Magnetometry with nitrogen-vacancy defects in diamond

The isolated electronic spin system of the Nitrogen-Vacancy (NV) centre in diamond offers unique possibilities to be employed as a nanoscale sensor for detection and imaging of weak magnetic fields. Magnetic imaging with nanometric resolution and field detection capabilities in the nanotesla range are enabled by the atomic-size and exceptionally long spin-coherence times of this naturally occurring defect. The exciting perspectives that ensue from these characteristics have triggered vivid experimental activities in the emerging field of "NV magnetometry". It is the purpose of this article to review the recent progress in high-sensitivity nanoscale NV magnetometry, generate an overview of the most pertinent results of the last years and highlight perspectives for future developments. We will present the physical principles that allow for magnetic field detection with NV centres and discuss first applications of NV magnetometers that have been demonstrated in the context of nano magnetism, mesoscopic physics and the life sciences.Comment: Review article, 28 pages, 16 figure

Quantitative stray field imaging of a magnetic vortex core

Thin-film ferromagnetic disks present a vortex spin structure whose dynamics, added to the small size (~10 nm) of their core, earned them intensive study. Here we use a scanning nitrogen-vacancy (NV) center microscope to quantitatively map the stray magnetic field above a 1 micron-diameter disk of permalloy, unambiguously revealing the vortex core. Analysis of both probe-to-sample distance and tip motion effects through stroboscopic measurements, allows us to compare directly our quantitative images to micromagnetic simulations of an ideal structure. Slight perturbations with respect to the perfect vortex structure are clearly detected either due to an applied in-plane magnetic field or imperfections of the magnetic structures. This work demonstrates the potential of scanning NV microscopy to map tiny stray field variations from nanostructures, providing a nanoscale, non-perturbative detection of their magnetic texture.Comment: 5 pages, 4 figure

Measuring the magnetic moment density in patterned ultrathin ferromagnets with submicron resolution

We present a new approach to infer the surface density of magnetic moments $I_s$ in ultrathin ferromagnetic films with perpendicular anisotropy. It relies on quantitative stray field measurements with an atomic-size magnetometer based on the nitrogen-vacancy center in diamond. The method is applied to microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements of $I_s$ with a few percent uncertainty and a spatial resolution in the range of $(100$ nm)$^2$, an improvement by several orders of magnitude over existing methods. As an example of application, we measure the modifications of $I_s$ induced by local irradiation with He$^+$ ions in an ultrathin ferromagnetic wire. This method offers a new route to study variations of magnetic properties at the nanoscale.Comment: 9 pages and 7 figures including main text and Supplemental Informatio

Spin relaxometry of single nitrogen-vacancy defects in diamond nanocrystals for magnetic noise sensing

We report an experimental study of the longitudinal relaxation time ($T_1$) of the electron spin associated with single nitrogen-vacancy (NV) defects hosted in nanodiamonds (ND). We first show that $T_1$ decreases over three orders of magnitude when the ND size is reduced from 100 to 10 nm owing to the interaction of the NV electron spin with a bath of paramagnetic centers lying on the ND surface. We next tune the magnetic environment by decorating the ND surface with Gd$^{3+}$ ions and observe an efficient $T_{1}$-quenching, which demonstrates magnetic noise sensing with a single electron spin. We estimate a sensitivity down to $\approx 14$ electron spins detected within 10 s, using a single NV defect hosted in a 10-nm-size ND. These results pave the way towards $T_1$-based nanoscale imaging of the spin density in biological samples.Comment: Main text with 4 figures together with supplemental informatio

The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry

The recent observation of current-induced domain wall (DW) motion with large velocity in ultrathin magnetic wires has opened new opportunities for spintronic devices. However, there is still no consensus on the underlying mechanisms of DW motion. Key to this debate is the DW structure, which can be of Bloch or N\'eel type, and dramatically affects the efficiency of the different proposed mechanisms. To date, most experiments aiming to address this question have relied on deducing the DW structure and chirality from its motion under additional in-plane applied fields, which is indirect and involves strong assumptions on its dynamics. Here we introduce a general method enabling direct, in situ, determination of the DW structure in ultrathin ferromagnets. It relies on local measurements of the stray field distribution above the DW using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire and find clear signature of pure Bloch DWs. In contrast, we observe left-handed N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co interface. This method offers a new path for exploring interfacial DMI in ultrathin ferromagnets and elucidating the physics of DW motion under current.Comment: Main text and Supplementary Information, 33 pages and 12 figure

Stray-field imaging of magnetic vortices with a single diamond spin

Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with nanometre scale spatial resolution remains an outstanding challenge. Recently, a technique has emerged that employs a single nitrogen-vacancy defect in diamond as an atomic-size magnetometer, which promises significant advances. However, the effectiveness of the technique when applied to magnetic nanostructures remains to be demonstrated. Here we use a scanning nitrogen-vacancy magnetometer to image a magnetic vortex, which is one of the most iconic objects of nanomagnetism, owing to the small size (~10 nm) of the vortex core. We report three-dimensional, vectorial and quantitative measurements of the stray magnetic field emitted by a vortex in a ferromagnetic square dot, including the detection of the vortex core. We find excellent agreement with micromagnetic simulations, both for regular vortex structures and for higher-order magnetization states. These experiments establish scanning nitrogen-vacancy magnetometry as a practical and unique tool for fundamental studies in nanomagnetism