Advancing a nuclear magnetic resonance force microscopy (NMRFM) probe and simulating NMRFM in thin films



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This research endeavor centers around the development of a nuclear magnetic resonance force microscopy (NMRFM) probe for investigating thin-film samples. Of particular interest is the conducting region that forms when lanthanum aluminate (LAO) is grown epitaxially on strontium titanate (STO). These materials are insulating in their bulk form. We propose NMRFM as a tool to detect whether there is diffusion of atoms across the interface, which could explain the emergence of the conducting region. While conventional scanning probe techniques are constrained to the surface of a sample, NMRFM features the non-invasive and subsurface detection capabilities of conventional nuclear magnetic resonance (NMR) spectroscopy. Unlike conventional NMR, for which a cubic millimeter-sized sample is required to produce a measurable signal, we can readily scale down NMRFM detection sensitivity, extending its application to smaller samples. In combination, these features suggest that NMRFM is well-suited to study the LAO/STO interface. Force detection of nuclear spins is made possible by coupling NMR spin flip sequences to a mechanical oscillator (cantilever). A small magnetic tip deposited on the cantilever establishes a large field gradient and an interaction force with the magnetic moments of the sample nuclei. This tip traces out constant-field slices perpendicular to the magnetic field. Within a particular slice, nuclear spins resonate with the perturbing oscillating field conventionally employed in NMR spectroscopy. We anticipate that evidence of atomic diffusion across the LAO/STO interface is limited to a 10-nanometer region. Before the reconfiguration outlined herein, this NMRFM probe previously resolved a sample whose smallest dimension was 60 microns. To develop this probe for thin films, we adopted the Interrupted OScillating Cyclic Adiabatic Reversal (iOSCAR) protocol. This method distinguishes the minuscule force interaction between the cantilever and the sample by implementing an NMR-induced modulation of the cantilever frequency. iOSCAR generates a distinguishable signal at an established frequency that is far from spurious artifact signals that limited the signal-to-noise ratio of the previous NMRFM protocol. This dissertation involves contextualizing NMR and NMRFM and assesses the need for further experimental investigation of LAO/STO. Furthermore, it details the evolution of an NMRFM probe to enable the exploration of thin-film samples using iOSCAR. While this research project largely involved the creation of experimental components, it concluded by modeling the expected experimental results. We created simulations of thin-film NMRFM, calculating the z component of the sample magnetization near an oscillating cantilever with a magnetic tip. These simulations explore the dynamic interactions between a thin-film sample and a cantilever as sample nuclei undergo magnetic resonance.



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