This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2017.2712860, IEEE
Transactions on Magnetics
FF-07
1
Improved efficiency of tapered magnetic flux concentrators with
double layer architecture
J. Valadeiro1,2, D.C. Leitao1,2, S. Cardoso1,2, P.P. Freitas1
1
INESC-MN - Instituto de Engenharia de Sistemas e Computadores –Microsistemas e Nanotecnologias, 1000-029 Lisboa,
Portugal
2
Physics Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa 1049-001 Portugal
Being able to increase the sensitivity of magnetoresistive sensors, by orders of magnitude, provides a route towards challenging
detection levels and opens the way for new applications. This work describes a novel architecture of magnetic flux concentrators to
achieve an improved guiding efficiency, combining materials with different magnetic properties and a vertical tapering. The novelty
consists in the concentration of the magnetic flux kept by the entire structure in a reduced cross section area within the vicinity of the
spinvalve sensor. Depending on the configuration of the double layer magnetic flux concentrators, average sensitivity gains of ~ 90x
and ~ 400x were obtained. This enhanced guiding efficiency also reduced the impact of a misalignment between the device sensing
direction and the applied magnetic field, since the device performance is not compromised until a misalignment angle  = 45º. This
further stabilization may arise from the vertical tapering of the magnetic flux concentrator, being consistent with 2D finite element
simulations.
Index Terms — magnetoresistive sensors, magnetic flux concentrators, sensitivity gain, vertical tapering
I. INTRODUCTION
R
heads industry have had a major role in the
development of soft magnetic materials technology,
broadening its applicability from magnetoresistive (MR)
sensors shielding to magnetic flux concentrators (MFCs) in
recessed read heads [1]. MFCs patterned with an appropriate
geometry concentrate the external magnetic field in the sensor
region, resulting in a sensitivity improvement and a
consequent enhancement of the minimum detection level. The
effective gain is defined by the ratio between the magnetic
field in the sensor region and its external value, depending on
geometrical parameters (e.g. MFCs dimensions) and magnetic
properties of the material (e.g. relative magnetic permeability)
[2-6]. The integration of MFCs is a reliable alternative for
low-field sensing applications when the device footprint is not
an issue. Biomedical applications [7] and highly sensitive
microscopy tools [8] offer many opportunities for ultra-low
magnetic field detection, where detection values in sub-pTesla
range are required. The inclusion of MFCs on MR sensing
elements increases their sensitivity values in more than one
order of magnitude, with a direct enhancement of the
minimum detection level. This strategy makes MR sensors
competitive for such low field detection applications, not
requiring cryogenic operating temperatures as in SQUIDs.
Sensitivity gains of MR sensors up to 30 times have been
reported for single layer MFCs patterned with funnel shape
and a steep profile (90o) at the gap region [6], while a
maximum sensitivity enhancement of 100 times was obtained
using the same geometry with a 3D tapered profile (45o)
[9][10]. In the former process, the thickness difference
between the MFC structure (~500 nm) and the spinvalve
sensor (~30 nm) limits the amount of magnetic field kept by
the sensing element since the majority of the concentrated flux
does not reach the sensor. Besides the larger gain, the latter
EAD
process requires the etching of 0.7 m thick of CoZrNb in the
gap region, demanding an accurate end-point control,
otherwise it damages the sensing elements and results in a
small fabrication yield. None of the presented examples of
MFCs address a statistical study about the reliable
reproduction of the devices or the dispersion of the reported
sensitivity gains.
This work proposes a novel MFC architecture with a
vertical tapering designed to achieve an improvement of the
magnetic flux concentration efficiency in the gap region, to
devise extremely sensitive magnetic detection tools. The
double layer MFC architecture combines materials with
different magnetic properties, with steep and vertical tapered
profiles. This strategy envisages the concentration of the
magnetic flux kept by the entire MFC structure (large
thickness at the entrance) in a reduced cross-sectional area in
the sensor region (thinner MFC structure at the pole). Average
sensitivity gains of ~90x and ~400x were obtained depending
on the double layer MFCs spatial configuration. Still, an
accurate control of the sensor operation point is required to
take advantage of such sensitivity enhancement. The use of
MFCs as a strategy to reduce the misalignment impact
between the external field and the device sensing direction
was also addressed.
