Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor

Abstract

High spatial resolution magnetic imaging has driven important developments in fields ranging from materials science to biology. However, to uncover finer details approaching the nanoscale with greater sensitivity requires the development of a radically new sensor technology. The nitrogenvacancy (NV) defect in diamond has emerged as a promising candidate for such a sensor based on its atomic size and quantum-limited sensing capabilities afforded by long spin coherence times. Although the NV center has been successfully implemented as a nanoscale scanning magnetic probe at room temperature, it has remained an outstanding challenge to extend this capability to cryogenic temperatures, where many solid-state systems exhibit non-trivial magnetic order. Here we present NV magnetic imaging down to 6 K with 6 nm spatial resolution and 3 μT/√Hz field sensitivity, first benchmarking the technique with a magnetic hard disk sample, then utilizing the technique to image vortices in the iron pnictide superconductor BaFe2(As0.7P0.3)2 with Tc = 30 K. The expansion of NVbased magnetic imaging to cryogenic temperatures represents an important advance in state-of-theart magnetometry, which will enable future studies of heretofore inaccessible nanoscale magnetism in condensed matter systems. and Lorentz transmission electron microscopy (TEM) 7 , and reciprocal space techniques including neutron scattering 8 have been successfully utilized to study magnetism in these systems. However, each of these techniques has limitations that must be considered. In MFM, a ferromagnetic tip must be placed in close proximity to a sample, which can perturb the magnetic order that is being probed.Scanning SQUIDs typically require a probe temperature of 10 K or lower, and generally offer micron-size spatial resolution, although recent studies have enhanced the resolution to submicron scales 9 . Lorentz TEM can provide images with high spatial resolution and magnetic contrast, but requires very thin samples, typically less than 100 nm thick. Neutron scattering requires the growth of large, high purity single-crystal samples, and is an ensemble-averaged measurement. There is therefore a significant opportunity to develop a real-space, non-invasive magnetic sensor capable of studying magnetic order at sub-10 nm spatial resolution and sub-T/Hz DC field sensitivities.The nitrogen vacancy (NV) defect center in diamond is an exceptionally versatile single spin system with unique quantum properties that have driven its application in diverse areas ranging from quantum information and photonics to quantum metrology [10][11][12][13][14][15][16][17][18][19] . Cryogenic scanning magnetometry stands out as potentially the most impactful application of NV centers, taking advantage of the exquisite magnetic field sensitivity and intrinsic atomic scale of the NV center for high resolution imaging 20 . The operation of an NV-based magnetic probe is dependent on a fundamentally different sensing principle than other imaging methods, namely the spin-dependent photolum...

Nanoscale Sensing Using Point Defects in Single-Crystal Diamond: Recent Progress on Nitrogen Vacancy Center-Based Sensors

“…In a further very recent approach, mounting structures holding an array of pillars are used. Consequently, several NV centers are usable, and vector magnetometry might be feasible [99]. However, this approach introduces the risk that a pillar that is not in direct contact with the sample is used and, thus, an unwanted stand-off distance strongly reduces the resolution.…”

“…More recently, versatile approaches with NV centers in single crystal scanning probes have been presented [99,132]. This technology significantly broadens the range of samples for which NV magnetometry can be applied: solid materials present unique phenomena that only occur at cryogenic temperatures such as superconductivity.…”

“…However, real superconductors present points at which magnetic fields penetrate and magnetic vortices form. NV nano-magnetometry was applied for a high-resolution, low-invasive imaging of superconducting vortices (6 K, [99], 4.2 K [132]). To achieve cryogenic operation, the NV microscope is enclosed in a liquid 4 He cryostat with optical access.…”

“…Particular care was taken to avoid heating by microwave (MW) and laser excitation. Superconducting vortices were imaged with a resolution below 100 nm with magnetic field sensitivity of 30 µT·Hz 1/2 [99] and 11.9 µT·Hz 1/2 [132], allowing in [132] to verify vortex models beyond the monopole approximation.…”

Sensing Magnetic Fields with a Giant Quantum Wave

“…APPLIED 7, 034026 (2017) 034026-5 nearly a kilogauss can be applied along the trap axis, in the plane of the sample. Nanoscale scanning NV diamond magnetometers are also compatible with samples from room temperature to cryogenic temperatures [44] but are additionally operable outside ultrahigh vacuum. They provide a field sensitivity of a few nT= ffiffiffiffiffiffi Hz p with resolution down to a few nanometers near surfaces at frequencies above 10 kHz [1,27,28] and approximately 2 μT= ffiffiffiffiffiffi Hz p at dc with more than tenfold improvements predicted [45].…”