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Philips Semiconductors
Electronic Compass Design using
KMZ51 and KMZ52
4. MAGNETORESISTIVE (MR) SENSORS FOR COMPASS APPLICATIONS
The intention of this section is to describe the basic principles of magnetoresistive sensors, which a compass
designer should know. A more detailed description of the magnetoresistive effect can be found in [3].
4.1 The Magnetoresistive Sensor Element
4.1.1 The Magnetoresistive Effect
Magnetoresistive (MR) sensors make use of the magnetoresistive effect, the property of a current carrying
magnetic material to change its resistivity in the presence of an external magnetic field. Figure 4 shows a strip of
ferromagnetic material, called permalloy (19% Fe, 81% Ni).
y
x
During deposition of the permalloy strip, a strong external magnetic field is applied parallel to the strip axis. By
doing this, a preferred magnetization direction is defined within the strip. In absence of any external magnetic
field, the magnetization always points into this direction. In Figure 4, this is assumed to be the x-direction, which
is also the direction of current flow. An MR sensor now relies on two basic effects:
•
The strip resistance R depends on the angle α between the direction of the current and the direction of the
magnetization.
•
The direction of magnetization and therefore α can be influenced by an external magnetic field Hy, where
Hy is parallel to the strip plane and perpendicular to the preferred direction.
When no external magnetic field is present, the permalloy has an internal magnetization vector parallel to the
preferred direction, i.e. α = 0. In this case, the strip resistance R has its maximum value Rmax. If now an
external magnetic field Hy is applied, the internal magnetization vector of the permalloy will rotate around an
angle α. At high field strengths, the magnetization tends to align itself parallel to Hy and the rotation angle α
approaches 90°. In this case, the resistance reaches its minimum value Rmin. The equation next to Figure 4
gives the functional dependence between R and α, where Ro = Rmin and ∆R = (Rmax-Rmin). Finally, the
function of R versus Hy is as follows:
∆
=
+
⋅
R
R
R
1
0
Figure 5a shows a diagram for equation (2). Ho is a parameter, which depends on material and geometry of the
strip. Equation (2) is defined for field strength magnitudes of Hy ≤ Ho. For Hy > Ho, R equals Ro. R
also material parameters. For permalloy, ∆ R is in the range of 2 to 3% of R
Hy
α
Current
Figure 4
The magnetoresistive effect in permalloy
2
H
y
−
H
o
Permalloy
Magnetization
(2)
12
Application Note
+ ∆ R cos² α
R = R
0
α = 0°
Õ
R
max
α = 90° Õ
R
min
.
0
AN00022
and ∆ R are
0
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