Magnetic sensors for bioassay: HTS SQUIDs or GMRs?

In this paper we compare the detection of magnetic microparticles by HTS SQUIDs and GMR sensors. Our prototype system uses a HTS SQUID array insulated/isolated from a nonmagnetic tube in which the sample flows at room temperature. This necessarily results in a liftoff between sensor and sample in the range of 2 mm. While HTS SQUIDs typically have an intrinsic noise sensitivity that is at least two orders of magnitude better than conventional GMR sensors (similar to 1 pT/root Hz for washer SQUIDs compared to similar to 100 pT/root Hz for the best commercial GMRs), the dipole like response of a magnetic microparticle in flow is such that this difference in noise performance can be compensated for by the reduction in standoff (order of 0.2 mm) when using a magnetoresistive sensor. Here we detail the two different approaches, present comparative results and discuss the relative merits of each setup.