Silicon Nanowires Feel the Force of Magnetic Resonance

Abstract

Viewpoint: Silicon Nanowires Feel the Force of Magnetic Resonance John Mamin and Daniel Rugar, IBM Research Division, Almaden Research Center, San Jose, CA 94024, USA Published February 13, 2012 | Physics 5, 20 (2012) | DOI: 10.1103/Physics.5.20 Silicon nanowire sensors offer a route to improving the sensitivity and spatial resolution of magnetic resonance force microscopy. Nanomechanical detection of nuclear magnetic resonance using a silicon nanowire oscillator John M. Nichol, Eric R. Hemesath, Lincoln J. Lauhon, and Raffi Budakian Phys. Rev. B 85, 054414 (2012) Published February 13, 2012 | PDF (free) +Enlarge image Figure 1 APS/Alan Stonebraker Figure 1 A silicon nanowire is used as an ultrasensitive cantilever in order to detect nuclear spins via magnetic resonance. A thin layer of polystyrene, containing weakly magnetic hydrogen nuclei, coats the end of the nanowire. A microfabricated wire serves as an electromagnet to produce a magnetic field gradient and thereby exert a force on the spins. Through a suitable modulation of the current in the wire, the interaction between the spins and the field gradient drives the silicon nanowire at its resonance frequency. The motion is then sensed with a laser. The hope is that the improved sensitivity afforded by this technique will eventually allow nanomagnetic resonance on the molecular scale. The technique of magnetic resonance force microscopy (MRFM) was conceived as a way to combine the 3D capabilities of magnetic resonance imaging (MRI) with the angstrom-scale resolution of scanning probe techniques, with the ultimate goal of imaging individual biological molecules in three dimensions [1]. The technique makes use of a sensitive mechanical oscillator to detect the minute magnetic forces associated with a small number of magnetic moments, or spins. Compared to conventional magnetic resonance detection methods that use inductive coils to sense nuclear spins, MRFM is about 100 million times more sensitive [2]. With such a dramatic improvement, it is now possible to image virus particles in 3D with 10 nanometer (nm) resolution, far beyond the millimeter resolution of medical MRI. Still, reaching the ultimate goal of single-molecule imaging will require innovations that radically enhance the technique’s sensitivity.