CHIP SCALE MAGNETIC SENSOR ARRAYS BASED ON MAGNETOVISCOUS EFFECT OF FERROFLUIDS

本项目将研究一种在室温下具有皮特斯拉磁场灵敏度的新型磁强计阵列的设计和实现。磁传感器的工作原理是利用微机械、剪切模式、体声波谐振器对铁磁流体中的磁粘性效应进行灵敏监测。其独特之处在于利用了迄今为止尚未开发的高频剪切波谐振器上的铁磁流体对外部施加磁场的响应，该磁场通过谐振器的谐振阻抗特性进行了灵敏的监测。铁磁流体由悬浮在载液中的~ 10nm大小的铁磁或铁磁纳米颗粒组成。铁磁流体纳米粒子与谐振腔表面的相互作用导致在界面处形成具有高磁化率的致密磁双层。外加磁场的作用会导致谐振器-铁磁流体界面处磁性双层的磁粘度变化，并可以利用微机械石英谐振器进行精确监测。高频微机械石英谐振器对谐振器表面粘弹性载荷的微小变化高度敏感，可用于磁粘性效应的量化，这是本文提出的磁强计的工作原理。智力优势：提出的工作旨在探索高频剪切波和磁场对悬浮在流体介质中的铁磁性纳米颗粒自发组织的相互作用，特别是关注与谐振器表面相邻的直接层。提出的实验和模型开发将试图探索磁粘性现象的界面起源。将实时粘弹性测量与磁通量集中器结构相结合，将为实现能够对磁场进行矢量测量的高灵敏度磁传感器和更好地理解磁流变效应提供设计。目前铁磁流体的实验技术采用的剪切速率要低5个数量级，因此无法对铁磁流体进行这样的界面研究。这项工作将展示高灵敏度、芯片级、能够分辨皮特斯拉磁场矢量的磁力计阵列。更广泛的影响：该项目将探索声波、磁场和铁磁性纳米颗粒在流体中的基本相互作用。对这一现象的深入理解将为理解热能、偶极相互作用能和纳米颗粒上的流体动力的作用提供工具。这项工作将阐明观察到的铁磁流体流变特性与纳米颗粒在铁磁流体-谐振腔界面上的团聚特性之间的关系。本方案中集成传感器的成功开发，有可能给生物磁场检测和成像带来革命性的变化。这项技术对生命科学研究和功能性脑成像的潜在影响是不可估量的，因为它创造了一个机会，可以生产一系列在室温下工作的便携式设备，而这些设备目前是不可用的。除了支持研究生培训外，拟议的工作还包括使用铁磁流体创建演示模型，向K-5学生演示磁场、偶极子和纳米粒子概念。

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.