MR BASICS – MEAS DEUTSCHLAND GMBH
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MR Basics
An Introduction to
Magnetoresistive Sensors
Axel Bartos, Armin Meisenberg
MEAS Deutschland GmbH
Hauert 13, D-44227 Dortmund, Germany
www.meas-spec.com
This application note provides a very basic introduction to the fundamentals of
magnetoresistive (MR) sensors for those users who may be unfamiliar with their
characteristics and modes of operation.
Magnetic sensors
Sensors that monitor properties such as
temperature, pressure, strain or flow
provide an output signal that is directly
related to the desired parameter.
Magnetic sensors, on the other hand,
differ from most of these detectors as
they very often do not directly measure
the physical property of interest. They
detect changes, or disturbances in
magnetic fields that have been created or
modified by objects or events. The
magnetic fields may therefore carry
information on properties such as
direction, presence, rotation, angle, or
electrical currents that is converted into
an electrical voltage by the magnetic
sensor. The minor amount of magnetic
sensors measure magnetic fields
absolutely, like earth field in compassing.
The output signal requires some signal
processing for translation into the desired
parameter. Obviously, a magnetic field
distribution depends on distance and the
form of the creating or disturbing object
(i.e. magnet, current etc.) or event. It is
therefore important always to consider
both sensor and creating object in the
application design. Although magnetic
sensors are somewhat more difficult to
use, they do provide accurate and reliable
data — without physical contact.
The magnetoresistance effect
Lord Kelvin discovered magnetoresistance in
1857 when he noticed the slight change in the
electrical resistance of a piece of iron when he
placed it in a magnetic field. But it took more
than 100 years before a first magnetoresisitve
(MR) sensor concept was reported by Hunt in
1971. And it lasted additional 20 years for IBM to
introduce the first MR head, which used a strip of
magnetoresistive material to detect bits, into a
hard disk drive in 1991. Earlier, MR sensors were
used in less demanding applications of price-tag
and badge readers (read-only) and magnetic tape
(1985).
The geometry of a Hunt element – a
magnetoresistive film with a sense
current I and magnetization vector M at a
singal determining angel α to the current
in the plane of the film – is depicted in
figure 1. A magnetic field Hy coupled
into the soft magnetic sensor material
MR BASICS – MEAS DEUTSCHLAND GMBH
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will change the resisitivity of the stripe,
which is probed by the sense current.
Fig. 1: The geometry of a Hunt element
The change in resistivity is found
experimentally to be
)cos1( 20 α⋅
∆
+⋅=
R
RRR (1)
where R0 is the resistivity with
magnetization perpendicular to the sense
current (α=90°). Due to its excellent
properties, the material to choose for
magnetic sensors utilizing the
magnetoresistance effect is Permalloy, a
Ni-Fe alloy with approximately 80% Ni
content. The value of the MR coefficient
∆R/R is typically 1.5-3% for Permalloy,
depending on stripe geometry and
preparation conditions.
An approximation between the signal
determining angle α and an applied
magnetic field in y direction Hy can be
given for Hy10 kA/m), the
magnetization vector in the sensor is
MR BASICS – MEAS DEUTSCHLAND GMBH
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always (almost) parallel to the applied
field. A common application for a
magnetoresistive high field sensor is a
contactless angular sensor, like the
KMT31 or KMT32B.
In low field applications the
magnetization vector is mainly
determined by the form of the strips,
because the magnetization shows a
natural preference for the longitudinal
direction. The external field causes a
twist α of the magnetization in the stripe,
which changes the resistance due to the
MR effect. Linear low field sensors like
the MR174B die and the KMY sensors
are typically working in this mode.
MR sensors with linearized transfer
curve
Applying low magnetic fields to a Hunt
element will lead only to small changes
of the magnetization and in turn, the cos
term in formula (1) will hardly change at
small changes of α. A Hunt element is
not sensitive at small field strengths.
In order to make the MR sensor sensitive
for low magentic fields, the MR transfer
curve (1) has to be modified. The most
common way is achieved by barber poles,
as shown in figure 2.
Small, highly conductive bars – the
barber poles -- are placed on top of the
Permalloy. They will shunt the current in
the Permalloy and will change the current
path due to their geometry, but they will
not change the magnetic behaviour. The
current between the barber pole gaps will
take the shortest path, i.e. perpendicular
to the barber poles.
Fig.2: Covering the MR stripe with highly
conductive barber poles will change the current
direction in the Hunt element but will not alter the
magnetical behaviour of the Permalloy.
Now, with no field present, the signal
determing angle will be 45°, i.e. α has to
be substituted by °+→ 45αα which
will change formula (1) to
αα 200 sin1sin −⋅⋅
∆
⋅±=
R
RRRR
(3)
Substituting formula (2) into (3) results in
2
00 1
−⋅
⋅
∆
⋅±=
eff
y
eff
y
H
H
H
H
R
RRRR
(4)
which now reveals linearity with Hy for
effy HH ⋅< 2
1
.
Fig.3: Characterisitc tranfer curves for MR
elements, (a) without and (b) with barber poles.