II. EXPERIMENTAL METHOD
The MFCs structure consists of: (i) a 100 nm layer of
Ni80Fe20 defined by ion milling etch at 45o angle, leading to a
tapered profile near the sensor; and (ii) a 500 nm layer of
Co93Zr3Nb4 defined by lift-off with a steep profile. The latter
is recessed d = 50 or 100 m relative to the Ni80Fe20 pole
below. Fig. 1(a) shows a schematic representation of the
0018-9464 (c) 2016 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.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2017.2712860, IEEE
Transactions on Magnetics
FF-07
2
(a)
(b)
(d)
1.0
Easy axis
M/Msat
0.5
(c)
Hard axis
0.0
Msat = 725 kA/m
-0.5
Hk = 1.5 mT
r = 803
NiFe 100 nm/
CoZrNb 500 nm
-1.0
-9
(e)
-6
-3
0
3
0.H (mT)
6
9
(f)
Fig. 1. (a) Schematic view of the device: top view and cross section, showing the double layer MFCs composition and vertical tapering (NiFe [100 nm] defined
with an angle of 45o in the pole region; recessed CoZrNb layer [500 nm] with a steep profile). A 5 m nominal gap was left between the MFCs poles, were the
SV sensor is placed. (b) Simulation of the magnetic flux distribution in the CoZrNb – NiFe transition, showing the field kept by the CoZrNb layer drives into the
NiFe layer (cross section). (c) Simulation of the magnetic flux distribution in the MFCs gap region, between the NiFe pole and the SV element (cross section).
(d) VSM characterization (M(H)) of the double layer materials (bulk) used as MFCs: [buffer/ NiFe (100 nm)/ CoZrNb (500 nm)]. (e)-(f) SEM images of the
fabricated devices showing the funnel shape of the patterned MFCs (left) and its vertical profile in the outer width edge (right).
device cross section, highlighting the vertical architecture of
the MFCs. This configuration allows a MFC thickness
reduction in the pole region and increases the flux
concentration efficiency in the sensor area since the field kept
by the thick Co93Zr3Nb4 is driven into the thin Ni80Fe20 layer,
as seen in the simulation of the magnetic flux distribution (fig.
1(b)-(c)). An unpatterned (continuous thin film) sample of the
used soft magnetic double layer was characterized by a
vibrating sample magnetometer (VSM). A relative
permeability r = 803 was obtained together with an intrinsic
anisotropy μ0Hk = 1.5 mT, setting the linear M(H) region (H
= 2Hk) where the MFCs gain is well controlled (Fig. 1(d)).
The NiFe and CoZrNb were deposited under an applied field
of 5 and 10 mT respectively, without post annealing. The
MFCs were patterned with a funnel shape and a nominal gap
distance of 5 m. The NiFe layer has a pole width of 40 m
and an outer width (length) ranging from 500-2000 m (2652420 m).
The spinvalve (SV) sensors in the gap were deposited by
ion beam with the following stack: Ta 2.0/ Ni80Fe20 3.0/
Co80Fe20 2.2/ Cu 2.1/ Co80Fe20 2.5/ Ir24Mn76 6.5/ Ta 10.0
(thickness in nanometer and alloy compositions in at.%). The
sensors were defined by optical lithography and ion-beam
milling, being patterned in single elements with an active area
of 40 x 2 m2. The metallic contacts were done by lift-off of
300 nm-thick AlSiCu films protected by 15 nm thick TiWN
layer. The sensors were then passivated with 50 nm of
sputtered Al2O3 which also protected them during the ion
milling etch required for the MFC first layer definition. After
the double layer MFC fabrication, vias for the metallic
contacts through the oxide were opened by ion milling
etching. Fig. 1(e)-(f) show scanning electron microscope
(SEM) images of the fabricated devices, being visible the
integration of the SV sensor with the double layer MFCs and
its vertical profile.
The device transfer curve [MR(H)] was measured using a
dc two-point probe method within a field range of ±14 mT and
±200 mT. The Helmholtz coils allowed an accurate MR(H)
characterization with a precision down to 3 T/mA.
III. RESULTS AND DISCUSSION
The MR(H) curves obtained for a SV sensor prior and upon
the integration of the double layer MFCs (MR = 8.4 % and
Rmin = 520 ) are shown in Fig. 2(a). In the absence of MFCs,
the SV sensor exhibits a linear and hysteresis free transfer
curve with a sensitivity S = 1.7 %/mT.
