We have demonstrated the use of highly sensitive magnetic tunnel junction (MTJ) sensors for the detection of individual micron-sized magnetic labels. By integrating the MTJ sensor into a microfluidic channel, we were able to detect the presence of moving superparamagnetic beads (Dynabeads® M-280) in real time by direct measurement of the magnetic dipole fields associated with single beads. The dipolar fields of a single bead were sufficient to obtain a signal of 80μV with signal to noise ratio of 24dB in an applied field of 15Oe. Our data show conclusively that MTJ sensors are very promising candidates for future applications involving the accurate detection and identification of biomolecules with magnetic labels.

We have systematically investigated the dependence of tunnel magnetoresistance in MgO-based magnetic tunnel junctions as a function of Ar pressure during sputtering. The MgO surface roughness, and therefore device magnetoresistance, depends strongly on Ar gas pressure. Magnetoresistance of up to 236% was achieved at room temperature after thermal annealing at 425°C and with optimal sputtering conditions. The long mean free path of target atoms at low background pressures increases their kinetic energy at the substrate surface, resulting in smooth surface morphology and correspondingly improved device performance.

We have demonstrated the detection of 2.5 M target DNA labeled with 16 nm Fe 3 O 4 nanoparticles ͑NPs͒ and 50 nm commercial MACS™ NPs using arrays of magnetic tunnel junction sensors with ͑001͒-oriented MgO barrier layers. Signal-to-noise ratios of 25 and 12 were obtained with Fe 3 O 4 and MACS™ NPs, respectively. These data show conclusively that MgO-based MTJ sensor arrays are very promising candidates for future applications involving the accurate detection and identification of biomolecules tagged with magnetic nanoparticles.