High-Gradient Nanomagnet-On-Cantilever Fabrication For Scanned Probe Detection Of Magnetic Resonance
Magnetic resonance force microscopy (MRFM) is a non-invasive, three-dimensional imaging technique that employs attonewton-sensitivity cantilevers to mechanically detect the interaction between the field gradient of a magnetic particle and magnetically-active sample spins. Achieving high sensitivity demands the use of a high field gradient. In order to study a wide range of samples, it is equally desirable to locate the magnetic tip on the cantilever. The work in this thesis centers on the development of nanomagnets on cantilevers that produce sufficiently large field gradients for nanometer-scale nuclear spin MRFM imaging and single electron spin detection. A new fabrication protocol is introduced to prepare nickel and cobalt nanomagnets on cantilevers. Custom attonewton-sensitivity cantilevers were batch fabricated. Nanomagnets were prepared separately on micrometer-scale silicon chips using electron beam lithography and electron beam deposition. Each magnet-tipped silicon chip was serially attached to a cantilever using focused ion beam manipulation. Frequency-shift cantilever magnetometry and superconducting quantum interference device magnetometry were used to assess the nanomagnet magnetization. X-ray photoelectron spectroscopy was used to determine the extent of oxidation damage. A cobalt nanomagnet-tipped chip attached to an attonewton-sensitivity cantilever was used to detect statistical fluctuations in the proton magnetization of a polystyrene film. MRFM signal was studied versus rf irradiation frequency and tip-sample separation. The tiptip field gradient ∂Bz /∂z of the nanomagnet was estimated to be between 4.4 and 5.4 MT m[-]1 , which is comparable to the gradient used in recent 4 nm resolution 1 H imaging experiments and nearly an order of magnitude larger than the gradients achieved in prior magnet-oncantilever MRFM experiments. These magnet-tipped cantilevers are projected to achieve a proton imaging resolution of 5 to 10 nm. The key design considerations and development of a new magnetic resonance force microscope are also discussed in this thesis. The microscope will use the newly-developed nanomagnet-tipped cantilevers to conduct high-resolution, three-dimensional MRFM imaging experiments at cryogenic temperatures, in high vacuum, and at magnetic fields up to 9 T. Overall, the work in this thesis has significantly advanced the capabilities of MRFM and has poised the field to begin conducting high-resolution imaging experiments on a broad range of previously-inaccessible samples.
magnetic resonance force microscopy; magnetometry; electron beam lithography
Marohn, John A.
Muller, David Anthony; Park, Jiwoong; Petersen, Poul B.
Chemistry and Chemical Biology
Ph.D. of Chemistry and Chemical Biology
Doctor of Philosophy
dissertation or thesis