Force-gradient detection of nuclear magnetic resonance

This thesis presents experiments in which magnetic resonance is detected as a force or a force gradient on a microcantilever, a technique known as magnetic resonance force microscopy (MRFM). A new type of MRFM is described with which unprecedented sensitivity for nuclear MRFM was achieved. These experiments represent an advance in the ongoing effort to reach single-nucleus sensitivity.; First an apparatus was built in which a millimeter-scale magnetic particle was used to exert a force on paramagnetic samples which were mounted on an atomicforce-microscope cantilever. This device was used to detect electron spin resonance in diphenyl-picrylhydrazyl at 77 kelvin, and nuclear magnetic resonance in ammonium nitrate at room temperature. These experiments formed the foundation for later high-sensitivity work by providing essential information about many aspects of the apparatus.; A more advanced set-up was then created for the demonstration of a new MRFM method in which the gradient of the force from spins in the sample alters the effective spring constant of the cantilever, causing a shift in its mechanical resonance frequency. Using a custom, magnet-tipped, low-spring-constant cantilever cooled to 4 kelvin, magnetization from 71Ga in GaAs was detected at a sensitivity of 7.5 x 10-21 J/T in a one-hertz measurement bandwidth, the highest nuclear-MRFM sensitivity ever reported at that time. The method has highly favorable spin-relaxation characteristics when compared with the other existing high-sensitivity MRFM technique.