a low saturation field of about
100 Oe and a high saturation magnetization of 18 kG. How-
ever, the FeGa films have a large FMR linewidth of 450-
600 Oe at X band and strong loss at microwave frequencies
[45]. The shortcoming indicates that the FeGa alloys are far
from being an ideal magnetic material to be incorporated into
RF/microwave magnetoelectric devices. Boron (B), as a well-
known metalloid element, has been added into the FeGa and
FeCo films to improve their magnetic properties. It has been
experimentally demonstrated that the addition of B in FeCo
films contributes to an excellent soft magnetization and mi-
crowave properties by refining the grain size and diminishing
magnetocrystalline anisotropy [46,47]. Moreover, the addition
of Boron in the composition of FeCoSiB and FeGaB thin films
contributes to the demonstration of giant ME coefficients and
low loss tangents in ME devices RF/microwave devices.
FeGaB material exhibits excellent performance on mag-
netic softness, saturation magnetization, saturation magne-
tostriction, piezomagnetic coefficient, E effect, magnetome-
chanical coupling factor and has been regarded as the most
ideal piezomagnetic film material for ME devices [48]. As
indicated in [48], with the addition of B rising from 0% to
21% in FeGa, the coercivity Hcdecreased from about 100 Oe
to 1 Oe and the effective anisotropy field Hkdropped from
120 Oe to 15 Oe due to the material phase transition (MPB).
The reduced coercivity and anisotropy significantly enhances
magnetic softness. Additionally, as shown in Fig. 3(a), the B
content induced phase transition results in a lower threshold
magnetic field to generate a magnetostrictive response. The
maximum magnetostriction of 70 ppm achieved at 12% B
content was three times of that of the binary FeGa films.
The FMR linewidth of FeGaB films also dropped dramatically
from 700 Oe for binary FeGa films to 16 Oe at 21% B content.
The maximum piezomagnetic coefficient of the FeGaB films
has been reported as about 7 ppm Oe-1, which is much higher
than multiple magnetostrictive materials,including Terfenol-D
(Tb-Dy-Fe), Galfenol (Fe-Ga) and Metglas (FeBSiC). The
comparison was made in [21] and graphically shown in Fig. 4.
The maximum piezomagnetic coefficient can be further en-
hanced to 12 ppm Oe-1 after annealing due to the effective
release of residual stress inside the film and the reduction of
total anisotropy energy [49].
III. INTEGRATED MAGNETIC AND MAGN ETOELECTRIC
FOR POWER, RF, AND MICROWAVE APPLICATIONS
Integrated magnetic materials play a significant role in
enabling high performance or unique features in multiple
power, RF and microwave devices. The static and dynamic
properties of these materials are widely utilized to achieve
high efficiency magnetic flux guide, frequency selectivity
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HE ET AL.: INTEGRATED MAGNETICS AND MAGNETOELECTRICS FOR SENSING, POWER, RF, AND MICROWAVE ELECTRONICS
TAB L E 3. Integrated Magnetic and Magnetoelectric Devices for Power, RF, and Microwave Applications
FIGURE 4. Piezomagnetic coefficients (dλ/dH) of different types of
magnetostrictive alloys. (Reproduced from [21]).
by spontaneous resonance, magnetostatic wave guide, spin
controlling, etc. As previously mentioned, by introducing
strain-mediated magnetoelectric composites, the magnetic
properties of these material can be altered by applying electric
field, which provides possibilities for novel electrostatically
highly energy-efficient, compact, miniature and lightweight
tunable devices for power, RF and microwave electronics. In
this section, several integrated magnetic and magnetoelectric
devices including high performance transformer, inductor
with laminated magnetic core, voltage tunable inductor, E-
and H- field tunable bandpass filter and NEMS ME antennas
are presented. The selected applications are good representa-
tives for power components, basic circuit components, RF and
microwave signal processing block and RF and microwave
signal transmitting/ receiving block, respectively. A summary
of the applications presented in this section is given in Table 3.
A. ON-CHIIP TRANSFORMER, INDUCTOR WITH
INTEGRATED MAGNETIC CORE AND VOLTAGE TUNABLE
INDUCTOR WITH ME COMPOSITE
The inductor and transformer are two types of devices whose
operations rely on the magnetic flux variation and energy
conversion. As the rapid development of integrated RF, mi-
crowave and power circuits, the high Q, compact induc-
tor with sufficiently large inductance and transformers with
high power efficiency and coupling coefficient are highly de-
manded. The integrated magnetic materials, serve as the mag-
netic core, can boost the inductance and the transformer cou-
pling condition by concentrating the magnetic flux generated
during the operation. The mostly used magnetic materials for
core are always high-permeability materials including permal-
loy (NiFe) [55], NiFeW [56], NiFeZn [57], NiZn ferrite [58],
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FIGURE 5. (a) Optical image of the on-chip inductor with solenoid structure. (b) Inductance measurement result of inductors with magnetic core and air
core. (c) Quality factor measurement results of inductors with magnetic core and air core. (Reproduced from [3]).
FeCoSiB [59], FeCoCu [60], FeGaB [3], [61], etc. However,
there are two limitations brought by the integrated magnetic
core: 1). The relatively thick magnetic core in on-chip de-
vices induces large eddy current loss during high frequency
operation, which limits the quality factor; 2). The FMR of the
magnetic core causes energy absorption in the corresponding
frequency range, which diminishes the power efficiency and
working frequency range. To mitigate the high eddy current
loss issue, the integrated magnetic core was engineered as a
laminated structure [61]–[63] with thinner magnetic material
layers and isolation spacer in between. The thinner magnetic
material has higher equivalent resistance and hence reduce the
eddy current induced by the time-varying magnetic field. As
for the loss caused by FMR, the Kittel [64] equation sug-
gests that by determining proper magnetic film dimensions,
the demagnetizing factor can be manipulated such that the
FMR frequency is higher, which subsequently expand the op-
erational frequency range [63]. The most commonly adopted
inductor and transformer coil structures include solenoid and
spiral/planner structure, where the solenoid structure [55],
[58], [61], [63] incorporates the magnetic core between the
wires at upper and lower layers whereas in the spiral/ planer
[65]–[68] structure the magnetic core layer is deposited be-
neath or between the planer wires to form the magnetic
circuit.
