Decoherence of Near-Surface Nitrogen-Vacancy Centers Due to Electric Field Noise.

We show that electric field noise from surface charge fluctuations can be a significant source of spin decoherence for near-surface nitrogen-vacancy (NV) centers in diamond. This conclusion is based on the increase in spin coherence observed when the diamond surface is covered with high-dielectric-constant liquids, such as glycerol. Double-resonance experiments show that improved coherence occurs even though the coupling to nearby electron spins is unchanged when the liquid is applied.

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.

Multipulse double-quantum magnetometry with near-surface nitrogen-vacancy centers.

We discuss multipulse magnetometry that exploits all three magnetic sublevels of the S=1 nitrogen-vacancy center in diamond to achieve enhanced magnetic field sensitivity. Based on dual frequency microwave pulsing, the scheme is twice as sensitive to ac magnetic fields as conventional two-level magnetometry. We derive the spin evolution operator for dual frequency microwave excitation and show its effectiveness for double-quantum state swaps.

Nanoscale nuclear magnetic resonance with a nitrogen-vacancy spin sensor.

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.

High-gradient nanomagnets on cantilevers for sensitive detection of nuclear magnetic resonance.

Detection of magnetic resonance as a force between a magnetic tip and nuclear spins has previously been shown to enable sub-10 nm resolution 1H imaging. Maximizing the spin force in such a magnetic resonance force microscopy (MRFM) experiment demands a high field gradient. In order to study a wide range of samples, it is equally desirable to locate the magnetic tip on the force sensor.

Isotope-selective detection and imaging of organic nanolayers.

Magnetic resonance force microscopy (MRFM) makes use of the spectroscopic nature of magnetic resonance to add unambiguous elemental selectivity to scanning probe microscopy. We show isotopic selectivity of MRFM for three nuclei, (1)H, (31)P, and (13)C, in organic materials. We also detect a roughly 1 nm thick layer of naturally occurring adsorbates on a gold surface by measuring the magnetic resonance signal of the hydrogen contained in the layer.

Nuclear double resonance between statistical spin polarizations.

We demonstrate nuclear double resonance for nanometer-scale volumes of spins where random fluctuations rather than Boltzmann polarization dominate. When the Hartmann-Hahn condition is met in a cross-polarization experiment, flip-flops occur between two species of spins and their fluctuations become coupled. We use magnetic resonance force microscopy to measure this effect between 1H and 13C spins in 13C-enriched stearic acid.

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 (1)H spin density within individual tobacco mosaic virus particles sitting on a nanometer-thick layer of adsorbed hydrocarbons.

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.

Nuclear spin relaxation induced by a mechanical resonator.

We report on measurements of the spin lifetime of nuclear spins strongly coupled to a micromechanical cantilever as used in magnetic resonance force microscopy. We find that the rotating-frame correlation time of the statistical nuclear polarization is set by the magnetomechanical noise originating from the thermal motion of the cantilever. Evidence is based on the effect of three parameters: (1) the magnetic field gradient (the coupling strength), (2) the Rabi frequency of the spins (the transition energy), and (3) the temperature of the low-frequency mechanical modes.

Role of spin noise in the detection of nanoscale ensembles of nuclear spins.

When probing nuclear spins in materials on the nanometer scale, random fluctuations of the spin polarization will exceed the mean Boltzmann polarization for sample volumes below about (100 nm){3}. In this Letter, we use magnetic resonance force microscopy to observe nuclear spin fluctuations in real time. We show how reproducible measurements of the polarization variance can be obtained by controlling the spin correlation time and rapidly sampling a large number of independent spin configurations.

Feedback cooling of a cantilever's fundamental mode below 5 mK.

We cool the fundamental mechanical mode of an ultrasoft silicon cantilever from a base temperature of 2.2 K down to 2.9+/-0.

Creating order from random fluctuations in small spin ensembles.

We demonstrate the ability to create spin order by using a magnetic resonance force microscope to harness the naturally occurring statistical fluctuations in small ensembles of electron spins. In one method, we hyperpolarized the spin system by selectively capturing the transient spin order created by the statistical fluctuations. In a second method, we took a more active approach and rectified the spin fluctuations by applying real-time feedback to the entire spin ensemble.

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 micro m 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 10(12) nuclear spins for MRI-based microscopy, or 10(7) electron spins for electron spin resonance microscopy.

Suppression of spin diffusion near a micron-size ferromagnet.

We have used the large gradients generated near the ferromagnetic tip of a magnetic resonance force microscope to locally suppress spin diffusion in a silica sample containing paramagnetic electron spins. By controlling the slice location with respect to the tip, the magnetic field gradient was varied from 0.01 to 36 mT/microm, resulting in a fourfold decrease in T-11 and a similar decrease in T(-1)(1 rho).

Detection and manipulation of statistical polarization in small spin ensembles.

We report the detection of the square root of N statistical polarization in a small ensemble of electron spin centers in SiO2 by magnetic resonance force microscopy. A novel detection technique was employed that captures the statistical polarization and cycles it between states that are either locked or antilocked to the effective field in the rotating frame. Using field gradients as high as 5 G/nm, we achieved a detection sensitivity equivalent to roughly two electron spins, and observed ultralong spin-lock lifetimes, as long as 20 s.

Electron spin relaxation near a micron-size ferromagnet.

Magnetic resonance force microscopy was used to study the behavior of small ensembles of unpaired electron spins in silica near a micrometer-size ferromagnetic tip. Using a cantilever-driven spin manipulation protocol and a magnetic field gradient greater than 10(5) T/m, signals from as few as 100 net spins within a 20 nm thick resonant slice could be studied. A sixfold increase in the spin-lattice relaxation rate was found within 800 nm of the ferromagnet, while no effect due to silica surface proximity was detected.

Noncontact friction and force fluctuations between closely spaced bodies.

Noncontact friction between a Au(111) surface and an ultrasensitive gold-coated cantilever was measured as a function of tip-sample spacing, temperature, and bias voltage using observations of cantilever damping and Brownian motion. The importance of the inhomogeneous contact potential is discussed and comparison is made to measurements over dielectric surfaces. Using the fluctuation-dissipation theorem, the force fluctuations are interpreted in terms of near-surface fluctuating electric fields interacting with static surface charge.

Magnetic dissipation and fluctuations in individual nanomagnets measured by ultrasensitive cantilever magnetometry.

Cantilever magnetometry with moment resolution better than 10(4)micro(B) was used to study individual nanomagnets. By using the fluctuation-dissipation theorem to interpret measurements of field-induced cantilever damping, the low frequency spectral density of magnetic fluctuations could be determined with resolution better than 1micro(B) Hz-1/2. Cobalt nanowires exhibited significant magnetic dissipation and the associated magnetic fluctuations were found to have 1/f frequency dependence.