基于金刚石NV色心的纳米尺度磁场测量和成像技术

Nanoscale magnetic field sensing and imaging based on nitrogen-vacancy center in diamond

纳米级分辨率的磁场测量和成像是磁学中的一种重要研究手段.金刚石中的单个氮-空位点缺陷电子自旋作为一种量子传感器,具有灵敏度高、原子级别尺寸、可工作在室温等诸多优势,灵敏度可以达到单核自旋级别,空间分辨率达到亚纳米.将这种磁测量技术与扫描成像技术结合,能够实现高灵敏度和高分辨率的磁场成像,定量地重构出杂散场.这种新型的磁成像技术可以给出磁学中多种重要的研究对象如磁畴壁、反铁磁序、磁性斯格明子的结构信息.随着技术的发展,基于氮-空位点缺陷的磁成像技术有望成为磁性材料研究的重要手段.

Magnetic field measurement and imaging with nanometer resolution is a key tool in the study of magnetism.There have been several powerful techniques such as superconducting quantum interference device, hall sensor, electron microscopy, magnetic force microscopy and spin polarized scanning tunneling microscopy. However, they either have poor sensitivity or resolution, or need severe environment of cryogenic temperature or vacuum. The nitrogen-vacancy color center（NV center） in diamond, serving as a quantum magnetic sensor, has great advantages such as long decoherence time, atomic size, and ambient working conditions. The NV center consists of a substitutional nitrogen atom and an adjacent vacancy in diamond. Its electronic structure of ground state is a spin triplet. The spin state can be initialized to mS= 0 state and read out by laser pulse, and coherently manipulated by microwave pulse. It is sensitive to the magnetic field by measuring the magnetic Zeeman splitting or quantum phase in quantum interferometer strategies. By using dynamical decoupling sequence to prolong the decoherence time, the sensitivities approach to nano tesla for a single NV center and pico tesla for the NV center ensemble, respectively. As a sensor with an atomic size, it reaches single-nuclear-spin sensitivity and sub-nanometer spatial resolution. Combining with scanning microscopy technology, it can accomplish high-sensitivity and high-resolution magnetic field imaging so that the stray field can be reconstructed quantitatively. The magnetic field is calculated from the two resonant frequencies by solving the Hamiltonian of NV center in order to obtain the value of stray field. Recently, this novel magnetic imaging technique has revealed the magnetization structures of many important objects in magnetism research. The polarity and chirality of magnetic vortex core are determined by imaging its stray field; laser induced domain wall hopping is observed quantitatively with a nanoscale resolution; non-linear antimagnetic order is imaged in real space by NV center. It was recently reported that magnetization of the magnetic skyrmion is imaged by NV center. The magnetization distribution is reconstructed from stray field imaging. With the topological number limited to one, the Néel type magnetization is uniquely determined.These results show that the magnetic imaging method has great advantages to resolve the emerging magnetic structure materials. The magnetic imaging technology based on the NV center will potentially become an important method to study magnetic materials under continuous development.