Single spins in diamond as local probes for magnetic fields

The nitrogen-vacancy (NV) center has been demonstrated to be a robust magnetic field sensor with high sensitivity and spatial resolution. We illustrate the applications and challenges of using single NV centers to image magnetic features both at ambient conditions and at low temperatures. In addition, we show different types of scanning probes including tips fabricated on bulk diamond membranes. The NV center is a point defect in diamond composed of a nitrogen atom next to a vacancy in a diamond lattice. The result of this configuration is a spin-1 defect that can be addressed and readout optically 1 . Here, we will demonstrate using single NV centers as scanning probes to detect the magnetic fields of individual FeCo nanoparticles. A single nanodiamond housing an NV center was attached to the end of a commercial atomic force microscope (AFM) cantilever 2 . Using a homebuilt AFM with confocal microscopy, the NV center was scanned over the FeCo nanoparticle at a fixed height while the electron paramagnetic resonance (EPR) of the NV center was measured optically. The resulting image is shown in Figure 1. Fitting the experimental image with the calculated field profile of the nanoparticle indicated that the NV-nanoparticle distance was ~ 25nm. Figure 1. Magnetic field profile of a FeCo particle (diameter < 100nm) with a static 30 mT magnetization field. The image was taken using a single NV center in a nanodiamond embedded at the tip of an AFM cantilever. In addition to scanning magnetometry under ambient conditions, we are interested in utilizing the NV centers to study nanoscale condensed matter physics at low temperatures. We used single NV centers in nanodiamonds to study the local magnetization dynamics of the helimagnet FeGe. FeGe is a skyrmion type material with potential applications in data processing and as information carriers. For such applications to be successful however, the local magnetization dynamics of the material must be understood and well controlled. To study these dynamics, we used both stationary NV magnetometry and magnetic force microscopy (MFM) to probe the helimagnetic ground state of FeGe 3 . The results are shown in Figure 2. The helimagnetic structure (λ~70 nm) can be obtained with MFM. With the NV center, we were able to measure the Néel temperature, TN, by detecting the onset of the magnetic stray field from the helimagnetic phase. Finally, we observed stochastic jump-like collective movements of the spin texture with both MFM and the NV center at all temperatures below TN and a drop in the NV contrast at TN. This drop in contrast may be due to dynamic magnetic fluctuations or transient local magnetic disorder with very large fields. Figure 2. NV magnetometry with single NV centers in nanodiamonds spread on the FeGe surface (left). Stationary NV magnetometry of the helical state of FeGe (middle) and scanning MFM of the same material (right). The EPR was monitored by optically detected magnetic resonance. In both cases, stochastic jumps in the local magnetization were observed (white arrows). One of the biggest challenges of scanning NV magnetometry is preparing the diamond probes. Although NV centers in nanodiamonds are easier to prepare, they have several shortcomings including low stability, poor spin properties (e.g. short T1 and T2), and low collection efficiency. One possible solution to these problems is using scanning diamond tips on bulk diamond substrate with shallowly implanted NV centers. Careful control of the shape and size of the diamond pillars leads to photon flux rates of up to 10 6 s -1 while having no impact on the spin properties 4 . We describe our efforts in making similar tips on platforms that can be attached to cantilevers for scanning (Figure 3). Figure 3. Diamond pillars with shallowly implanted NV centers (left) and the same pillars fabricated onto breakable levers for scanning use (right). REFERENCES 1. Schirhagl, R. et. al., “Nitrogen-Vacancy centers in diamond: nanoscale sensors for physics and biology,” Annu. Rev. Phys. Chem., Vol. 65, 83-105, 2014. 2. Degen, C. L., “Scanning magnetic field microscope with diamond single-spin sensor,” Appl. Phys. Lett., Vol. 92, 243111, 2008. 3. Dussaux, A. et. al., “Observation of local magnetization dynamics in the helimagnet FeGe,” arXiv:1503.06622 4. Momenzadeh, S. A. et. al., “Nanoengineered diamond waveguide as a robust bright platform for nanomagnetometry using shallow nitrogen vacancy centers,” Nano. Lett., Vol. 15, No. 1, 165-169, 2015.