Philips Magnetoresistive Sensor - Manual (Page 19)

2000 Sep 06 19
Philips Semiconductors
Magnetoresistive sensors for
magnetic field measurement
General
this also considerably enlarges H
k
. If a small temperature
coefficient of
∆ρ
is required, NiCo alloys are preferable.
The amorphous alloy CoFeB has a low
∆ρ/ρ
, high H
k
and
slightly worse thermal stability but due to the absence of
grain boundaries within the amorphous structure, exhibits
excellent magnetic behaviour.
APPENDIX 2: SENSOR FLIPPING
During deposition of the permalloy strip, a strong external
magnetic field is applied parallel to the strip axis. This
accentuates the inherent magnetic anisotropy of the strip
and gives them a preferred magnetization direction, so that
even in the absence of an external magnetic field, the
magnetization will always tend to align with the strips.
Providing a high level of premagnetization within the
crystal structure of the permalloy allows for two stable
premagnetization directions. When the sensor is placed in
a controlled external magnetic field opposing the internal
aligning field, the polarity of the premagnetization of the
strips can be switched or ‘flipped’ between positive and
negative magnetization directions, resulting in two stable
output characteristics.
The field required to flip the sensor magnetization (and
hence the output characteristic) depends on the
magnitude of the transverse field H
y
. The greater this field,
the more the magnetization rotates towards 90˚ and
therefore it becomes easier to flip the sensor into the
corresponding stable position in the ‘-x’ direction. This
means that a smaller -H
x
field is sufficient to cause the
flipping action
As can be seen in Fig 22, for low transverse field strengths
(0.5 kA/m) the sensor characteristic is stable for all positive
values of H
x
, and a reverse field of approximately 1 kA/m
is required to flip the sensor. However at higher values of
H
y
(2 kA/m), the sensor will also flip for smaller values of
H
x
(at 0.5 kA/m). Also illustrated in this figure is a
noticeable hysteresis effect; it also shows that as the
permalloy strips do not flip at the same rate, the flipping
action is not instantaneous.
The sensitivity of the sensor reduces as the auxiliary field
H
x
increases, which can be seen in Fig 22 and more
clearly in Fig 23. This is because the moment imposed on
the magnetization by H
x
directly opposes that of H
y
,
resulting in a reduction in the degree of bridge imbalance
and hence the output signal for a given value of H
y
.
Fig.21 Sensor characteristics.
handbook, halfpage
MLC130
0
2 4 2 4
O
(mV)
H (kA/m)
y
V
10
10
reversal
of sensor
characteristics
Fig.22 Sensor output ‘V
o
’ as a function of the
auxiliary field H
x
.
MLC131
0
1 2 3 1
O
(mV)
H (kA/m)
x
H =
2 kA/m
y
0.5 kA/m
V
50
100
100
50
2 3