Biomedical Engineering Reference
In-Depth Information
Fig. 7.2
(
a
)
Representative
structure
of
GMR
SV
film
stack
annotated
with
thicknesses.
(
b
) Optical microscopy image of fabricated serpentine GMR SV
One drawback of the basic multilayer GMR film stack is the high magnetic field
needed to fully switch the magnetization from the parallel to the antiparallel state
(up to 2,000 Oe), which tends to cause the sensors to have poor low-field sensitivity
[
4
], limiting their utility as biosensors. GMR spin-valves (GMR SV) overcome
this drawback by introducing a synthetic antiferromagnet to the film stack. This
antiferromagnet formed by the CoFe/Ru/CoFe structure pins the magnetization with
exchange coupling. The other ferromagnetic layer, called the free layer, rotates
freely with the applied magnetic field. A pinning layer, typically PtMn or IrMn,
defines the magnetization of the synthetic antiferromagnetic and is also used to
direct the microstructural texture of the subsequent thin films. As can be seen in
Fig.
7.2
, GMR SV sensors are elaborately engineered film stacks, typically only a
few tens of nanometers thick, passivated with an ultrathin oxide [
5
].
The transfer curve of a GMR SV sensor is shown in Fig.
7.3
a. The sensor
adopted in this work has a minimum resistance of 2;190 in the parallel state and
a maximum resistance of 2;465 in the antiparallel state, corresponding to 12 %
MR ratio. The sensitivity of the sensor was calculated by differentiating the transfer
curve (Fig.
7.3
b). The sensor is most sensitive when no field is applied, tapering
off as the field strength is increased. The saturation field is only 200 Oe, rendering
GMR SV excellent low-field sensors.
The quest for higher MR ratios has led researchers to more exotic materials and
even more elaborate film stacks. However, possibly the simplest way to increase the
MR ratio is by changing the mode of operation. The original devices passed current
vertically through the device as depicted in Fig.
7.1
, referred to as the current-
perpendicular-to-plane (CPP) mode of operation. The CPP mode of operation has
a limited MR ratio because the electrons only pass through the ferromagnetic
layers once and have limited opportunities to undergo spin-dependent scattering.
In contrast is the current-in-plane (CIP) mode where the current flows parallel to
the Cu layer and, given that the length of the sensor is significantly longer than
the thickness, has many more opportunities to scatter. Figure
7.2
b is an image of a
device operated in CIP mode where each segment of the sensor is very long (90m).
Typically GMR SV sensors often exhibit a 10-15 % MR ratio at room temperature.
