Scanning gradiometry with a single spin quantum magnetometer

Abstract

Quantum sensors based on spin defects in diamond have recently enabled detailed imaging of nanoscale magnetic patterns, such as chiral spin textures, two-dimensional ferromagnets, or superconducting vortices, based on a measurement of the static magnetic stray field. Here, we demonstrate a gradiometry technique that significantly enhances the measurement sensitivity of such static fields, leading to new opportunities in the imaging of weakly magnetic systems. Our method relies on the mechanical oscillation of a single nitrogen-vacancy center at the tip of a scanning diamond probe, which up-converts the local spatial gradients into ac magnetic fields enabling the use of sensitive ac quantum protocols. We show that gradiometry provides important advantages over static field imaging: (i) an order-of-magnitude better sensitivity, (ii) a more localized and sharper image, and (iii) a strong suppression of field drifts. We demonstrate the capabilities of gradiometry by imaging the nanotesla fields appearing above topographic defects and atomic steps in an antiferromagnet, direct currents in a graphene device, and para- and diamagnetic metals.

Imaging ferroelectric domains with a single-spin scanning quantum sensor

“…In this work, we apply scanning NV microscopy to map static electric stray fields above surfaces with sub-100 nm resolution. Using mechanical oscillation of the tip 12 to overcome the challenges of static screening 19 and low coupling to electric fields 10,11 , we reach an excellent sensitivity of 0.24 kV cm −1 Hz −1/2 , on a par with the sensitivities demonstrated for a.c. field detection in bulk diamond 11 . We illustrate the impact of our approach by imaging patterned domains in application-relevant 6 ferroelectric thin films and by mapping the natural domain configuration in a prototypical improper ferroelectric.…”

“…Here, we use a scanning nitrogen-vacancy (NV) microscope, well known for its use in magnetometry 9 , to image domain patterns in piezoelectric (Pb[Zr 0.2 Ti 0.8 ]O 3 ) and improper ferroelectric (YMnO 3 ) materials through their electric fields. Electric field detection is enabled by measuring the Stark shift of the NV spin 10,11 using a gradiometric detection scheme 12 . Analysis of the electric field maps allows us to discriminate between different types of surface charge distributions, as well as to reconstruct maps of the three-dimensional electric field vector and charge density.…”

“…This a.c. electric signal is proportional to the electric field gradient in the oscillation direction 12 . In addition to alleviating static screening, the a.c. detection also improves sensitivity by at least an order of magnitude 11,12,18 . For the spin-echo pulse sequence shown in Fig.…”

“…Owing to our gradiometric detection scheme, the signal is maximum near vertical edges of the pattern. This directionality reflects the horizontal oscillation direction of the sensor, and other oscillation directions (such as a tapping mode) may be used to acquire different spatial signatures 12 . To verify that the signal is indeed due to electric fields, we purposely misalign the bias field to θ B = 95° (Supplementary Fig.…”

Diamond surface engineering for molecular sensing with nitrogen—vacancy centers

“…This phenomenon can be exploited for sensing environmental noise via changes in coherence at specific frequencies, essentially using the NV as a magnetic spectrum analyzer. Such sequences have also been used in scanning probe experiments to detect signals originating from AC-modulated single spins, 328 currents, 329 magnetic gradients 330,331 and electric fields. 84 For noise with frequency f , refocusing will occur for sequences with τ = 1/(2 f ) as well as for harmonics τ ′ = N /(2 f ), where N = 2 n + 1.…”