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Sensor linearity
The linearity of the sensor output signal
depends on the relation of the real signal
amplitude to the maximum output voltage
amplitude. Fig. 4 shows the linearity
deviation in relation to this quotient (in
percent):
0%
2%
4%
6%
8%
10%
0% 20% 40% 60% 80% 100%
signal ampl. in % of max. ampl.
lin
ea
rit
y
de
v
ia
tio
n
in
%
Fig. 4: Linearity of MR174B (KMY20S/M &
KMZ20S/M)
Sensor stability
The magnetostatic energy is the same for
a magnetic domain whether it is for
example parallel HM ↑↑ or antiparallel
HM ↑↓ to the external field. In other
words, magnetic domains can fluctuate
between two directions in a stable
environment. This is not an issue in case
of high field sensors, as the transfer curve
is quadratic in α. But it has dramatic
effects in case of a barber pole sensor, as
now the output signal changes also sign.
For this reason, low field
magnetoresistive sensors like the
barberpole sensor have to be stabilized
(biased) by an external additional field
(Hx), which is favorable oriented along
the MR stripe (i.e. x-direction). The only
task of this field is to define a
preferrential direction for the alignment
of the magnetic domains. The bias field
must be strong enough that disturbing
fields are not able to switch domains. It
has been found empirical, that bias field
strengths greater than appr. 2.5 kA/m
ensure proper performance of the sensor.
Fig. 5: The characteristic curves of a barber pole
sensor for different bias fields
Smaller bias fields
One should keep in mind that a biasing
field will change the sensitivity of the
sensor. This is shown in figure x and in
table x.
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Hx [kA/m]
se
n
si
tiv
ity
[(m
V/
V)
/(k
A
/m
)]
Fig. 6: Sensitivity versus bias field
In some applications a high sensitivity is
desired. In this case it is possible to work
without a bias field. To do so, the sensor
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must be well preconditioned:
immediately before the measurement, the
magnetization is flipped by a short
magnetic pulse in a defined x-direction
(premagnetisation). To prevent the stripe
magnetization of flipping during the time
between premagnetisation and
measurement, external fields must be
limited to approx. less than 0.5 kA/m.
Bias field Operating Sensitivity max. field Remark
Hx field range S Hy,max
in kA/m in kA/m in mV/V/kA/m in kA/m
0 0.35 14 0.5 Premagnetization necessary
1 0.5 10.5 0.5 Premagnetization necessary
2 1.1 6.3 1 Premagnetization recommended
3 1.4 4.9 ∞ Stable
5 2 3.4 ∞ Stable
Tab. 2: Sensitivity and recommended operating area
Permanent magnets and barber pole
sensors
The stabilizing Hx-field is usually
generated by a permanent magnet. Using
KMY20S or KMZ20S the customer has
to apply a permanent magnet to generate
the required bias field. KMY20M and
KMZ20M are provided with an internal
Barium-ferrite-magnet. The maximum
external field strength is only limited by
the stability of the permanent magnet
material. In the case of the KMY20M and
KMZ20M-types disturbing fields higher
than approx. 40 kA/m (500 Gauss) can
change the magnetization direction of the
permanent magnet irreversible. This can
lead to a permanent change in the offset
voltage and to a destruction of the sensor
function.
This limitation can be extended by use of
S-type sensors in combination with other
magnets, like for example rare earth
magnets, which have to be provided by
the user.
Temperature
Both ohmic and magnetoresistance
originate from scattering processes of the
conducting electrons. As all scatter
processes are temperature dependent, the
bridge resistance and MR effect ∆R/R
show a temperature dependence as well.
Temperature coefficients are usually
related to two temperatures via
%100)(
)()(
)(
1
1
12
12
⋅
−
⋅
−
=
TX
TXTX
TT
TCX
for X= bridge resistance (BR), and signal
voltage ∆R/R (SV). If not otherwise
stated, usually T1 = -25 °C and T2 = 125
°C.
In case of Permalloy both bridge resistant
and amplitude temperature coefficient
have roughly the same value, but differ in
sign TCBR≈-TCSV.
This fact enables the user to compensate
the temperature dependence of sensitivity
by using constant current supply. In this
case the supply voltage increases with
increasing temperature and increasing
bridge resistance. This effect causes an
MR BASICS – MEAS DEUTSCHLAND GMBH
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increasing output voltage, which
compensates the loss in sensitivity.
Another important value is the
temperature coefficient of the offset. This
temperature coefficient is caused by
small differences in the temperature
behaviour of the four bridge resistors. In
practice, a drift in the output voltage is
observed, which can not be separated
from the regular output signal caused by
magnetic fields. In applications using
DC-signal coupling the temperature
coefficient of the offset will thus limit the
measurement accuracy.
Permalloy is a very robust material that
can withstand very high temperatures up
to approx. 300°C when protected by a
coating. In this case, packaging will be
the limiting factor.
Contact
Axel Bartos
MEAS Deutschland GmbH
Hauert 13
D-44227 Dortmund
Germany
www.hlplanar.com
www.meas-spec.com
phone: +49 (231) 9740-0
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