Scanning nitrogen vacancy center magnetometry and its applications in nanomagnetism

Spintronics uses not only the charge of electrons, but also their spin, to develop efficient, low-energy sensors and information processing and storage devices. Born at the end of the 1980s with the discovery of giant magnetoresistance, it is now developing in many directions, including the use of non-collinear magnetic textures such as domain walls or skyrmions, antiferromagnetic or two-dimensional Van der Waals materials, or spin waves (known as magnonics) to store, transmit and process information.

These developments have increased the need for high-performance magnetic imaging techniques, capable of detecting and quantitatively characterizing all these magnetic objects on the nanometric scale. Among the techniques available, local probe microscopy has the advantage of offering spatial resolution ranging from a few tens of nanometers down to atomic resolution, while being sufficiently compact to be used in the laboratory.

This article presents in detail one of them, NV center scanning magnetometry, which relies on the use of the nitrogen-lacune defect in diamond (the NV center, for Nitrogen-Vacancy) as a quantum sensor to measure the leakage magnetic field generated by magnetic textures. This measurement is based on the optical detection of the Zeeman effect, which shifts the spin energy levels of the NV center in the presence of a magnetic field, and provides answers to crucial questions in spintronics, such as determining the internal structure of domain walls, observing and measuring magnetization in two-dimensional materials, or imaging antiferromagnetic textures.

This article begins with a discussion of the conditions of use and comparison of several local-probe magnetic imaging techniques, including NV magnetometry, followed by an overview of the main nanomagnetic objects studied using them. This is followed by a comprehensive description of the properties of the NV center, enabling it to be used as a non-perturbative, highly sensitive magnetic field sensor. How to integrate this sensor into a magnetometer consisting of an atomic force microscope coupled to a confocal microscope is also detailed, with emphasis on the necessary calibration procedure for the tips used. The versatility of this technique is illustrated by a presentation of the different imaging modes of NV magnetometry, their usage regimes, and their implementation. The various procedures for analyzing experimental data are also outlined, to provide an overview of all the steps involved in the rigorous use of NV magnetometry. Finally, recent developments enabling a substantial improvement in magnetic sensitivity are discussed, opening the door to the measurement of ever weaker magnetic signals.

In addition to magnetometry, the experimental protocols...