In 2014, Gao [3] reported a series of integrated induc-
tors using multilayer FeGaB (100 nm)/Al2O3(5 nm)/FeGaB
(100 nm) sandwich structure as the magnetic core. The au-
thors adopted solenoid structure for the inductor design and
the optical image of the as-fabricated device is shown in
Fig. 5(a). As shown in Fig. 5(b), compared with the air core
inductor with same dimensions, the magnetic core boosted the
inductance by nearly 100% in the frequency range of up to 3
GHz. The inductance showed a flat characteristic with a peak
inductance of over 3 nH, which indicated that the laminated
magnetic core was of help to eliminate the eddy current loss
in high frequency range and the energy loss contributed by
FMR. The quality factor in the frequency range of up to 3
GHz is shown in Fig. 5(c), where the peak quality factor of
the inductor with magnetic core was over 10 and showed a
better performance compared to the air core inductor in the
relatively low frequency.
Mullenix [62] reported an on-chip micro-transformer with
integrated laminating magnetic core in 2013. The device turns
ratio is 1:1. As shown in the Fig. 6(a), the transformer has a
dual-coils solenoid structure. The authors fabricated a series
of devices with 8 turns coils, 16 turns coils, 32 turns coils,
and 32 turns coils with air core for comparison. The cross-
sectional view of the coil is shown in Fig. 6(b), a 2.8 µm thick
NiFe/AlN laminated film was adopted as magnetic core and
the optimized thickness of the insulating AlN layer was deter-
mined to be 7 nm. The inductances of a pair of inductors with
a 500 µm×5.6 µm core connected in series with difference
turn numbers are shown in Fig. 6(c). The 32 turns AC in the
plot indicates the measurement result of air core inductor. The
introducing of the magnetic core boosted the inductance by
60 times, namely 565 nH in the operating frequency range.
Fig. 6(d) shows the maximum Q factor achieved by 32 turns
magnetic core transformer was 6.3 and the maximum coupling
factor is 0.97, indicating a superior coupling between the
primary and secondary coil. The results show the advantage
of adopting laminating magnetic core since the pure magnetic
material core with same thickness has lower roll-off frequency
than the designed operation range.
Tunable RF inductors provide another possible solution for
tackling the trade-off between inductance, quality factor and
operating frequency range, since the external electric signal
can lead to the control and improvement of these 3 parameters
at the same time. There have been two main branches of con-
trol strategies for tunable inductors in microelectromechani-
cal systems (MEMS). One mechanism is to adjust the turns
or numbers of inductor coils. Based on this, Park changed
the turns or length of inductor coils by MEMS switch and
achieved an operation frequency of 2.4 GHz, a Q-factor of
3 and extremely large tunability of 187.5% [69]. The main
drawback of this MEMS switch control strategy is the discrete
tunability of inductance limited by the number of switches
[69, 70]. In order to overcome this shortcoming, the inductor
coupling control method with a secondary coil was put for-
ward and applied to the fabrication of related devices, with
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FIGURE 6. (a) Optical image of the on-chip transformer. (b) Cross-sectional view of the coil. Measured (c) inductance, (d) quality factor and coupling
coefficient for the devices with 500 μm X 5.6 μm core with 8, 16, and 32 turns. The “32 Turns AC” refers to the air core transformer of identical geometry
with 32 turns. (Reproduced from [62]).
an operating frequency ranging from 4.35 GHz to 7 GHz,
a quality factor of ∼10 and an inductance tunability from
∼30% to ∼90% [71], [72]. However, the related work was
greatly limited by the complicated structure and fabrication
procedure. Another perspective is to change the properties
of the magnetic core or the magnetic field distribution in
it. Following this direction, researchers tried to control the
magnetic flux by thermal actuation [73] or metal shield [74].
DC current was also widely used to modify the properties of
magnetic core. For example, Salvia realizes a 40% increase
in inductance, an inductance tunability of 15% and a Q-factor
from 5 to 11 in the tunable on-chip inductors operating up to
a frequency of 5 GHz using patterned permalloy laminations
[67]. Unfortunately, both methods exhibit limited inductance
tunability, high power consumption, significant thermal effect
and excessive noise [67], [75]–[77].
With the development of magnetoelectric materials, re-
searchers tended to use ME coupling to adjust the perme-
ability of magnetic core and tune the inductance, aimed at
improving the energy efficiency, reducing joule heating and
increasing inductance tunability. The CME tuning mechanism
is elaborated in Section II. In order to ensure higher induc-
tance density and efficient utilization of magnetic film, the
most popular spiral and solenoid structures have been widely
used in electrostatically tunable inductor. A solenoid type
magnetoelectric inductor fabricated with a layered multifer-
roic composite core was proposed by Lou, which consisted of
two layers of Metglas magnetic ribbons and one PZT piezo-
electric slab [78]. The inductance showed a strong electric
field tunability, with maximum inductance change of 450%,
250%, and 50% for operation frequencies of 1 kHz, 100 kHz,
and 5 MHz, respectively. The estimated energy consumption
to achieve maximum tuning 450% is only about 1.3 mJ, in-
dicating that the fabricated tunable magnetoelectric inductors
were essentially passive and energy efficient and achieved a
giant tunability. Due to the decreased permeability, increased
skin depth and reduced core eddy current loss, the quality fac-
tor is improved from about 3 at zero electric field to about 8.5
at an external field of 12 kV/cm. The giant tunability achieved
in such magnetoelectric inductors with low power consump-
tion makes them ideal candidates for novel power efficient and
compact electronic devices. In order to extend the operation
frequency range and enhance the quality factor by reduc-
ing the eddy current loss, Lin fabricated PZT/Metglas/PZT
voltage tunable inductors with single layer Metglas film and
multiple piezoelectric slabs [79]. An 80% increase in Q-factor,
10 times of extension in operational frequency and 3 times of
enhanced quality factor were observed. In addition, a 150%
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FIGURE 7. Integrated gigahertz FeGaB/Al2O3/PMN-PT voltage tunable ME inductor. (a) Optical image and structure model of integrated voltage tunable
inductor with 3.5 turn and a magnetic film of 340 μm X 800 μm. (b) Measured inductance and (c) Q-factor of integrated FeGaB/PMN-PT tunable inductor
with applied electric field from 0 to 10 kV/cm. (Reproduced from [13]).
improvement in quality factor and 100 times of extension in
operational frequency could be found in ME inductors with
a single PZT slab, whereas a 235% improvement in quality
factor and 200 times of extension in operational frequency
were realized in its double PZT slabs counterpart.
