Theoretical and Experimental Investigations in MEMS-Based Force-Detected NMR

Citation

Elgammal, Ramez Ahmed (2005) Theoretical and Experimental Investigations in MEMS-Based Force-Detected NMR. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4FSQ-YP59. https://resolver.caltech.edu/CaltechETD:etd-02182006-145814

Abstract

This thesis describes a method of mechanically detecting magnetic resonance. The detector consists of a ferromagnet harmonically bound to a mechanical resonator and measures a magnetic force of interaction with a nearby sample via dipole-dipole coupling. Flexural modes of vibration of the resonator are induced by inversion of the sample magnetization at the mechanical resonance frequency of the device. In this method, a nominally homogeneous field at the sample allows coherent spectroscopy over the entire sample volume.

Sensitivity analyses suggest that encoding an NMR signal into mechanical oscillations favors inductive detection at the micron scale and below with Brownian motion of the detection being the predominant source of noise and azimuthal eddy currents being the predominant source of damping. As such, the design issues of a MEMS-based spectrometer optimized for 50 micron samples have been investigated. Finite element methods were used and the results for magnetic softening effects, mechanical stresses, field homogeneity, magnet design, radiofrequency excitation, and the utility of capacitive transduction to provide tuning of the oscillator’s mechanical resonance frequency and active shimming are discussed. A piezeoelectrically actuated microvalve is proposed as part of a microfluidic device to allow shuttling of liquid samples. We present a new means of fiber-optic interferometry for geometrically confined regions in which the light exits transverse to core axis. The use of a composite magnetic array of packed nanoparticles may reduce the damping by 10⁴.

The portability of the spectrometer will allow in situ spectroscopy and towards that end 14N overtone experiments were simulated. Force-detection of this transition is superior not only at reduced size scales, but over a broad range of magnetic field strengths. The line narrowing observed by detecting the overtone transition should allow detailed spectroscopic analysis not possible by observing the quadrupolar broadened first-order spectrum. Simulations for a representative class of tholins suggest that the overtone linewidths is of order tens of kHz.

We conclude by discussing the feasibility of nanoscale NMR using torque detection of spin-locked, transverse magnetization, include a derivation of the signal-to-noise and detector optimization, and comment on the fundamental limitations of quantum statistical noise.

Thesis Files