In the seminal virus-imaging experiment carried out by Christian Degen, Dan Rugar, and coworkers at IBM [1], a magnetic resonance force microscope was used to detect and image nuclear spin fluctuations. Because the number of nuclear spins observed in their experiment was so small, the signal from spin magnetization fluctuations was much larger that the signal from the average Curie-law magnetization. It is remarkable that the IBM team was able to detect spin fluctuations and then harness them perform nanoscale magnetic resonance imaging, “nano-MRI”. One way to improve the sensitivity and resolution of the nano-MRI experiment is to eschew detecting spin fluctuations and instead observe the average spin magnetization. This strategy requires significantly improving the spin polarization [2].
In a previous paper, Corinne Isaac and coworkers in the Marohn group showed how to increase the average spin polarization of protons in a magnetic resonance force microscope experiment using the dynamic nuclear polarization (DNP) effect [3]. One challenge that Corinne had to address in this experiment was aligning, in vacuum at cryogenic temperatures, a fragile magnet-tipped cantilever to a coplanar waveguide whose centerline was only < 10 μm wide. Corinne has now published a paper in the Review of Scientific Instruments documenting numerous alignment techniques that she and her collaborators developed for use in nano-MRI and related experiments.
Isaac, C. E.; Curley, E. A.; Nasr, P. T.; Nguyen, H. L. & Marohn, J. A.“Cryogenic Positioning and Alignment With Micrometer Precision in a Magnetic Resonance Force Microscope”, Rev. Sci. Instrum., 2018, 89(1), 013707 [10.1063/1.5008505][arXiv:1710.01442].
The paper’s abstract reads
Aligning a microcantilever to an area of interest on a sample is a critical step in many scanning probe microscopy experiments, particularly those carried out on devices and rare, precious samples. We report a series of protocols that rapidly and reproducibly align a high-compliance microcantilever to a < 10 μm sample feature under high vacuum and at cryogenic temperatures. The first set of protocols, applicable to a cantilever oscillating parallel to the sample surface, involve monitoring the cantilever resonance frequency while laterally scanning the tip to map the sample substrate through electrostatic interactions of the substrate with the cantilever. We demonstrate that when operating a cantilever a few micrometers from the sample surface, large shifts in the cantilever resonance frequency are present near the edges of a voltage-biased sample electrode. Surprisingly, these “edge-finder” frequency shifts are retained when the electrode is coated with a polymer film and a ~10 nm thick metallic ground plane. The second series of methods, applicable to any scanning probe microscopy experiment, integrate a single-optical fiber to image line scans of the sample surface. The microscope modifications required for these methods are straightforward to implement, provide reliable micrometer-scale positioning, and decrease the experimental setup time from days to hours in a vacuum, cryogenic magnetic resonance force microscope.
Corinne’s DNP experiment employed a high-compliance cantilever with a relatively large, micron-scale magnetic tip. Future experiments will employ a 100 nm diameter magnetic nanorod tip [4]. The sensitivity of these experiments will be limited by non-contact friction arising from electric field fluctuations in the sample interacting with trapped charges or patch charges on the cantilever [5]. Sensitivity can be improved by applying a thin metal coating to the sample, to act as a Faraday cage shielding the cantilever from the sample’s electric field fluctuations. In her RSI paper, Corinne shows how to align a magnet-tipped cantilever with a coplanar waveguide buried beneath a metal-coated sample. This is an exciting achievement that we will certainly use in future nano-MRFM experiments.
Congratulations Corinne!
| [1] | Degen, C. L.; Poggio, M.; Mamin, H. J.; Rettner, C. T. & Rugar, D. “Nanoscale Magnetic Resonance Imaging”, Proc. Natl. Acad. Sci. U.S.A., 2009, 106 (5), 1313–1317. [10.1073/pnas.0812068106] |
| [2] | Butler, M. C.; Weitekamp, D. P. “Sensitivity of Force-Detected NMR Spectroscopy With Resonator-Induced Polarization”, Phys. Rev. B, 2013, 87(6), 64413 [10.1103/PhysRevB.87.064413]. |
| [3] | Isaac, C. E.; Gleave, C. M.; Nasr, P. T.; Nguyen, H. L.; Curley, E. A.; Yoder, J. L.; Moore, E. W.; Chen, L. & Marohn, J. A.“Dynamic Nuclear Polarization in a Magnetic Resonance Force Microscope Experiment”, Phys. Chem. Chem. Phys., 2016, 18, 8806 - 8819 [10.1039/C6CP00084C][arXiv:1601.07253]. |
| [4] | Longenecker, J. G.; Mamin, H. J.; Senko, A. W.; Chen, L.; Rettner, C. T.; Rugar, D. & Marohn, J. A.“High-Gradient Nanomagnets on Cantilevers for Sensitive Detection of Nuclear Magnetic Resonance”, ACS Nano, 2012, 6, 9637 - 9645 [10.1021/nn3030628][PMCID:PMC3535834]. |
| [5] | Kuehn, S.; Loring, R. F. & Marohn, J. A.“Dielectric Fluctuations and the Origins of Noncontact Friction”, Phys. Rev. Lett., 2006, 96, 156103 [PhysRevLett.96.156103][PMCID:PMC1941717]. |
This work was funded by the U.S. Army Research Office and the U.S. National Science Foundation.