Though the PZT/Metglas/PZT ME inductors show a high
performance in quality factor, inductance density and a gi-
ant tunability of 450% in the kilohertz frequency band, the
Metglas/PZT multiferroic heterostructures are not applicable
for the gigahertz applications due to their low FMR fre-
quency. In addition, these inductors fail to realize compati-
ble integration on CMOS circuits, which prevents their ap-
plications in RF/microwave electronics. To realize gigahertz
range frequency operation, Chen has recently demonstrated a
voltage tunable GHz integrated magnetic inductor by using
the FeGaB/Al2O3/PMN-PT magnetoelectric composites, as
shown in Fig. 7(a) [13]. Compared with materials like NiZn,
the high permeability further improved by annealing, FMR
frequency of 1.85 GHz with a narrow FMR bandwidth of
16∼20 Oe, large magnetostriction of FeGaB films lead to high
inductance density, operation band in GHz range, relatively
low magnetic loss and high frequency tunability, respectively.
The inductor showed a relatively constant inductance and
quality factor value in the frequency range of 0.5 to 2 GHz.
The alumina spacer reduced the thickness of single-layer mag-
netic material, thereby increasing the sheet resistance and re-
ducing the eddy current loss. The E-field tunability test results
shown in fig. 7 (b) and (c) indicate that in this frequency range,
with an applied electric field up to 10 kV/cm, the inductance
rises from 1.2 to 3.5 nH and the quality factor rises from
2.7 to 5.2. The device achieved a maximum inductance contin-
uous tunability of 191% at 1.5 GHz, which was significantly
larger than other integrated inductors tuned by magnetic flux,
DC bias current and coil coupling.
B. INTEGRATED RF AND MICROWAVE FILTERS AND
VOLTAGE TUNABLE FILTERS WITH ME COMPOSITES
Filter is an essential component in RF/microwave signal pro-
cessing circuits and blocks. There are several types of filters
where the integrated magnetic and magnetoelectric materials
function as a core part. In a typical filter based on lumped
element [80,81], the magnetic materials serve as the core of
inductors in the filter topology and the function is similar as
forementioned inductor components. FMR absorption and
magnetostatic wave manipulation properties of magnetic
materials are also widely utilized for achieving band-stop and
band-pass characteristics. Band-stop filters utilizing FMR
absorption typically consist of a transmission line that couples
with the magnetic materials. When the frequency of electro-
magnetic wave propagating in the transmission line coincides
with FMR frequency of the material, a maximum attenuation
and hence a stop band is generated. Multiple materials includ-
ing transition metal [82], ferrite [83]–[85] and soft magnetic
composite [4] are adopted for these band-stop filters. Mag-
netostatic wave manipulation property in ferrite is utilized to
achieve non-reciprocal band-pass characteristic [14], which
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HE ET AL.: INTEGRATED MAGNETICS AND MAGNETOELECTRICS FOR SENSING, POWER, RF, AND MICROWAVE ELECTRONICS
FIGURE 8. Integrated NiZn/PMN-PT dual E- and H-field tunable ME band-pass filter. (a) Schematics of integrated tunable inductor. (b) S12 and S21 under
abiasmagneticfieldof400Oe.(c)S
11 under different bias electric fields. (d) S21 under different bias magnetic fields. (Reproduced from [14]).
will be elaborated later in this part. The third type of integrated
filter utilizes the mechanical resonance to realize frequency
selectivity [86], where the input excitation signal is trans-
formed into acoustic wave at the input resonator and propagate
to the output resonator thorough the released material layer.
Reconfigurability is a crucial feature in filter design consid-
ering the need for wide band operation, the size of the signal
processing block and the yielding cost. MEMS switches [87]–
[89] are adopted to realize the reconfigurability. However,
this mechanism suffers from the tuning discontinuity which
limits its flexibility. The integrated filters that incorporate the
magnetic materials mentioned above are usually magnetically
tunable. In [4], He presented an integrated tunable band-stop
filter that achieved a tunability of 70% under a low magnetic
field of 400 Oe working in C-band. However, for some higher
frequency filter applications, the relatively lossy ferrite has to
be adopted and a large magnetic field is required for tunabil-
ity [83]. Meanwhile, the instrument for applying a magnetic
field is usually bulky and power consuming, which makes the
magnetically tunable filters an inferior choice. Integrated ME
materials offer a possibility to achieve on-chip electric field
tunable filter devices. The advantages of ME tuning mech-
anism are continuous tuning, low power consumption and
easily-defined tuning effect range (determined by the size of
ME materials). Current efforts are still committed to mitigate
the magnetic material performance degradation induced by
the deposition or bonding of the piezoelectric phase in ME
materials.
In 2015, the first integrated dual E- and H- field tunable
band-pass filter was demonstrated in an inverted-L-shape mi-
crostrip structure with a multiferroic heterostructure formed
by a spin-sprayed NiZn ferrite film and a PMN-PT piezoelec-
tric slab, as shown in Fig. 8(a) [14]. The signal from the input
port is transmitted as magnetostatic surface wave (MSSW).
