Daniel Rugar

Single spin detection by magnetic resonance force microscopy.

Magnetic resonance imaging (MRI) is well known as a powerful technique for visualizing subsurface structures with three-dimensional spatial resolution. Pushing the resolution below 1 µm remains a major challenge, however, owing to the sensitivity limitations of conventional inductive detection techniques. Currently, the smallest volume elements in an image must contain at least 1012 nuclear spins for MRI-based microscopy, or 107 electron spins for electron spin resonance microscopy. Magnetic resonance force microscopy (MRFM) was proposed as a means to improve detection sensitivity to the single-spin level, and thus enable three-dimensional imaging of macromolecules (for example, proteins) with atomic resolution. MRFM has also been proposed as a qubit readout device for spin-based quantum computers. Here we report the detection of an individual electron spin by MRFM. A spatial resolution of 25 nm in one dimension was obtained for an unpaired spin in silicon dioxide. The measured signal is consistent with a model in which the spin is aligned parallel or anti-parallel to the effective field, with a rotating-frame relaxation time of 760 ms. The long relaxation time suggests that the state of an individual spin can be monitored for extended periods of time, even while subjected to a complex set of manipulations that are part of the MRFM measurement protocol.

Quality factors in micron- and submicron-thick cantilevers

Micromechanical cantilevers are commonly used for detection of small forces in microelectromechanical sensors (e.g., accelerometers) and in scientific instruments (e.g., atomic force microscopes). A fundamental limit to the detection of small forces is imposed by thermomechanical noise, the mechanical analog of Johnson noise, which is governed by dissipation of mechanical energy. This paper reports on measurements of the mechanical quality factor Q for arrays of silicon-nitride, polysilicon, and single-crystal silicon cantilevers. By studying the dependence of Q on cantilever material, geometry, and surface treatments, significant insight into dissipation mechanisms has been obtained. For submicron-thick cantilevers, Q is found to decrease with decreasing cantilever thickness, indicating surface loss mechanisms. For single-crystal silicon cantilevers, significant increase in room temperature Q is obtained after 700/spl deg/C heat treatment in either N/sub 2/ Or forming gas. At low temperatures, silicon cantilevers exhibit a minimum in Q at approximately 135 K, possibly due to a surface-related relaxation process. Thermoelastic dissipation is not a factor for submicron-thick cantilevers, but is shown to be significant for silicon-nitride cantilevers as thin as 2.3 /spl mu/m.

Near‐field optical data storage using a solid immersion lens

A near‐field optical technique, using a new type of solid immersion lens (SIL), has been developed and applied to the writing and reading of domains in magneto‐optic material. The SIL is a truncated glass sphere which serves to increase the numerical aperture of the optical system by n2, where n is the index of refraction of the lens material. Using a SIL made from n=1.83 glass and illuminating with 780 nm light, we have achieved a 317 nm spot size. We have resolved a 500 nm period grating, and written and read 350 nm diameter magnetic domains. The technique should be capable of a 125 nm focused spot size using blue light.

Attonewton force detection using ultrathin silicon cantilevers

A measured force resolution of 5.6×10−18 N/Hz at 4.8 K in vacuum using a single-crystal silicon cantilever only 600 A thick is demonstrated. The spring constant of this cantilever was 6.5×10−6 N/m, or more than 1000 times smaller than that of typical atomic force microscope cantilevers. The cantilever fabrication includes the integration of in-line tips so that the cantilever can be oriented perpendicular to a sample surface. This orientation helps suppress cantilever snap-in so that high force sensitivity can be realized for tip-sample distances less than 100 A.