A. Improved device sensitivity
The integration of the double layer MFCs (outer width = 1500
m, d = 50 m) increases the transfer curve transition slope,
0018-9464 (c) 2016 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.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2017.2712860, IEEE
Transactions on Magnetics
FF-07
0.H (mT)
-75 -50 -25 0
8
S = 1.7 %/mT
6
2
Sensitivity gain
= 104 x
4
MR (%)
6
4
25 50 75
Hexch = 33 mT
SV with double
layer MFCs
0
MR (%)
-10
-5

0
300
150
SV sensor
2 Hexch = 12 mT
(a)
0
-15
8
400x
Gain
S = 175 %/mT
SV sensor
4
450
MR = 8.4 %
Rmin = 520 
5
10
15
Sensitivity (%/mT)
6
SV sensor with
double layer MFCs
MR = 8.4 %
d = 50 m
d = 100 m
S = 170 %/mT
Gain = 100 x
S = 730 %/mT
Gain = 430 x
0
800
600
400
200
0
6
4
2
0
2
MFC dimension
outer
= 1500 m
width
(b)
0
-0.4
-0.2
90x
8
MR (%)
MR (%)
8
3

0.0
0.2
0.4
Fig. 2.(a) Effect of the double layer MFC inclusion in the magnetotransport
curve of a single SV sensor. Inset: Influence of the MFCs inclusion in the
exchange field of the SV sensor. (b) Detailed view of the obtained curves
upon the inclusion of double layer MFCs with different NiFe - CoZrNb
separations (d = 50, 100 m). Characterization within [-0.3, 0.3] mT range
with 0.01 mT steps.
yielding a sensitivity enhancement of 104x (S = 175 %/mT).
The observed coercivity of the final device remained
unchanged upon the MFCs inclusion (20.Hc ~ 0.3 mT). The
coercivity is more notorious upon the MFCs integration as a
consequence of the higher measurement precision (0.01 mT
steps), while for the isolated SV sensor a coarser
characterization was performed (0.2 mT steps). However,
M(H) for continuous thin films of NiFe – CoZrNb
(unpatterned, fig. 1(d)) indicates a small remanent
magnetization at the hard axis. Assuming a similar behavior
for the patterned structure, such remanence was previously
seen to lead to a field in gap which polarizes the SV sensor
[11]. Such effect translates into a shift of both MR curve
branches masking the expected reduction in the coercivity of
the final device. Despite the coercivity, a correct selection of
the curve branch allows the device to operate with a large
sensitivity.
To evaluate the MFCs effect on the SV element exchange
bias field (Hex), a MR(H) characterization in a wider field
NiFe - CoZrNb distance:
d = 50 m
d = 100 m
500
1000
1500
MFCs outer width (m)
2000
Fig. 3 Summary of the device features – MR, sensitivity and sensitivity gain
– obtained upon the inclusion of the double layer MFCs with different
dimensions (outer width and length). A comparison of the results depending
on the NiFe - CoZrNb distance d is also presented. An average sensitivity
gain ~ 90x (~ 400x) is obtained for d = 50 m (d = 100 m).
range was performed [inset Fig. 2(a)]. A 0.Hex reduction from
33 mT to 12 mT (factor ~ 3x) was observed upon double layer
MFCs inclusion. The maximum gain of the MFCs occurs
within the linear transition range of the material H=2Hk, with
an abrupt reduction for external applied fields larger than Hk
[12]. The presence of magnetostatic coupling between the
pinned layer and the MFCs is not taken into account, but is
well known to affect the calculated gain [13].
The influence of NiFe - CoZrNb separation (d = 50,
100 m) in the device transfer curve is presented in Fig. 2(b),
where a finer characterization of the obtained curves within
the [-0.3, 0.3] mT range (0.01 mT step) is shown. Both
presented curves have a square shape, exhibiting a coercivity
0.Hc < 0.1 mT. Taking the sensitivity as the average value of
the two transition branches in the transfer curve, a sensitivity
enhancement of 430x (100x) is obtained upon the integration
of MFCs with d = 100 (50) m. With the configuration where
d = 100 m, a sharper transition between the saturation states
occurs (within 0.01 mT), yielding sensitivity gains of ~
hundreds of times. However, besides these gain values, the
device is always in a saturation state at remanence with both
MFCs configurations, implying a fine control of its operation
point to place it in one of the transition branches.
Fig. 3 summarizes the main sensor features (MR,
sensitivity and sensitivity gain) upon the inclusion of MFCs
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Transactions on Magnetics
FF-07
Fig. 4 (a) Schematic view of the performed experiment, where  is defined
between the device sensing direction and the applied external field. (b)
Normalized sensitivity dependence of the characterized devices with the
angle . Angular stability results obtained for: single SV sensor, SV sensor
with double layer MFCs, and SV sensor with single layer MFCs.
Comparison of the experimental results with the trend obtained from 2D
finite element method simulation, taking into account the NiFe – CoZrNb
unpatterned r and the MFCs funnel shape (no vertical tapering considered).
with different dimensions. While MFCs with a NiFe - CoZrNb
separation d = 50 m allowed an average sensitivity gain ~
90x, this increased to ~ 400x when a pole separation of
100 m was used. A considerable dispersion of the sensitivity
gain is observed for both configurations. This dispersion
suggests the need of local magnetic control of the MFCs
elements, particularly in the pole region. The possible
presence of defects in the MFCs poles and the small
remanence in gap affect the structure magnetic domains and
consequently the obtained gain value. An overall trend of the
obtained sensitivity gains with the MFCs dimensions (outer
width and length) was not observed for the studied geometries.
