CHIP SCALE MAGNETIC SENSOR ARRAYS BASED ON MAGNETOVISCOUS EFFECT OF FERROFLUIDS

This project will investigate the design and implementation of a novel magnetometer array capable of picoTesla magnetic field sensitivity at room temperature. The principle of operation of the magnetic sensor is based on the sensitive monitoring of the magnetoviscous effects in ferrofluids using micromachined, shear mode, bulk acoustic wave resonators. The uniqueness is in the exploitation of, hitherto unexplored, response of a ferrofluid atop a high-frequency shear wave resonator to externally applied magnetic field that is sensitively monitored through the at-resonance impedance characteristics of the resonator. Ferrofluids consist of ~10 nm sized ferri- or ferro-magnetic nanoparticles suspended in a carrier liquid. Interaction of the ferrofluid nanoparticles with the surface of the resonator is considered to result in the formation of a dense magnetic double layer at the interface with a high magnetic susceptibility. Application of external magnetic fields results in changes in magnetoviscosity of the magnetic double layer at the resonator-ferrofluid interface and can be accurately monitored using micromachined quartz resonators. High frequency micromachined quartz resonators are highly sensitive to small changes in the viscoelastic loading at the resonator surface and can be used for the quantification of the magnetoviscous effect and is the working principle of the magnetometer proposed here.

Intellectual Merit: The proposed work aims at exploring the interaction of high frequency shear waves and magnetic fields on the spontaneous organization of ferromagnetic nanoparticles suspended in a fluid medium focusing specifically on the immediate layers adjacent to the resonator surface. The proposed experiments and model development will attempt to explore the interfacial origins of the phenomenon of magnetoviscosity. Combining real-time viscoelastic measurements with magnetic flux concentrator structures will provide the designs for realizing high-sensitivity magnetic sensors capable of vector measurements of magnetic field and a greater understanding of the magneto-rheological effects. Current experimental techniques on ferrofluids employ shear rates that are five orders of magnitude lower and preclude such interfacial investigations on ferrofluids. This work will demonstrate high-sensitivity, chip-scale, magnetometer arrays capable of resolving pico-Tesla magnetic field vectors.

Broader Impact: This project will explore the basic interaction of acoustic waves, magnetic fields, and ferromagnetic nanoparticles in a fluid. A deeper understanding of this phenomenon will provide the tools for understanding the role of thermal energy, dipole interaction energy, and hydrodynamic forces on nanoparticles. This work will elucidate the relationship between the observed rheological properties of the ferrofluids and the agglomeration characteristics of the nanoparticles at ferrofluid-resonator interface. The successful development of the integrated sensors in this proposal has the potential to revolutionize the biomagnetic field detection and imaging. The potential impact of this technology on life science research and functional brain imaging, in particular, is immeasurable because it creates an opportunity to produce an array of portable devices operating at room temperature that are currently unavailable. In addition to supporting graduate student training the proposed work includes the creation of demonstration models using ferrofluids to demonstrate magnetic fields, dipoles and nanoparticle concepts to K-5 students.