Research about passivation layer of SiO2 in GMR sensors for magnetic bead detection

Abstract

More and more research has been conducted on the detection of biomolecules by using magnetic beads as magnetic tags combined with a giant magneto resistance (GMR) sensor. In studies that involve detection using magnetic particles, larger particle sizes are preferred due to their larger magnetic moment, which facilitates detection. Hence, micrometer-scaled magnetic beads are commonly utilized as detection targets. However, in this study, the thickness of the passivation layer was reduced to enable the detection of magnetic beads with a particle size of 200 nm, which is smaller than the usual size. Magnetic beads with an average particle size of 200 nm and high saturation magnetization were synthesized. The impact of magnetic bead size on the sensor surface was investigated by simulating the stray magnetic field generated by magnetic beads ranging from 100 to 1000 nm, taking into account the properties of the magnetic beads. The simulation results reveal that the presence of magnetic beads reduces the total magnetic field on sensor surface, necessitating thinner passivation layers for smaller particle sizes. For the self-developed magnetic beads used in this study, a passivation layer thickness within 200 nm is necessary. To detect the 200 nm magnetic beads, a GMR sensor with a passivation layer thickness of 10–40 nm was fabricated using MEMS technology. The experimental results show that the sensor signal is maximized at an external magnetic field of –20 Oe, and the sensor signal output increases with decreasing passivation layer thickness.