The forward and backward wave transmissions happen on
bottom and top surface of the ferrite film. The passband fre-
quency of the filter is determined by the dispersion relation of
the MSSW. Due to the discrepancy of radiation resistance on
the top and bottom surfaces of the ferrite film, a non-reciprocal
transmission characteristic can be observed [90], [91]. By ide-
ally rotating the NiZn ferrite film by 45 degrees, the standing
wave resonance and the multi-passband are diminished since
the reflected wave at the edge of ferrite film is converted to
magnetostatic back volume wave (MSBVW) which will decay
fast at the filter operating frequency. The detail of the filter
operation principle is presented in [92]. As shown in Fig. 8(b),
the 15.8 dB difference of S12 and S21 in band shows the
non-reciprocal transmission. The central frequency could be
electrically tuned from 2.075 GHz to 2.295 GHz when the
out-of-plane electric field rises from 0 kV/cm to 4 kV/cm,
as shown in Fig. 8(c), namely an E-field frequency tunability
of 55 MHz/(kV/cm). In addition, a tunable central frequency
from 3.78 GHz to 5.27 GHz could be realized by increasing
the DC magnetic field from 100 Oe to 400 Oe, corresponding
to an H-field frequency tunability of about 5 MHz/Oe, as
shown in Fig. 8(d).
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FIGURE 9. Integrated FeGaB/Al2O3/AlN dual E- and H-field tunable ME band-pass filter. (a) Schematics of integrated tunable filter. (b) S11 and S21 of the
tunable filter at zero bias field. (c) Measured resonant frequency as a function of DC magnetic field. (d) Measured resonant frequency as a function of DC
voltage across the thickness direction of the AlN film. (Reproduced from [86]).
In 2016, an integrated RF tunable bandpass filter based on
two coupled elliptic-shape nano-mechanical resonators with
FeGaB/Al2O3/AlN multiferroic heterostructure on a bottom
Pt electrode were reported [86] and schematically shown in
Fig. 9(a). The filter relies on the acoustic wave propagation
to achieve signal transmission between the two ports, the
two resonators work in the contour mode whose resonance
frequency is determined by the width of the coupled structure.
The filter exhibited a return loss of -11.2 dB, an insertion loss
of 3.4 dB and a quality factor of 252 at the central frequency
of 93.165 MHz, as shown in Fig. 9(b). The tunability test
showed a H-field frequency tunability of 5 kHz/Oe and an
E-field frequency tunability of 2.3 kHz/V, presented in fig.
9(c), (d), respectively. The tunability is realized based on the
E effect, where the Young’s modulus of FeGaB changes
with applied fields.
C. STATE- OF-ART INTEGRATED ME ANTENNAS
Conventional antennas rely on the direct excitation of current
or voltage to control the motion of electrons inside resonators
for radiation and typically have sizes ranging from one-tenth
to one-half of the electromagnetic (EM) wavelength λ0[93],
[94]. Multiple miniaturization techniques have been applied
to design miniature antennas with different structures, such as
dipoles, monopoles, slot, Z-type, metamaterial loaded anten-
nas [95]. Electrically small antenna performance is restricted
by the Chu–Harrington limit [96]–[99]. The inherent high
quality factor (Q-factor) of these antennas limits both the
radiation efficiency and operating bandwidth.
The concept of ME antennas was first put forward in [100,
101]. Unlike the conventional antennas that use oscillating
charges to induce EM waves, the ME antennas use magnetic
dipole moment oscillations that are acoustically actuated at
their electromechanical resonance (EMR) rather than the EM
wave resonance. Therefore, though there is no proof that the
Chu–Harrington limit has been overcome, the antenna di-
mensions of the mechanical antennas can be decoupled with
the wavelength of EM waves. Since the velocity of acoustic
waves is orders of magnitude slower than EM waves at the
same operation frequency, the application of ME antennas
will lead to 1–2 orders of miniaturization in antenna dimen-
sion. In addition, ME mechanical antennas based on novel
ME composites could maintain a high radiation efficiency,
wide bandwidth, low power loss and even high transmission
rates and adjustable multiband under appropriate design and
nonlinear modulation strategy, which may become an excel-
lent solution to very low frequency (VLF) communication
[102], broadband wearable and implantable biomedical de-
vices [103]–[106] and multiband portable devices [107].
The ME antennas consisting of one layer of piezoelectric
material and one layer of magnetostrictive material rely on the
bulk acoustic wave (BAW) resonator to transfer the dynamic
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FIGUR E 10. (a) Illustration and explanation of the new antenna
mechanism. (b) Illustration and explanation of the ground plane effect.
(Reproduced from [108]).
strain across different layers. The illustration of the novel ME
antenna mechanism is shown in Fig. 10(a) [108]. On the trans-
mitting side, by applying RF electric field, the mechanical
resonance would induce an alternating strain wave/acoustic
wave that can be directly transferred to the upper ferromag-
netic thin film. The acoustic wave would then induce a dy-
namic change of the magnetization due to the strong piezo-
magnetic effect and generate a magnetic current for radia-
tion. Reciprocally, on the receiving side, the RF magnetic
field component of the electromagnetic wave can induce an
acoustic wave on the ferromagnetic layer. The acoustic wave
is then transferred to the piezoelectric layer and converted
to the electric voltage output. Different from the cancelling
effect of imaging currents in the ground plane of radiation in
conventional EM antennas, the usage of magnetic currents for
radiation in ME antennas would provide a 3 dB gain enhance-
ment due to in-phase image currents attaching on the ground,
as shown in Fig. 10(b). This ground plane immunity property
can provide a variety of applications on the metallic surface
and the human body which is also considered as a ground
plane.
The first batch of ME antennas were fabricated by Nan in
2017, based on laterally-vibrating nano plate resonator (NPR)
and vertically-vibrating film bulk acoustic resonator (FBAR)
[15]. The ME Antenna with NPR consisted of a rectangu-
lar resonating plate with a single-finger bottom Pt electrode
and a thin film FeGaB/AlN active resonant heterostructure,
as shown in Fig. 11(a). As shown in Fig. 11(c), a high ME
coupling coefficient of αME =6kVOe
−1cm−1can be ob-
tained at EMR frequency fr,NPR of 60.68 MHz without DC
bias magnetic field, indicating a strong ME coupling. The
strong ME coupling leads to a high electromechanical trans-
duction efficiency, low loss and effective interaction between
acoustic resonance and RF magnetic field component of EM
waves, which contributes to a high-quality factor Q of 930
and an electromechanical coupling coefficient (kt2) of 1.35%,
as shown in Fig. 11(b). The ME NPR antenna achieves 1–2
orders of magnitude miniaturization over state-of-the-art com-
pact antennas without performance degradation.
