Force-gradient detected nuclear magnetic resonance and the origins of noncontact friction.

摘要

Magnetic resonance is a ubiquitous technique for the interrogation of chemical and biological systems. Despite this prominence, the intrinsic low sensitivity of inductive detection has prohibited the application of magnetic resonance to individual cells and molecules. Magnetic resonance force microscopy (MRFM) has been proposed as a route to magnetic resonance imaging with single nucleus sensitivity. MRFM brings the possibility of subsurface, non-destructive, chemically specific imaging, to the atomic length scale.;We have demonstrated a new MRFM measurement protocol: detecting the presence of nuclear magnetic moments as a frequency shift in a micromechanical oscillator. Our method obviates the need for long, coherent manipulation of spin magnetization at the oscillator frequency. In doing so, we lift the restriction that samples studied by MRFM exhibit long spin-lock lifetimes and reduce the radio frequency irradiation duty cycle. Using this technique we have demonstrated a sensitivity of ∼ 105 proton magnetic moment equivalents by detecting magnetic resonance from 108 71Ga nuclear magnetic moments at 4.4K and 7T using a custom fabricated single crystal silicon cantilever. At the time of publication this represented the most sensitive NMR measurement by a general method.;The dominant source of noise in all high sensitivity MRFM measurements to date has been noncontact friction between the tip of the cantilever and the sample. Prior to our work, no physical mechanism of noncontact friction had been experimentally validated. We have shown that noncontact friction can arise from dielectric fluctuations within the sample. Using high sensitivity, custom fabricated, single crystal silicon cantilevers we have measured energy losses over poly(methyl methacrylate), poly(vinyl acetate), and polystyrene thin films at room temperature. A new theoretical analysis relating noncontact friction to the dielectric response of the film was consistent with our experimental observations. This work constituted the first direct, mechanical detection of friction due to dielectric fluctuations.