Force Detection of Nuclear Magnetic Resonance

Abstract

Micromechanical sensing of magnetic force was used to detect nuclear magnetic resonance with exceptional sensitivity and spatial resolution. With a 900 angstrom thick silicon nitride cantilever capable of detecting subfemtonewton forces, a single shot sensitivity of 1.6 x 10(13) protons was achieved for an ammonium nitrate sample mounted on the cantilever. A nearby millimeter-size iron particle produced a 600 tesla per meter magnetic field gradient, resulting in a spatial resolution of 2.6 micrometers in one dimension. These results suggest that magnetic force sensing is a viable approach for enhancing the sensitivity and spatial resolution of nuclear magnetic resonance microimaging.

Magnetic resonance force microscopy with a ferromagnetic tip mounted on the force detector

“…The ultra-high force sensitivity of the resonator (demonstrated in the atomic force microscope) enables much higher sensitivity to spin magnetization in MRFM and thus much higher spatial resolution than is achievable in conventional MRI. MRFM has been successfully demonstrated in electron spin resonance (ESR) [3][4][5][6][7], ferromagnetic resonance (FMR) [8], and nuclear magnetic resonance (NMR) [9,10]. For instance, spatial resolution of ∼ 1 µm has been achieved in NMR [9,10], already an order of magnitude better than conventional MRI.…”

“…MRFM has been successfully demonstrated in electron spin resonance (ESR) [3][4][5][6][7], ferromagnetic resonance (FMR) [8], and nuclear magnetic resonance (NMR) [9,10]. For instance, spatial resolution of ∼ 1 µm has been achieved in NMR [9,10], already an order of magnitude better than conventional MRI. Further increase of the sensitivity and resolution can be achieved by performing MRFM at cryogenic temperatures, by increasing the field gradient and by improving the performance of the micro-mechanical resonators [11].…”

Advances in mechanical detection of magnetic resonance

“…A flurry of activity followed the initial demonstration that microcantilevers could be used to detect electron spin resonance 36 and nuclear magnetic resonance. 37 The purpose of this section is to detail key experiments and ideas in magnetic resonance force microscopy that have been developed since the comprehensive review of Ref. 3.…”

“…36 To improve sensitivity, a custom-fabricated, low spring constant, 90-nm-thick silicon nitride cantilever with a record low intrinsic friction was harnessed in the first demonstration of mechanically detected NMR. 37 These experiments inspired the development of cantilevers with vastly improved sensitivity.…”

“…36͒ or by performing cyclic adiabatic rapid passages via sweeping the frequency of the applied rf or the static magnetic field ͑when T 1 Ͼ f c −1 . 37 In order to avoid the spurious response of the cantilever to the modulated rf, a number of alternative schemes of modulating magnetization were developed. These schemes employed joint anharmonic modulation of rf frequency and field ͑either electronically 51 or mechanically 52 ͒ and parametric amplification of the spin signal.…”