The ME FBAR antenna was composed of a suspended Fe-
GaB/AlN ME circular disk, as shown in Fig. 11(d). Different
from ME NPR antennas, the ME FBAR antenna operated
at GHz in thickness extensional vibration mode, as shown
in Fig. 11(e). The ME FBAR antennas exhibited a peak re-
turn loss of 10.26 dB, a Q-factor of 632 and a mechanical
resonance frequency of 2.53 GHz. The operating frequency
(same as the mechanical resonance frequency) f0,FBAR can be
changed by adjusting the thickness Tof the ME heterostruc-
ture due to the relation f0,FBAR ∝1
TEeq
ρeq , where Trepresents
the thickness of the FBAR resonator; Eeq is the equivalent
Young’s modulus; ρeq is the equivalent density of the ME
composite. The FBAR antenna achieved an antenna gain of
-18 dBi at the resonance frequency of 2.53 GHz based on the
measured radiated signal (S12) and received signal (S21), as
shown in Fig. 11(f). Compared with the state-of-art conven-
tional compact antennas operating at the same frequency, the
dimension of the ME antenna is 1 or 2 orders smaller due to
the guided acoustic wave with much lower velocity compared
to the EM wave.
The designed and fabricated NPR and FBAR ME antennas
could realize different modes of vibration and radiation at
both very high frequency (VHF, 60 MHz) and ultra-high fre-
quency (UHF, 2.525 GHz) operation frequency bands. More-
over, the similar microfabrication process of both NPR and
FBAR based antennas on the same silicon wafer allows the
integration of broadband ME antenna arrays working in the
frequency range from tens of MHz (NPR with large lateral
dimensions) to tens of GHz (FBAR with thinner AlN thick-
ness) on one chip. These ultra-compact ME antennas serve
as the potential candidates for future communication systems,
internet of things, wearable antennas, bio-implantable and
bio-injectable antennas, smart phones and wireless commu-
nication systems.
IV. I NTEGRATED MAGNETIC AND MAGNETOELECTRIC
FOR SENSING APPLICATIONS
During the past decades, the ultra-sensitive magnetic field
sensors were highly demanded and utilized in various field
including biomedical applications, geographic detection,
electric malfunction diagnosis and information technologies,
etc. For practical usage, more figures of merit should be
considered, including working bandwidth, temperature
stability, linearity, power consumption and cost [109].
Currently the best limit of detection (LOD) of single digit
fT/Hz1/2 at 1 Hz was achieved by superconducting quantum
interference device (SQUID) magnetometer [110]. However,
the SQUID requires near-zero working temperature and hence
bulky and expensive. Integrated magnetic and magnetoelectric
sensors, which take the advantage of its silicon/CMOS
compatibility and size suitable for large sensor arrays, have
drawn much interest in recent years and notable progresses
have been made. The magnetic or magnetoelectric materials
integrated in these sensor applications typically have electric
or magnetic properties that are sensitive to the external weak
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FIGUR E 11. Magnetoelectric (ME) nanoplate resonator (NPR) and film bulk acoustic resonator (FBAR) antennas with gigantic ME coupling. ME NPR
antenna: (a) Scanning electron microscopy (SEM) images of the fabricated ME NPR antenna. (b) Admittance curve and Butterworth–van Dyke model
fitting of the ME NPR. The inset shows the schematic of the cross-section of the ME heterostructure. (c) ME coupling coefficient (left axis) and the
induced ME voltage (right axis) versus the frequency of RF magnetic field excitation HRF. FBAR antenna: (d) SEM photo. (e) return loss S22 and (f)
radiating characteristic (S12) and receiving characteristic (S21 ) at resonance of the FBAR device. (Reproduced from [15]).
TAB L E 4. Integrated Magnetic and Magnetoelectric Devices for Magnetic Field Sensing Applications
magnetic field, which directly induces a measurable electric
output in thin film-based devices, or changes the mechanical
properties of the sensor structure in MEMS devices and
subsequently the electric output. The prevalent integrated
magnetic sensors include ME sensor [11], [111]–[113],
magnetoresistance (MR) sensor [114], [115], permanent
magnet-based sensor [116]–[120], and fluxgate sensor [121].
In this section, the principle and typical sensor structure
of first three types of sensors will be elaborated where the
integrated magnetic or magnetoelectric material serves as
the sensing element. In the fluxgate sensors, the magnetic
material serves as core which shares the similar function as
integrated inductors and transformers. A cross comparison of
the sensors discussed in this work is given in Table 4.
A. INTEGRATED ME SENSOR
The two-phase ME composite that consists of a piezoelec-
tric/electrostrictive phase and a magnetostrictive phase is ca-
pable to directly convert the magnetic moment to the electric
polarization and subsequently the output voltage through the
mechanical coupling between the two phases, which enables
the possibility of its sensing application. The sensing perfor-
mance of the composite is majorly determined by 1). The ma-
terial properties of both phases including the piezoelectric and
magnetostrictive constant, stiffness, dielectric constant and
permeability. 2). The mechanical structure issues including
the volume and thickness ratio of two phases, the mechanical
coupling condition. 3). The operating mode which determines
the mechanical deformation and polarization directions [122].
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Earlier works [123], [124] on ME sensors focused on form-
ing the bulk ME laminate structure by epoxy bonding to
detect the external weak magnetic field thorough direct ME
coupling, where a strain induced by the magnetic field in mag-
netostrictive phase is transferred to piezoelectric phase and
reflected at the electric output. However, the major drawback
of this scheme is the narrow detectable bandwidth considering
the mechanical resonance characteristic of the sensor and the
high flicker noise in the low frequency range. A DC bias field
is also needed for optimizing the ME coupling coefficient
in the measurement frequency range. Moreover, the epoxy
glued ME sensors suffer from the temperature stability and
device fatigue issues. Some efforts have been put into optimiz-
ing the sensor configuration and structure to achieve higher
sensitivity by optimizing sensor geometry [125], improv-
ing the lamination process [126], performing a differential
sensor configuration [127] and improving the output signal
processing [128].
