Magnetic bead sensors based on the planar Hall effect in thin films of exchange-biased permalloy have been fabricated and characterized. Typical sensitivities are $\text{3\hspace{0.5em}μ}\mathrm{V/Oe} \mathrm{mA}.$ The sensor response to an applied magnetic field has been measured without and with coatings of commercially available 2 μm and 250 nm magnetic beads used for bioapplications (Micromer-M and Nanomag-D, Micromod, Germany). Detection of both types of beads and single bead detection of 2 μm beads is demonstrated, i.e., the technique is feasible for magnetic biosensors. Single 2 μm beads yield 300 nV signals at 10 mA and 15 Oe applied field.

On-chip spin-valve sensors $\text{(2×6\hspace{0.17em}μ}{\mathrm{m}}^2)$ were used to detect the binding of streptavidin-functionalized superparamagnetic labels to sensor surface-bound biotin. Both micron-sized and nanometer-sized labels were studied. The detection of biomolecular recognition was demonstrated using 2 μm Micromer^®-M and 250 nm Nanomag^®-D labels, with signals ranging from ∼300 μV to ∼2 mV (8 mA sense current; ∼15 Oe in-plane magnetizing fields). For smaller labels, detection of biomolecular recognition was not achieved. The capability of detecting single labels was demonstrated for label moments down to ${\text{2×10}}^{\text{−12}}\hspace{0.17em}\mathrm{emu}$ (2 μm labels), corresponding to signals of 100–400 μV. Although, theoretical calculations suggest that the minimum measurable moment is in the order of ${\text{6×10}}^{\text{−15}}\hspace{0.17em}\mathrm{emu},$ due to noise limitations of the present setup, this limit is in the order of ${\text{2×10}}^{\text{−13}}\hspace{0.17em}\mathrm{emu},$ corresponding to a single 250 nm label. On-chip tapered aluminum current line structures were used for placement of magnetic labels at sensor sites.

This paper describes how magnetoresistive materials can be optimized and integrated in magnetic field sensor devices. Giant magnetoresistive (GMR) and tunnel magnetoresistive (TMR) can provide advantageous solutions for a wide range of applications, of which some are described here. Considerations on field sensitivities, intrinsic noise mechanisms, biasing, and integration schemes are presented.

Superparamagnetic labels, 400 nm dextran iron oxide particles and 2 μm polymer encapsulated iron oxide microspheres, with biomolecules immobilized on the surface, e.g., the enzyme horseradish peroxidase (20–40 molecules per label) were controllably placed on chip sites $\text{(5×15\hspace{0.17em}μ}{\mathrm{m}}^2)$ using tapered Al current lines (10–20 mA current) and moved to and from adjacent spin valve sensors $\text{[2×6\hspace{0.17em}μ}\mathrm{m}{,}^2,$ magnetoresistance (MR) ∼5%]. Average MR signals of 1.2 and 0.6 mV were obtained for the detection of bulk numbers of 400 nm and 2 μm labels respectively using an on-chip field of 15 Oe and a sense current of 5 mA. The moment per label was calculated at ${\text{5×10}}^{\text{−13}}\hspace{0.17em}\mathrm{emu}$ for the 400 nm labels and ${\text{5×10}}^{\text{−12}}\hspace{0.17em}\mathrm{emu}$ for the 2 μm labels, illustrating the higher density of the 400 nm particles. MR signals of ∼100 μV were obtained for single 2 μm labels positioned over the spin valve sensor using an on-chip field of 15 Oe and 8 mA sense current. The corresponding sensor saturation occurred at ∼1 mV, with a noise level of ∼10 μV. The estimated maximum MR signal for one 2 μm label directly on top of the sensor was ∼400 μV. Biotechnological applications include high sensitivity biosensors and biochips for protein and DNA screening.