B. Improved resilience to misalignment
As a consequence of its field guidance capability, the
MFCs were also evaluated as a strategy to minimize the
impact of misalignment between the device sensing direction
and the signal-of-interest direction. In this work, the angular
dependency of the device transfer curve (and sensitivity) with
the direction of the external magnetic field was addressed. The
angle () of the external field, measured relative to the SV
sensing axis (Fig. 4(a)), was consecutively changed in the [0,
90]º range with an increment  = 15º. This experiment was
performed for devices with three different specifications: (i)
single SV sensor; (ii) SV sensor with double layer MFCs; and
(iii) SV sensor with single layer MFCs, defined with the same
shape but without vertical tapering (500 nm-thick CoZrNb,
step profile in the gap region as described elsewhere [3][6]).
Fig. 4(b) shows the dependence of the devices normalized
4
sensitivity with the angle . The sensitivity of a SV sensor
reduces to 80% of its initial value already for  = 15º. This
angle increases to  = 45o upon the inclusion of the double
layer MFCs, denoting advantageous angular stability in the
device performance. For the same conditions, single layer
MFCs only operate within an angular range up to  = 30o. The
developed MFCs increase the angular operation range
rendering the device insensitive to misalignments relative to
the applied field without compromising its performance.
Fig. 4(b) also displays the field amplification gain
dependence obtained through 2D finite element method
(FEM) simulations. The MFCs geometry was the same used in
the fabrication process (funnel shape, pole width = 40 m,
outer width = 1000 m, length = 675 m) but without
accounting with the vertical tapered profile. A single material
was considered taking the bulk r = 803 of the NiFe/ CoZrNb
double layer. To further approximate the simulated model to
the fabricated device, a stripe of magnetic element was placed
in the MFCs gap (SV total dimensions 50 x 2 m2, r = 480).
The MFCs and the magnetic element are placed in a squared
air box (r = 1, length = 5000 m), being subjected to a
constant magnetic field 0.H = 10 mT. The field gain is
obtained from the ratio between the magnetic flux calculated
in the MFC pole – SV middle point and the fixed field applied
outside the MFCs.
For misalignments  < 45º, the simulated curve approaches
the experimental results of the device with single layer MFCs.
This behavior suggests that for a low misalignment range [0,
45]º the further stabilization obtained upon the double layer
MFCs arises from its vertical tapering and consequent
enhanced guiding efficiency, exhibiting a deviation from the
simulated curve which only accounts for the in-plane funnel
shape geometry. Consequently, the close matching of the
simulated curve with the experimental data for single layer
MFCs (sharp profile in the pole) supports the vertical tapering
influence in the sensitivity stabilization. However, for  > 45º
the simulated curve tends to the experimental results of double
layer MFCs, suggesting that for larger misalignments the
MFCs in-plane geometry and the materials magnetic
properties (r) are the main contribution factors for the
observed trend. Therefore the vertical tapering of the proposed
double layer MFCs architecture provides an enhanced
sensitivity stabilization, resulting in a wider angular operating
range without sensitivity loss.
IV. CONCLUSION
This paper addresses the enhanced guiding efficiency of
MFCs obtained through a double layer architecture with a
vertical tapering. The presented strategy allows a vertical
concentration of the flux together with an in-plane magnetic
flux concentration. Sensitivity gains surpassing the values for
single layer concentrators were achieved, being obtained an
average gain ~ 400x for a NiFe – CoZrNb distance of 100 m.
The impact of a misalignment between the device sensing
direction and the external magnetic field was minimized for a
larger angular range upon the inclusion of the developed
MFCs. The device performance is not compromised within a
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2017.2712860, IEEE
Transactions on Magnetics
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5
misalignment range [0, 45]º. This further stabilization arises
from the MFCs vertical tapering.
ACKNOWLEDGMENT
Work partially supported by FCT-project EXCL/CTMNAN/0441/2012 and EU-FP7-ICT project nº 600730
(Magnetrodes). J. Valadeiro acknowledges FCT for
scholarship grant PD/BD/113956/2015 within the Doctoral
Programme AIM-Advanced Integrated Microsystems and
support through POPH. D C Leitao acknowledges financial
support through FSE/POPH. INESC-MN acknowledges FCT
funding through the Associated Laboratory IN.
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