Biomedical Engineering Reference
In-Depth Information
Fig. 7.3
GMR SV characterization:
(
a
) transfer
curve of magnetic field versus resistance;
(
b
) sensitivity curve
The lower-field sensitivity of GMR SV sensors made them very attractive for
many sensor applications, such as the read head in hard disk drives, current sensing,
earth field sensing, and biosensing. GMR SV sensors replaced the inductive read
heads in hard disk drives in the late 1990s and were key in enabling to the rapid
increase in areal density and larger hard drives [
6
]. These GMR SV sensors have
since been replaced by magnetic tunnel junction (MTJ) sensors, which exhibit even
higher MR ratios. MTJs rely on an entirely different quantum mechanical effect
known as tunneling magnetoresistance (TMR). Structurally the devices look very
similar to a GMR SV, but the conductive Cu layer is replaced with an insulator.
The orientation between the pinned layer and the free layer changes the probability
that electrons can tunnel through the oxide, thus modulating the conductivity of
the device. The MR ratio of these devices is often 100 % or more with the current
record at 604 % at room temperature and 1,144 % at 5 K [
7
]. Presently, the main
issue limiting the adoption of MTJs as biosensors is that with the large area of the
devices, a single pin-hole defect renders the device unusable.
Despite the differences in the origin of the magnetoresistance, all magne-
toresistive biosensors can be made to operate in a similar fashion. A magnetic
immunoassay tethers MNP tags to the surface of the sensor. The underlying
magnetically responsive biosensor detects the stray field from the MNP tags through
a change in resistance. Since the stray field of the MNP tags falls off rapidly as the
distance between the sensor and the tags increases, the magnetoresistive sensors can
be referred to as proximity-based sensors.