To tackle the above-mentioned bonding issues, thin-film
technologies are adopted to deposit the two phases in the ME
composite, which enables a more desirable bonding condition
and hence improves the ME coupling coefficient. The mostly
used soft magnetic materials for magnetostrictive phase in-
clude FeGa [129], FeCoSiB [130] and FeGaB [131], while
the piezoelectric phase is typically AlN [132], PZT [129]
and PVDF [133]. The frequency conversion technique was
first proposed by Jahns [134] and was widely adopted [135],
[136] to expand the operating frequency range of the ME
sensor, particularly in the low frequency regime. The tech-
nique utilizes the non-linear magnetostrictive response and
converts the low frequency magnetic field component to a
frequency-mixing component close to the resonance by ap-
plying a modulation magnetic field at resonance frequency.
The frequency mixing component intensity was utilized to
determine the weak magnetic field under detect.
Integrated ME sensors has its unique advantage in form-
ing the well-arranged sensor array, compatibility of CMOS
process and power efficiency. Currently, the principle of in-
tegrated ME sensor includes E effect and the direct ME
coupling. Like the bulk ME sensors, the integrated ME sensor
that relies on the direct ME coupling can only detect the AC
magnetic field with frequency close to its resonance. An ex-
ternal DC bias magnetic field is also required to optimize the
ME coupling and LOD. The E integrated ME sensor, on the
other hand, utilizes the E effect in magnetostrictive phase,
where the Young’s modulus changes with the magnetic field
under detect. The mechanical characteristic of the sensor is
then altered and can be detected by optical detection of the de-
flection [111], close-loop measurement of the resonance fre-
quency [112], open-loop measurement for sensor admittance
[131] and direct output voltage readout through a directional
coupler [113]. The E ME sensor provides the possibility
for wideband operation and low frequency/DC magnetic field
detection. The two types of integrated sensors are exemplified
below in detail to illustrate the device structures and working
principles.
In [11], Su proposed to use AlScN/FeCoSiB laminate ME
structure for sensing application based on direct ME coupling.
The simplified sensor structure is shown in Fig. 12(a), where a
1µm thick AlScN layer and a 2 µm thick FeCoSiB layer were
deposited on a poly-silicon cantilever. The Mo top electrode
and the Ti/Pt bottom electrode are not shown in the schematic.
The polysilicon cantilever structure was obtained through a
TMAH etching process. The as-fabricated sensor was pack-
aged and mounted on the test board shown in Fig. 12(b) with
a wafer-level transient-liquid-phase process. A series of AlN
based sensors were also fabricated for performance compar-
ison. Fig. 12(c) demonstrates the ME coefficient measure-
ment results of sensors with different piezoelectric phase. The
AlScN material provides a factor of 2 improvement on the ME
coefficient compared to its AlN counterpart, reached a value
of 1580 V/cm·Oe at 8185 Hz. The electric noise spectrums
of both sensors are given in Fig. 12(d) with the reference
audio noise. The peak position that appears in the spectrum
coincides with that of ME coefficient. The LOD of each sensor
was calculated by finding the ratio of the ME coefficient to the
electric noise density at resonance. The best LOD achieved
was 55 pT/Hz1/2 . It was also shown that despite the increase
in ME coefficient for AlScN based sensor, the LOD did not get
a similar boost at resonance. This was caused by the scaling
up thermomechanical noise with larger sensor deflection in
AlScN based sensor. It was also reported the working fre-
quency of such sensors is tunable by an external applied DC
electric field to the piezoelectric phase.
In 2013, Nan [131] reported a 215 MHz NEMS resonator to
detect DC magnetic field based on E effect. The structure of
the resonator is shown in Fig. 13(a). The resonator adopted an
AlN/(FeGaB/Al2O3)×10 ME heterostructure with interdigital
Pt electrode at bottom. The FeGaB soft magnetic multilayer
has lower eddy current loss compared with one single layer
and the interdigital electrode with designed pitch width drives
the resonator in extension vibration mode. With fixing pitch
width and equivalent density, the resonance frequency of the
sensor is purely determined by the effective Young’s modules,
which is affected by the DC magnetic field under detect due
to the E effect. Fig. 13(b) shows the admittance spectrum
under different applied DC magnetic field, presenting the
effect of changing Young’s modules electrically. The peak
admittance and quality factor Q under different applied DC
magnetic field is plotted in Fig. 13(c). The quality factor has
a minimum at 15 Oe, which is caused by the variation of
the magnetic domain wall state and the corresponding loss.
With large magnetic field applied, the magnetic domain wall is
eliminated, reducing the magnetic loss, and hence increasing
the quality factor. To test the sensing capability, the resonator
was biased under 5 Oe DC magnetic field where the reso-
nance frequency was at the most sensitive point to the applied
field. The resonator was then excited at a single frequency
for admittance measurement under a superimposed tiny DC
field as low as 50 pT. The measurement result is shown in
Fig. 13(d), where a 300 pT LOD was determined. The 10-
layer FeGaB/Al2O3magnetostrictive material also showed a
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FIGUR E 12. (a) Schematic of the MEMS AlScN resonant magnetoelectric sensor. (b) photograph of the packaged ME chip on a test board. (c) Measured
ME coefficient αME of AlScN and AlN based ME sensors at their mechanical resonance. (d) Voltage noise density spectra for AlScN (blue) and AlN
(orange) based ME sensors as well as a reference channel (black). (Reproduced from [11]).
self-bias performance which offered a LOD of 600 pT with no
DC magnetic field bias.
B. INTEGRATED PERMANENT MAGNET-BASED SENSOR
The permanent magnets represent a group of ferromagnetic
or ferrimagnetic materials that exhibit strong magnetization
after the removal of external field. The most commonly used
permanent hard magnet materials include ferrites, transition
metal alloys and rare-earth alloys [137]. Unlike the bulk per-
manent magnets fabrication, the silicon compatible integra-
tion of permanent magnets suffers the following difficulties
and challenges: 1). The harsh fabrication conditions including
the high temperature and pressure are not suitable for on-
chip process; 2). The traditional deposition methods includ-
ing electrodeposition [138], magnetron sputtering [139] and
pulsed laser deposition (PLD) [140] typically generate thin
film material with the thickness in the range of nanometer
to micrometer, which fairly limits the sample performance;
3). The wafer-level patterning of the as-deposited permanent
magnetic material may be hard to conduct or require specific
etching process that would damage the pre-exist structures.
