Micro-magnetometry for susceptibility measurement of superparamagnetic single bead

We have fabricated a micro-planar Hall resistive (PHR) sensor consisting of a thin magnetic multilayer structure, and characterized the magnetic susceptibility of a single superparamagnetic bead (Dynabeads® M-280 of 2.8 μm). The sensor arm length with an active sensing junction of 3 μm × 3 μm was optimized using a finite element method (FEM) simulation to minimize its induced field effect over the junction area under an applied field, and the field sensitivity of the fabricated 7 μm arm length sensor was measured to be 0.075 μV/A m−1 (6.0 μV/Oe) in the low field region. An average voltage change of 7.6 μV with a standard deviation of 0.26 μV was observed in the sensor during the repeated bead droplet-washing procedure. The magnetic susceptibility of a single bead was calculated to be 0.65 (SI), which agreed with the measured susceptibility by a SQUID magnetometer. This novel approach provides an inexpensive micro-magnetometry method for measuring the magnetic susceptibility of magnetic micro-sized objects, which is applicable in widespread laboratories.