Crystal defects in diamond have emerged as unique objects for a variety of applications, both because they are very stable and because they have interesting optical properties. Embedded in nanocrystals, they can serve, for example, as robust single-photon sources or as fluorescent biomarkers of unlimited photostability and low cytotoxicity. The most fascinating aspect, however, is the ability of some crystal defects, most prominently the nitrogen-vacancy (NV) center, to locally detect and measure a number of physical quantities, such as magnetic and electric fields. This metrology capacity is based on the quantum mechanical interactions of the defect's spin state. In this review, we introduce the new and rapidly evolving field of nanoscale sensing based on single NV centers in diamond. We give a concise overview of the basic properties of diamond, from synthesis to electronic and magnetic properties of embedded NV centers. We describe in detail how single NV centers can be harnessed for nanoscale sensing, including the physical quantities that may be detected, expected sensitivities, and the most common measurement protocols. We conclude by highlighting a number of the diverse and exciting applications that may be enabled by these novel sensors, ranging from measurements of ion concentrations and membrane potentials to nanoscale thermometry and single-spin nuclear magnetic resonance.

We investigate spin and optical properties of individual nitrogen-vacancy centers located within 1-10 nm from the diamond surface. We observe stable defects with a characteristic optically detected magnetic resonance spectrum down to lowest depth. We also find a small, but systematic spectral broadening for defects shallower than about 2 nm. This broadening is consistent with the presence of a surface paramagnetic impurity layer [Tisler et al., ACS Nano 3, 1959] largely decoupled by motional averaging. The observation of stable and well-behaved defects very close to the surface is critical for single-spin sensors and devices requiring nanometer proximity to the target.

We report on Fourier spectroscopy experiments performed with near-surface nitrogen-vacancy centers in a diamond chip. By detecting the free precession of nuclear spins rather than applying a multipulse quantum sensing protocol, we are able to unambiguously identify the NMR species devoid of harmonics. We further show that by engineering different Hamiltonians during free precession, the hyperfine coupling parameters as well as the nuclear Larmor frequency can be selectively measured with high precision (here 5 digits). The protocols can be combined to demonstrate two-dimensional Fourier spectroscopy. The technique will be useful for mapping nuclear coordinates in molecules en route to imaging their atomic structure.Nitrogen-vacancy (NV) centers in diamond have opened exciting perspectives for the ultrasensitive detection of nuclear magnetic resonance (NMR), with possible applications to molecular structure imaging and chemical nanoanalytics [1][2][3]. NMR signals are detected by placing an analyte on a diamond chip engineered with a surface layer of NV centers, and measuring the weak magnetic dipole fields of nuclei via optically detected magnetic resonance [4,5]. Examples of the rapid recent progress in NV-NMR include the detection of small numbers of nuclei within voxels of a few (nm) 3 [6,7], the detection of multiple nuclear isotopes [8,9] and naturally occurring adsorption layers [6], the observation of surface diffusion and molecular motion [10,11], scanning imaging with < 20 nm spatial resolution [9,12], and the spatial mapping of up to 8 internal 13 C nuclei [13]. One of the far goals of NV-NMR is the detection and three-dimensional localization of individual nuclei in single molecules deterministically placed on the diamond chip [1,13,14].Sensitive detection of nuclear magnetic signals is possible with multipulse sequences that consist of a series of π pulses (see Fig. 1a). These sequences act like a narrow-band lock-in amplifier [15] whose demodulation frequency f = 1/(2τ ) is set by the delay time τ between the pulses [14,16,17]. By varying τ a frequency spectrum of the magnetic field can be recorded. Multipulse spectroscopy of NMR signals has been reported for many nuclear isotopes, including 1 H, 13 C, 14 N, 15 N, 19 F, and possibly 29 Si and 31 P [3, 6-9, 12, 18]. These experiments have, however, also revealed some important shortcomings of the method, including a modest spectral resolution [2,19] and ambiguities in peak assignments due to signal harmonics [18]. The fundamental reason for both effects is the indirect way nuclear spin signals are detected via their influence on the electronic spin.A more natural way for measuring NMR signals is to observe the free nuclear precession in the absence of microwave or radio-frequency pulses, reminiscent of the "free induction decay" in conventional NMR Fourier spectroscopy. The free nuclear precession can be detected by performing two consecutive nuclear spin measurements and incrementing the duration t 1 between the measurements. Mamin et al. [2] have ...

Chiral magnetic interactions induce complex spin textures including helical and conical spin spirals, as well as particle-like objects such as magnetic skyrmions and merons. These spin textures are the basis for innovative device paradigms and give rise to exotic topological phenomena, thus being of interest for both applied and fundamental sciences. Present key questions address the dynamics of the spin system and emergent topological defects. Here we analyse the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining magnetic force microscopy, single-spin magnetometry and Landau–Lifschitz–Gilbert simulations we show that the nanoscale dynamics are governed by the depinning and subsequent motion of magnetic edge dislocations. The motion of these topologically stable objects triggers perturbations that can propagate over mesoscopic length scales. The observation of stochastic instabilities in the micromagnetic structure provides insight to the spatio-temporal dynamics of itinerant helimagnets and topological defects, and discloses open challenges regarding their technological usage.

Charge transport in nanostructures and thin films is fundamental to many phenomena and processes in science and technology, ranging from quantum effects and electronic correlations in mesoscopic physics, to integrated charge-or spin-based electronic circuits, to photoactive layers in energy research. Direct visualization of the charge flow in such structures is challenging due to their nanometer size and the itinerant nature of currents. In this work, we demonstrate non-invasive magnetic imaging of current density in two-dimensional conductor networks including metallic nanowires and carbon nanotubes. Our sensor is the electronic spin of a diamond nitrogen-vacancy center attached to a scanning tip. Using a differential measurement technique, we detect DC currents down to a few µA above a baseline current density of ∼ 2 · 10 4 A/cm 2 . Reconstructed images have a spatial resolution of typically 50 nm, with a best-effort value of 22 nm. Current density imaging offers a new route for studying electronic transport and conductance variations in two-dimensional materials and devices, with many exciting applications in condensed matter physics.