Numerous efforts have been put into developing new powder-
based on-chip permanent magnet fabrication methodologies
to overcome these challenges including spin casting/screen
printing [141], dry-packing [142] and 3-dimensional (3D)-
printing [143]. The Ongoing research on powder-based per-
manent magnet integration is focusing on improving the pack-
ing density and the micro-structure of the magnetic powder
which is essential to the magnet performance.
With the advance of fabrication technique, the integrated
permanent magnet is utilized and serves as a sensing part
that responds to the external magnetic field under detect. The
mechanical force or torque generated is transferred to piezo-
electric phase and converted to electric outputs for readout
[116]–[118]. Compared to optical detection [119], [120], the
piezoelectric readout scheme does not need the optical gener-
ation, coupling and collimation instrument, which is favorable
for outdoor detection in harsh environment. In 2019, Niekiel
[118] reported a fully integrated permanent magnet-based sen-
sor that adopted the cantilever structure. The schematic of sen-
sor working principle and the structure are shown in Fig. 14(a)
and (b) respectively. When the integrated NdFeB magnet was
magnetized along the vertical direction and with the magnetic
field under detect applied along the horizontal direction, a
torque can be generated and drive the cantilever whose de-
flection was monitored by the AlN piezoelectric layer. Two
types of devices with different cantilever length are shown in
Fig. 14(c), where the electrodes were wire-bonded to a PCB
for the measurement. The sensitivity frequency spectrums are
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HE ET AL.: INTEGRATED MAGNETICS AND MAGNETOELECTRICS FOR SENSING, POWER, RF, AND MICROWAVE ELECTRONICS
FIGUR E 13. (a) Schematic of NEMS ME resonator. (b) Admittance curves of the NEMS sensor at various bias DC magnetic fields. (c) Resonance frequency
and admittance amplitude at the resonance frequency as a function of DC magnetic field. (d) Sensitivity measurement results under 5 Oe DC magnetic
field bias. (Reproduced from [131]).
plotted in Fig. 14(d). The peak sensitivity of sensor 1 and 2 are
34.6 kV/T and 37.1 kV/T, respectively. The sensors showed
a high quality factor oscillator characteristic which boosted
the sensitivity in band. The vertically magnetizing permanent
magnet configuration also offers superior sensitivity perfor-
mance compared to its horizontal counterparts which rely on
the magnetic field gradient for sensing. To determine the LOD
of the sensors, the electric noise density was measured and
divided by the sensitivity, it is shown in fig. 14(e) that the LOD
at resonance is 7.2 pT/Hz1/2 for sensor 1 and 7.3 pT/Hz1/2 for
sensor 2.
C. INTEGRATED MAGNETORESISTVE MATERIALS AND
SENSORS
Magnetoresistance effect refers to the effect that the resistance
of the thin film material varies under the external magnetic
field. The 3 major categories of MR including anisotropic
magnetoresistance (AMR), giant magnetoresistance (GMR)
and tunnel magnetoresistance (TMR). The AMR is attributed
to the anisotropic differences in the scattering of s electrons
off d electrons resulting from spin-orbit interaction [144] and
always happens in the transition metal and transition metal
alloys [145]. The GMR and TMR effect rely on the spin-
dependent electronic transport. All the MR effects have been
utilized for sensing applications and commercialized prod-
ucts are available. Typically, the MR sensors are used in
magnetic read head in magnetic hard disk drive, automotive
and consumer electronics including current leakage detection,
positioning, and biotechnology, etc [146]. The noise energy
spectrums of commercially available MR sensor products
are shown in Fig. 15 [147]. Note the term ’detectivity’ as
the Y-axis name in the plot refers to the noise floor level
. The sensor types corresponding to the product part numbers
are marked in the caption of the plot. It is shown among all the
MR sensors, AMR sensor has lowest noise performance in the
low frequency range. However, due to the physical nature of
these MR effect, the AMR have the lowest magnetoresistance
change, namely around 2%−4% [144], compared to ∼10% in
GMR device under low magnetic field, and as high as 220%
[148], [149] to 600% [150] in TMR structures.
A typical GMR device consists of two ferromagnetic layers
with a non-magnetic conductive layer in between, whereas
for the TMR case, the non-magnetic layer in between is a
semiconductor or an isolator layer and the electron travels
through the layered structure by tunneling effect. The two
ferromagnetic layers are deposited to a designed thickness
such that a spontaneous anti-parallelly aligned magnetization
is achieved in these two layers. The external applied field
forces the magnetization in both ferromagnetic layers, which
subsequently decreases the overall resistance of the structure
due to the spin-dependent electron transport. The theory of the
spin-induced electric difference is elaborated by two current
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FIGUR E 14. (a) Working principle of the integrated permanent magnet-based sensor. (b) Cross-sectional view of the sensor structure. (c) Images of the
sensors mounted on PCB. (d) Frequency dependency of sensor sensitivity. (e) Frequency dependency of limit of detection. (Reproduced from [118]).
FIGUR E 15. The noise energy density of commercially available MR
sensors, in which NVE AAL002 is GMR sensor; NVE SDT is TMR sensor;
Honeywell HMC1001 is AMR sensor. (Reproduced from [141]).
models proposed by Mott [151]. Since the first discovery of
GMR effect by Fert in 1988 using Fe/Cr superlattice [152],
numerous efforts have been put into improving the magnetore-
sistance changing ratio of GMR/TMR thin film layered struc-
ture. In 1991, Dieny [153] proposed a structure named spin
valve where the magnetization of one of ferromagnetic layers
mentioned above was pined by an anti-ferromagnetic layer
(typically Mn alloy) adjacent to it through the spin interaction,
which overcame the issue that the previous structures were
only sensitive to high magnetic field. To further reduce the
field required to reverse the magnetization direction in ferro-
magnetic layers, a synthetic anti-ferromagnetic layer structure
that consists of two ferromagnetic layer (typically FeCo alloy)
and a coupling Ru layer was proposed by Parkin [154]. It was
found then by Parkin [155] introducing a Co-rich alloy layer
between the ferromagnetic layer and spacer of simple GMR
structure doubled the MR changing amount. The TMR effect
under room temperature was first demonstrated by Moodera
[156] in FeCo/Al2O3/Co structure where Al2O3acted as in-
sulting barrier for tunneling effect. The MgO isolation layer
was also widely used since then.
