Nanoscale magnetic resonance imaging

Abstract

We have combined ultrasensitive magnetic resonance force microscopy (MRFM) with 3D image reconstruction to achieve magnetic resonance imaging (MRI) with resolution <10 nm. The image reconstruction converts measured magnetic force data into a 3D map of nuclear spin density, taking advantage of the unique characteristics of the "resonant slice" that is projected outward from a nanoscale magnetic tip. The basic principles are demonstrated by imaging the (1)H spin density within individual tobacco mosaic virus particles sitting on a nanometer-thick layer of adsorbed hydrocarbons. This result, which represents a 100 million-fold improvement in volume resolution over conventional MRI, demonstrates the potential of MRFM as a tool for 3D, elementally selective imaging on the nanometer scale.

Fig. 1.

Configuration of MRFM apparatus. (A) Tobacco mosaic virus particles, attached to the end of an ultrasensitive silicon cantilever, are positioned close to a magnetic tip. A rf current i_rf passing through a copper microwire generates an alternating magnetic field B_rf that induces magnetic resonance in the ¹H spins of the virus particles. The resonant slice represents those points in space where the field from the magnetic tip (plus an external field) matches the condition for magnetic resonance. Three-dimensional scanning of the tip with respect to the cantilever, followed by image reconstruction is used to generate a 3D image of the spin density in the virus sample. (B) Scanning electron micrograph of the end of the cantilever. Individual tobacco mosaic virus particles are visible as long, dark rods on the 0.8-μm × 1.3-μm-sized sample platform. (C) Detail of the magnetic tip.

Fig. 2.

Details of the resonant slice and associated point spread function (PSF). (A) Three-dimensional representation corresponding to the conditions B_res = ω₀/γ = 2.697 T and B_ext = 2.482 T. The center of the tip apex is assumed to be at the origin of the coordinate system. The resonant slice is the hemispherical “shell” outlined in red, representing the region of space for which B₀(r) lies within B_res ± Δω_rf,peak/γ (here 2.697 ± 0.014 T). Regions to the left and right of the tip (shaded red) contribute most to the signal because this is where the lateral gradient G(r) is largest. (B) Cross-sections of the point spread function at the 4 tip-sample spacings used in the imaging experiment. The PSF was calculated by using Eq. 2, assuming A = 1. The color scale reflects the force variance per spin (zN = zeptonewton = 10⁻²¹ N). The size of the tip apex (r_a = 100 nm) is indicated by a dotted circle. At z = 24 nm, the PSF lobe thickness reaches a minimum of ≈4.8 nm (FWHM). The small left–right asymmetry is due to a slight (1.7°) tilt of the sample plane with respect to the magnetic tip.

Fig. 3.

Spin signal scan data and resulting 3D reconstruction of the hydrogen (proton) density distribution. (A) Raw scan data presented as x–y scans of the spin signal at 4 different tip-sample spacings. Pixel spacing is 8.3 nm × 16.6 nm in x × y, respectively. Each data point represents the spin signal variance obtained during a 1-min integration. B_ext = 2.482 T. (B) A more finely sampled line scan showing 4-nm lateral resolution. The scanned region is indicated by the dashed line in A. B_ext = 2.432 T. (C) Reconstructed 3D ¹H spin density. Black represents very low or zero density of hydrogen, whereas white is high hydrogen density. The image is the result of the Landweber reconstruction, followed by a 5-nm smoothing filter. (D) Horizontal slice of C, showing several TMV fragments. (E) Scanning electron micrograph of the same region. (F) Cross-section showing 2 TMV particles on top of a hydrogen-rich background layer adsorbed on the Au surface. (G) Reconstruction is improved if this background layer is included as a priori information by assuming a thin, uniform plane of ¹H density as the starting point of the reconstruction.

Fig. 4.

Imaging results for a second sample region. (A) Three-dimensional reconstruction of ¹H spin density for virus particles sitting on adsorbed layer of hydrocarbons. (B) Representative horizontal slice from the 3D reconstruction showing the distribution of hydrogen in the plane located 13 nm above the hydrocarbon layer. Several virus particles are evident. (C) Corresponding scanning electron micrograph.