360
T. Vemulkar and R. P. Cowburn
GMR Sensor
Capture anƟbody
Protein
DetecƟon anƟbody
SPION
SPION stray field
Fig. 15.3 A schematic immunoassay utilizing SPIONs and a GMR type sensor. The sensor top
surface is biofunctionalized with a capture antibody. The sample solution containing the protein
of interest is poured over the sensor, allowing the protein to be captured by the capture antibody.
The sensor surface is then washed and a detection reagent consisting of a complementary detection
antibody attached to a SPION is used to confirm the presence of the analyte. Binding of the detection
antibody is confirmed by a resistance change in the sensor caused by the stray field of the SPION
The first approach uses magnetically soft materials magnetized in the plane of the
particle with weak intrinsic in-plane anisotropy and a symmetric planar shape minimizing in-plane configurational anisotropy [103]. For circular disc configurations in
the 200 nm–2 μm diameter range and thicknesses ranging from approximately 10–
60 nm (aspect ratios of thickness/diameter typically ranging from 4–40) [104–106],
a magnetic flux closure state may be exhibited at remanence known as a magnetic
vortex. In such a configuration, the magnetization vector of the disc remains parallel
to the nearest disc edge at all points.
The second approach makes use of Ruderman-Kittel-Kasuya-Yoshida (RKKY)
[107–109] coupling between adjacent magnetic layers in a multilayer thin film.
A phenomenon originally of interest in magnetic memory and logic elements
[110–112], it has been of great interest in the space of lithographically fabricated
microdiscs. In RKKY coupled systems, two adjacent magnetic layers may be coupled
either ferromagnetically or antiferromagnetically (AF) [113, 114] depending on the
thickness and composition of the interlayer separating the magnetic layers [115, 116].
A typical example of an RKKY coupling interlayer that can be sandwiched between
two magnetic layers is 0.5–1 nm of Ru [116, 117]. When the thickness of the interlayer is tuned to lie in the AF regime, the two magnetic layers lie antiparallel (AP) to
each other in the absence of a magnetic field. This AP configuration arising from the
T. Vemulkar and R. P. Cowburn
GMR Sensor
Capture anƟbody
Protein
DetecƟon anƟbody
SPION
SPION stray field
Fig. 15.3 A schematic immunoassay utilizing SPIONs and a GMR type sensor. The sensor top
surface is biofunctionalized with a capture antibody. The sample solution containing the protein
of interest is poured over the sensor, allowing the protein to be captured by the capture antibody.
The sensor surface is then washed and a detection reagent consisting of a complementary detection
antibody attached to a SPION is used to confirm the presence of the analyte. Binding of the detection
antibody is confirmed by a resistance change in the sensor caused by the stray field of the SPION
The first approach uses magnetically soft materials magnetized in the plane of the
particle with weak intrinsic in-plane anisotropy and a symmetric planar shape minimizing in-plane configurational anisotropy [103]. For circular disc configurations in
the 200 nm–2 μm diameter range and thicknesses ranging from approximately 10–
60 nm (aspect ratios of thickness/diameter typically ranging from 4–40) [104–106],
a magnetic flux closure state may be exhibited at remanence known as a magnetic
vortex. In such a configuration, the magnetization vector of the disc remains parallel
to the nearest disc edge at all points.
The second approach makes use of Ruderman-Kittel-Kasuya-Yoshida (RKKY)
[107–109] coupling between adjacent magnetic layers in a multilayer thin film.
A phenomenon originally of interest in magnetic memory and logic elements
[110–112], it has been of great interest in the space of lithographically fabricated
microdiscs. In RKKY coupled systems, two adjacent magnetic layers may be coupled
either ferromagnetically or antiferromagnetically (AF) [113, 114] depending on the
thickness and composition of the interlayer separating the magnetic layers [115, 116].
A typical example of an RKKY coupling interlayer that can be sandwiched between
two magnetic layers is 0.5–1 nm of Ru [116, 117]. When the thickness of the interlayer is tuned to lie in the AF regime, the two magnetic layers lie antiparallel (AP) to
each other in the absence of a magnetic field. This AP configuration arising from the