Although the GMR/TMR effect offers great potential in
sensing application thanks to their large MR change under
external magnetic field, it has some major drawbacks in-
cluding the non-linear response and hysteresis characteristic.
Typically, a DC magnetic bias generated by the Helmholtz
coils or permanent magnet can mitigate the issue [157]. Dif-
ferential [114] and bridge [115] sensor configurations have
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HE ET AL.: INTEGRATED MAGNETICS AND MAGNETOELECTRICS FOR SENSING, POWER, RF, AND MICROWAVE ELECTRONICS
FIGUR E 16. (a,b) Microscopy image of a serial TMR sensor. (c) Stacking structure of TMR sensor. (d) Outputs for one serial TMR sensor and (e) four serial
TMR sensors connected in a full Wheatstone bridge circuit at room temperature. (f) Noise spectrum of one serial TMR sensor and four serial TMR sensors
connected in a full Wheatstone bridge circuit. (Reproduced from [115]).
also been adopted to improve the sensitivity and diminish the
noise.
In [115], Jin fabricated and utilized the on-chip TMR sen-
sors for magnetic field leakage detection. The optical images
of the as-fabricated devices connected in series are shown
in Fig. 16(a) and (b), and the layered sensor structure with
constituent and thicknesses in nanometer of each layer is
schematically shown in Fig. 16(c). The TMR sensor adopted
synthetic ferromagnetic layers and the magnetization of the
upper one was pinned by IrMn material. The MgO layer was
adopted as tunneling barrier. The magnetic field sensitivity
was tested for one serial TMR sensor and four serial TMR
sensors connected in Wheatstone bridge configuration are
shown in Fig. 16(d) and (e), respectively. The sensitivity of
one sensor was 0.49 V/Oe and that of the 4-element bridge
is 0.37 V/Oe at zero field. Although some compromises were
made on the sensitivity, the 4-element bridge showed better
linear range by 6 Oe and nearly one order lower intensity in
noise energy spectrum which is shown in Fig. 16(f).
V. CONCLUSION
During the past several decades, integrated magnetic and mag-
netoelectric materials are widely utilized and enabling high-
efficiency, compact and tunable on-chip devices, which shows
a bright future for power, RF, microwave and sensing applica-
tions. Ongoing research is focusing on controlling the mag-
netic damping to either achieving high efficiency in magneto-
static wave based high frequency devices, or to control the fer-
romagnetic behavior and consequently domain wall dynamics
and spin transfer torque switching [158]. The integration of
highly dense hard magnet has long been another difficulty for
the community which is in the needs of more versatile fab-
rication methodologies with accurate positioning and defin-
able pattern on silicon. As for the integrated magnetoelectric,
surely the materials with high piezoelectric and magnetostric-
tive coefficients are highly demanded for stronger ME cou-
pling. The stress elimination in the two-phase ME composite
is also desirable to optimize the device performance, which
requires deposition condition, releasing process control and
even the post-fabrication annealing. Some researchers also
commit to get better understanding on physical basis of strain-
mediated magnetization and the radiation mechanism of ME
antennas. For practical applications, biomedical stimulus and
sensing incorporating the NEMS ME sensors and antennas
has become a research hotspot. Considering the miniaturiza-
tion brought by the novel ME devices on transmitting and
sensing devices, they are good candidates for invasive probing
and non-invasive wearable products in healthcare domain.
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YIFAN HE received the B.E. degree in electri-
cal engineering from Tianjin University, Tianjin,
China, in 2014, and the M.S. degree in electri-
cal and computer engineering from Northeastern
University, Boston, MA, USA, in 2016, where
he is currently working toward the Ph.D. degree
in electrical engineering. His research interests
include magnetic material and its application in
RF/microwave devices, energy harvesting devices.
BIN LUO received the B.E. degree in electri-
cal engineering from the Huazhong University of
Science and Technology, Wuhan, China, in 2020.
He is currently working toward the Ph.D. degree
in electrical engineering with Northeastern Uni-
versity, Boston, MA, USA. His research inter-
ests include gas discharge plasma and lightning
physics, electrical breakdown and lightning pro-
tection, computational chemical kinetics, fluid dy-
namics and electromagnetics, novel magnetic, fer-
roelectric, and multiferroic materials and their ap-
plications in RF/microwave electronics and spintronics, sensors and quantum
devices.
NIAN-XIANG SUN (Fellow, IEEE) received the
Ph.D. degree from Stanford University, Stanford,
CA, USA. He was a Scientist with IBM, Armonk,
NY, USA, and Hitachi Global Storage Technolo-
gies, San Jose, CA, USA. He is currently a Profes-
sor with the Electrical and Computer Engineering
Department and the Director of the W.M. Keck
Laboratory for Integrated Ferroics, Northeastern
University, Boston, MA, USA, and the Founder
and a Chief Technical Advisor of Winchester Tech-
nologies, LLC, Burlington, MA, USA. He has au-
thored or coauthored more than 280 publications. He holds more than 20
patents and patent applications. His research interests include novel magnetic,
ferroelectric, and multiferroic materials, devices, and subsystems. One of his
articles was selected as the Ten Most Outstanding Full Papers in the Past
Decade (2001–2010) in Advanced Functional Materials. Dr. Sun was the re-
cipient of the NSF CAREER Award, the ONR Young Investigator Award, and
the Søren Buus Outstanding Research Award. He is an editor of the Sensors
and IEEE TRANSACTIONS ON MAGNETICS and a Fellow of the Institute of
Physics and the Institution of Engineering and Technology. He has given more
than 180 plenary/keynote/invited presentations and seminars.
22 VOLUME 1, NO. 4, OCTOBER 2021