We report an experimental study of the longitudinal relaxation time (T1) of the electron spin associated with single nitrogen-vacancy (NV) defects hosted in nanodiamonds (ND). We first show that T1 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 T1-quenching, which demonstrates magnetic noise sensing with a single electron spin. We estimate a sensitivity down to ≈ 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 T1-based nanoscale imaging of the spin density in biological samples. PACS numbers:The ability to detect spins is the cornerstone of magnetic resonance imaging (MRI), which is currently one of the most important tools in life science. However, the sensitivity of conventional MRI techniques is limited to large spin ensembles, which in turn restricts the spatial resolution at the micrometer scale [1, 2]. Extending MRI techniques at the nanoscale can be achieved at sub-Kelvin temperature with magnetic resonance force microscopy, through the detection of weak magnetic forces [3, 4]. Another strategy consists in directly sensing the magnetic field created by spin magnetic moments with a nanoscale magnetometer. In that context, the electron spin associated with a nitrogen-vacancy (NV) defect in diamond has been recently proposed as an ultrasensitive and atomic-sized magnetic field sensor [5]. In the last years, many schemes based on dynamical decoupling pulse sequences have been devised for sensing ac or randomly fluctuating magnetic fields with a single NV spin [6][7][8][9]. These protocols recently enabled nuclear magnetic resonance measurements on a few cubic nanometers sample volume [10,11] and the detection of a single electron spin under ambient conditions [12].An alternative approach for sensing randomly fluctuating magnetic fields -i.e. magnetic noise -is based on the measurement of the longitudinal spin relaxation time (T 1 ) of the NV defect electron spin. Using an ensemble of NV defects and a T 1 -based sensing scheme, Steinert et al. recently demonstrated magnetic noise sensing with a sensitivity down to 1000 statistically polarized electron spins, as well as imaging of spin-labeled cellular structures with a diffraction-limited spatial resolution (≈ 500 nm) [13]. Bringing the spatial resolution down to few nanometers could be achieved by using a single NV defect integrated in a scanning device, e.g. with a nanodiamond (ND) attached to the tip of an atomic force microscope (AFM) [14,15]. With this application in mind, we study here the T 1 time of single NV defects hosted in NDs, as a function of ND size and magnetic environment. We first report a decrease of T 1 over three orders of magnitude when the ND size is reduced from 100 to 10 nm. This behavior is explained by ...

We demonstrate quantitative magnetic field mapping with nanoscale resolution, by applying a lock-in technique on the electron spin resonance frequency of a single nitrogen-vacancy defect placed at the apex of an atomic force microscope tip. In addition, we report an all-optical magnetic imaging technique which is sensitive to large off-axis magnetic fields, thus extending the operation range of diamond-based magnetometry. Both techniques are illustrated by using a magnetic hard disk as a test sample. Owing to the non-perturbing and quantitative nature of the magnetic probe, this work should open up numerous perspectives in nanomagnetism and spintronics.The ability to map magnetic field distributions with high sensitivity and nanoscale resolution is of crucial importance for fundamental studies ranging from material science to biology, as well as for the development of new applications in spintronics and quantum technology [1][2][3]. In that context, an ideal scanning probe magnetometer should provide quantitative magnetic field mapping at the nanoscale under ambient conditions. In addition, the magnetic sensor should not introduce a significant magnetic perturbation of the probed sample.Over the last decades, different roads have been taken towards ultra-sensitive detection of magnetic fields including superconducting quantum interference devices (SQUIDs) [1], semiconductor-based Hall probes [1] and optical magnetometers [2]. Even though extremely high sensitivity has been achieved with these devices, their spatial resolution remains limited at the micron-scale. Prominent approaches to reach nanoscale resolution are scanning-tunneling microscopy [4], mechanical detection of magnetic resonance [5], nanoSQUIDs [6], X-ray microscopy [7] and magnetic force microscopy (MFM) [8]. Since the latter technique operates under ambient conditions without any specific sample preparation, it is now routinely used for mapping magnetic field gradients around magnetic nanostructures. However, besides introducing an inevitable perturbation of the studied magnetic sample owing to the intrinsic magnetic nature of the probe [7], MFM does not provide quantitative information about the magnetic field distribution.Here we follow a recently proposed approach to magnetic sensing based on optically detected electron spin resonance (ESR) [10]. It was shown that this method applied to a single nitrogen-vacancy (NV) defect in diamond could provide an unprecedented combination of spatial resolution and magnetic sensitivity under ambient conditions [6,11,12,14]. The principle of the measurement * Electronic address: vjacques@lpqm.ens-cachan.fr is similar to the one used in optical magnetometers based on the precession of spin-polarized atomic gases [2]. The applied magnetic field is evaluated by measuring the Zeeman shifts of the NV defect spin sublevels. In this article we demonstrate quantitative magnetic field mapping with nanoscale resolution, by applying a lock-in technique on the ESR frequency of a single NV defect placed at the apex of ...