Magnetic Resonance Force Microscopy

The proposal of magnetic resonance force microscopy (MRFM) and its subsequent realization combine the physics of magnetic resonance imaging (MRI) with the techniques of scanning probe microscopy. Driven by the ultimate goal of imaging a single nuclear spin and the promise of a molecular structure microscope, such work is being pursued in a handful of laboratories world-wide [1]. The promise of this technique was demonstrated by a landmark experiment in 2004 in which an IBM group led by Dr. Dan Rugar measured the force of flipping a single isolated electron spin using a nanoscale cantilever [2]. This achievement was not only among the first single-spin measurements, but also showed the versatility of nanoscale force sensors beyond conventional applications such as atomic force microscopy (AFM) and magnetic force microscopy (MFM). Progress continued with the report in 2009 of three-dimensional MRI of proton spins in a biological specimen (tobacco mosaic virus particles) with a spatial resolution under 5 nanometers [3].

We continue to work on improving MRFM detection sensitivity [4] and on applying the method to nanometer-scale volumes of quadrupolar nuclear spins [5]. Furthermore, we have been using this technique to study the peculiar regime of nanometer-scale spin ensembles, in which statistical fluctuations in the nuclear polarization dominate the thermal polarization [6]. In particular, we have recently shown that by monitoring, controlling, and capturing statistical spin fluctuations, we can produce polarizations that are both larger and have a narrower distribution than the thermal polarization [7]. Such schemes may provide a route for enhancing the sensitivity to the weak magnetic signals produced by nanometer-scale volumes. In addition, since at the nanometer-scale spin noise dominates and has a random phase, such an initialization protocol is required for most nuclear magnetic resonance (NMR) measurements to be implemented.

Another area potentially impacted by this work concerns the realization of quantum information processing with solid-state electron spins. The dephasing time of qubit spins is often limited by the random nuclear field distribution in the host material. Our ability to initialize and narrow nanoscale nuclear spin distributions could extend these times and thus facilitate protocols for quantum information processing. In particular, the size of nuclear spin ensembles accessible by MRFM, ~105, is roughly equivalent to the number of nuclear spins contained in a typical solid-state quantum dot (QD).

Finally, our demonstrated ability to detect nanometer-scale volumes of Ga and As nuclei using MRFM [5] shows that — in principle — sub-surface, isotopically selective imaging of semiconductors nanostructures is possible. For this reason, we are currently working on nanoMRI by MRFM in a variety III-V nanostructures. This prospect is particularly exciting since conventional methods such as scanning electron microscopy (SEM) and tunnelling electron microscopy (TEM) lack isotopic contrast.

 

[1] M. Poggio and C. L. Degen, Nanotechnology 21, 342001 (2010).
[2] D. Rugar, et al., Nature 430, 329 (2004).
[3] C. L. Degen et al., Proc. Nat. Acad. Sci. U.S.A. (2009).
[4] F. Xue et al., Appl. Phys. Lett. 98, 163103 (2011).
[5] F. Xue et al., Phys. Rev. B 84, 205328 (2011)
[6] P. Herzog et al., Appl. Phys. Lett. 105, 043112 (2014).
[7] P. Peddibhotla et al., Nature Phys. 9, 631 (2013).