1. Mechanical Detection Of Electron Spin Resonance From Nitroxide Spin Probes, 2. Ultrasensitive Cantilever Torque Magnetometry Of Magnetization Switching In Individual Nickel Nanorods

摘要

In 2004 mechanical detection of magnetic resonance was used to detect a single electron spin. However, the demonstration required a carefully chosen sample and the techniques developed there are not immediately applicable to detecting single electron spins in organic spin labels and probes widely used in biology. In this dissertation ultrasensitive mechanical detection of magnetic resonance is extended to detect electron spin resonance from TEMPAMINE, a nitroxide spin probe. Using an ultrasensitive cantilever with a spherical nickel tip 4 [MICRO SIGN]m in diameter 400 [MICRO SIGN]B sensitivity was demonstrated in a force gradient experiment and a route to single nitroxide spin detection outlined. A necessity for reaching single spin sensitivity is controlling the close approach surface force and frequency noise the cantilever experiences. Using a nickel nanorod with 100 nm x 100nm cross section batchfabricated to overhang the tip of an ultrasensitive cantilever by 350 nm, the lowest surface force noise ever achieved in a scanned-probe experiment was demonstrated and the magnetic tip used to detect electron spin resonance. Unfortunately, the surface frequency noise experienced by the batch-fabricated tip was extremely large and the demonstrated sensitivity of the batch fabricated tip is poor. To take advantage of the extremely low surface force noise experienced by the overhanging tip, a technique based on non degenerate parametric amplification that converts a modulated frequency shift into an on resonance amplitude was developed and demonstrated. Mechanical detection requires a very high quality magnetic tip, however, the tip must be small and located at the end of a fragile cantilever. A non destructive way to determine the magnetic moment and anisotropy constant for the magnetic tip is cantilever torque magnetometry. Prior studies have investigated in-plane switching and here the in-plane to out-of-plane transition is studied. Multiple sharp, simultaneous transitions in cantilever frequency, dissipation and jitter were observed as the external field was swept. A quantitative model for the frequency shift at high field and qualitative models for the frequency shift and dissipation peaks near the switching field were developed.