Nanoscale Nuclear Magnetic Resonance with a Nitrogen-Vacancy Spin Sensor

Nanoscale NMR with Diamond Defects Although nuclear magnetic resonance (NMR) methods can be used for spatial imaging, the low sensitivity of detectors limits the minimum sample size. Two reports now describe the use of near-surface nitrogen-vacancy (NV) defects in diamond for detecting nanotesla magnetic fields from very small volumes of material (see the Perspective by Hemmer). The spin of the defect can be detected by changes in its fluorescence, which allows proton NMR of organic samples only a few nanometers thick on the diamond surface. Mamin et al. (p. 557) used a combination of electron spin echoes and pulsed NMR manipulation of the proton spins to detect the very weak fields. Staudacher et al. (p. 561) measured statistical polarization of a population of about 104 spins near the NV center with a dynamical decoupling method. The optical response of the spin of a near-surface atomic defect in diamond can be used to sense proton magnetic fields. [Also see Perspective by Hemmer] Extension of nuclear magnetic resonance (NMR) to nanoscale samples has been a longstanding challenge because of the insensitivity of conventional detection methods. We demonstrated the use of an individual, near-surface nitrogen-vacancy (NV) center in diamond as a sensor to detect proton NMR in an organic sample located external to the diamond. Using a combination of electron spin echoes and proton spin manipulation, we showed that the NV center senses the nanotesla field fluctuations from the protons, enabling both time-domain and spectroscopic NMR measurements on the nanometer scale.

Nanoscale magnetic resonance imaging

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 1H 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.

Force detection of nuclear magnetic resonance

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 1013 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.

Nuclear magnetic resonance imaging with 90-nm resolution

Magnetic resonance imaging (MRI) is a powerful imaging technique that typically operates on the scale of millimetres to micrometres. Conventional MRI is based on the manipulation of nuclear spins with radio-frequency fields, and the subsequent detection of spins with induction-based techniques. An alternative approach, magnetic resonance force microscopy (MRFM), uses force detection to overcome the sensitivity limitations of conventional MRI. Here, we show that the two-dimensional imaging of nuclear spins can be extended to a spatial resolution better than 100 nm using MRFM. The imaging of 19F nuclei in a patterned CaF(2) test object was enabled by a detection sensitivity of roughly 1,200 nuclear spins at a temperature of 600 mK. To achieve this sensitivity, we developed high-moment magnetic tips that produced field gradients up to 1.4 x 10(6) T m(-1), and implemented a measurement protocol based on force-gradient detection of naturally occurring spin fluctuations. The resulting detection volume was less than 650 zeptolitres. This is 60,000 times smaller than the previous smallest volume for nuclear magnetic resonance microscopy, and demonstrates the feasibility of pushing MRI into the nanoscale regime.

Proton magnetic resonance imaging using a nitrogen–vacancy spin sensor

Magnetic resonance imaging, with its ability to provide three-dimensional, elementally selective imaging without radiation damage, has had a revolutionary impact in many fields, especially medicine and the neurosciences. Although challenging, its extension to the nanometre scale could provide a powerful new tool for the nanosciences, especially if it can provide a means for non-destructively visualizing the full three-dimensional morphology of complex nanostructures, including biomolecules. To achieve this potential, innovative new detection strategies are required to overcome the severe sensitivity limitations of conventional inductive detection techniques. One successful example is magnetic resonance force microscopy, which has demonstrated three-dimensional imaging of proton NMR with resolution on the order of 10 nm, but with the requirement of operating at cryogenic temperatures. Nitrogen-vacancy (NV) centres in diamond offer an alternative detection strategy for nanoscale magnetic resonance imaging that is operable at room temperature. Here, we demonstrate two-dimensional imaging of (1)H NMR from a polymer test sample using a single NV centre in diamond as the sensor. The NV centre detects the oscillating magnetic field from precessing protons as the sample is scanned past the NV centre. A spatial resolution of ∼12 nm is shown, limited primarily by the scan resolution.

Silicon dopant imaging by dissipation force microscopy

Noncontact damping of a cantilever vibrating near a silicon surface was used to measure localized electrical dissipation. The dependence of the damping on tip-sample distance, applied voltage, carrier mobility, and dopant density was studied for n- and p-type silicon samples with dopant densities of 1014–1018 cm−3. Dopant imaging with 150 nm spatial resolution was demonstrated.