Magnetoviscoelastic Ferrofluid-Based Magnetometer

In this paper, we present a novel concept for magnetic sensing that is based upon the sensitive monitoring of the magnetoviscoelastic effects in magnetic nanoparticle containing ferrofluids using micromachined shear mode bulk acoustic wave quartz crystal resonators (μ QCR). The unique sensor concept is based on the application of, hitherto unexplored, magnetic field induced viscoelastic response of a thin interfacial ferrofluid layer placed atop a high-frequency shear wave quartz resonator, which can be sensitively monitored through the at-resonance impedance characteristics of the resonator. The high magnetic susceptibility of ferrofluid suspensions results in the modulation of the viscoelasticity due to applied magnetic fields. A bias magnetic field perpendicular to the resonator surface was applied to realize a dense agglomeration of the ferrofluid particles at the immediate interface of the resonator surface. Viscoelastic changes due to in-plane incident magnetic field shifts the at-resonance conductance characteristics of μ QCR, and is tracked in real time to achieve a novel magnetic sensing mechanism to detect and quantify the low-frequency low strength magnetic fields. For improved sensitivity, the in-plane sensed magnetic flux density is concentrated using a high relative permeability (μr = 7000) thin film of Metglas (Fe₈₅B₅Si₁₀) deposited on the resonator electrode. Furthermore, by patterning the Metglas film in a bow-tie shape and aligned at the center of the μ QCR electrode, both 2-D vector sensing and improvement in the sensitivity were achieved. Using these improvements, a minimum detectable field of 1.5 nT/√Hz at 1 Hz has been experimentally demonstrated.