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Hall-effect biosensors with a magnetic-field sensitivity of 23nT∕(Hz)1∕2 (30×30μm2, at a drive current of 400μA) were fabricated using pseudomorphic AlGaAs∕InGaAs∕GaAs heterostructures for detection of DNA labeled with magnetic beads. Magnetic beads with diameters of 120 nm were selectively immobilized onto the surfaces of the Hall biosensors using highly specific hybridization of complementary oligonucleotides and were subsequently detected by measurement of the ac magnetic susceptibility of the beads.

A compact, high sensitivity and low cost biosensing platform based on InSb thin film micro-Hall effect biosensors [30 ×30 µm2; magnetic field sensitivity of 1.8 nT/(Hz)1/2] was developed and used for the detection of DNA molecules labeled with magnetic beads. Magnetic beads (∼200 nm diameter) were functionalized and selectively immobilized onto Hall biosensor surfaces via highly specific hybridization of complementary oligonucleotides and subsequently detected by magnetic susceptibility measurements. The potential for parallel biomolecular analysis with 5 ×5 µm2 linear sensor arrays was demonstrated.

A unique variable temperature scanning Hall probe system (VT-SHPM) has been developed for large area, nanometre scale lateral resolution imaging of ferromagnetic domains, in which piezoelectric actuators are maintained at ambient temperature and only the semiconducting micro-Hall probe and sample are cooled. Its design overcomes the limitations of conventional cryogenic SHPM systems, where the piezoelectric actuators are cooled thus reducing the maximum scan area to about 10 µm × 10 µm due to a reduction of the piezoelectric constant at cryogenic temperatures. The VT-SHPM operates between 300 and 77 K and consists of a liquid nitrogen glass Dewar housing the sample and the semiconducting micro-Hall probe (active area = 900 nm × 900 nm; magnetic sensitivity at 300 K 6 mG Hz−1/2) used to detect surface magnetic fields. The VT-SHPM was used to image perpendicularly magnetized domains at the surfaces of 5.5 µm thick single crystal bismuth substituted garnet thin films between 300 and 77 K. Areas of about 120 µm × 130 µm were readily imaged by synchronizing the action of x–y piezoelectric actuators and corresponding stepper motors to move and scan the sample at a constant height of 0.25 µm over the surface of the micro-Hall probe. The average width of magnetic domains in the garnet thin films was found to increase as the measurement temperature decreased.

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