Ferromagnetic resonance detection for magnetic microbead sensors

This paper presents a novel detection scheme for magnetic beads used to label target molecules in immunoassay based sensors. The beads are detected inductively using a microwave circuit consisting of a slotline and coplanar waveguide (CPW) fabricated in a single metal layer. The waveguides are terminated at a short-circuited junction that serves as the active sensor area. When the slotline is excited by an input radio frequency (RF) signal, ac magnetic fields are generated at the junction. These fields are orthogonal to the propagation mode allowed in CPWs. As a result, the signal from the slotline does not nominally couple into the CPW. In the presence of a bead immobilized at the junction, fields from the slotline are distorted and the signal is coupled to the output at the CPW. The output signal is further enhanced by exciting ferromagnetic resonance in the bead. Simulation results indicate a single bead to be detectable with a sensitivity of 1-10 mu V/V depending on its location in the active sensor area and the waveguide geometry. The distinctive advantages of this detection technique are ease of implementation, requiring simple and inexpensive fabrication processes; and suitability for integration in lab-on-a-chip systems.