**1. Introduction**

Understanding magnetic properties at the microscopic level plays an important role in the development of modern science and technology [1, 2]. For instance, writing and reading information using nanometer size magnetic bits is the heart of massive data storage indispensable in the modern information technology [1]. Magnetic resonance imaging (MRI) which is an important medical tool of imaging the inner structures of human body is also based on sensing the magnetic response of minuscule protons with respect to radio frequency (RF) electromagnetic waves [2]. For fundamental research, on the other hand, studying magnetic phases and spin textures at the nanometer scale are one of the hottest topics in solid-state physics due to the recent discovery of exotic materials and topological phases [3–5]. Therefore, it is not too much to say that the continuous advances in modern science and technology strongly reply on the precise sensing and control of magnetism at the atomic level.

The paradigm of modern science and technology seems to shift from chargebased devices to spin-based systems. Nonetheless studying spins is a lot more difficult than electric charges mainly due to the lack of sensitive measurement techniques of magnetic field. For instance, the size of magnetic bits used in MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and STT-MRAM (Spin-Transfer Torque Magnetic Random-Access Memory) are less than 10

nanometers and eventually reaches at the level of single spins requiring sensitive detection of individual spins with high spatial resolution [6, 7]. However, detecting single electron spin takes more than 13 hours even with the best magnetometer [8], while sensing single electron charge takes only 1 picosecond [9, 10]. This motivates to develop new magnetic sensors with high magnetic field sensitivity and high spatial resolution.

**2. Background of diamond NV center**

*DOI: http://dx.doi.org/10.5772/intechopen.84204*

uncertainty [27].

**Figure 2.**

**51**

The diamond NV center is a hetero-molecular defect in a diamond crystal consisting of a substitutional nitrogen defect combined with an adjacent carbon vacancy [15, 16] (**Figure 2a**). It is a color center as it absorbs photons in the visible range of wavelength (e.g. 532 nm) and emits photons of a broad range of wavelength (e.g. 632–800 nm). The NV center can exist in a natural diamond, but it can be created in more controllable fashion, for instance, by implanting nitrogen ions into the diamond. Subsequent high temperature annealing (e.g. at 800°C) results in the thermal migration of carbon vacancies and NV centers are formed once the vacancies meet the implanted nitrogen impurities. The density and location of NV centers in diamond are well controlled with various techniques. **Figure 2b** shows an example of precise positioning of NV centers less than a few hundred of nanometers

*Atomic Scale Magnetic Sensing and Imaging Based on Diamond NV Centers*

When negatively charged, the NV center has total six electrons (i.e. three electrons from three carbons, two electrons from the substitutional nitrogen and one electron from the diamond lattice). Four of them form pairs and the remaining two unpaired electrons make spin triplet states (i.e. S = 1) in the ground energy level. The spin triplet states are split into ms = 0 and ms = 1 whose separation is about 2.9 GHz at room temperature due to the crystal field and spin–spin interaction [17, 18] (this is called zero-field splitting). As shown in **Figure 3a**, the degenerated ms = 1 states are split further if there is non-zero magnetic field along the NV crystal axis (i.e. the quantized axis of NV spin). Sensing magnetic field (i.e. magnetic field component along the NV axis) is realized by measuring the amount of

The optical response of the NV center varies depending on its spin states. When

The energy levels of NV center are located well within the bandgap of diamond (i.e. 5.3 eV) making it effectively isolated from the hosting material and enabling to preserve its intrinsic quantum properties. Thus, the NV center has exceptionally long spin coherence times even at room temperature [28] (e.g. T2 > 1 ms). In

*Physical properties of diamond NV center. (a) Crystal structure of NV center in a diamond lattice. NV center consists of nitrogen substitutional defect and carbon vacancy. (b) Precise formation of NV centers using focused ion beam (FIB) implantation of nitrogen. Reprint with permission from [27]. Copyright (2013) Wiley-VCH*

pumping. On the other hand, for the case of ms = 1 states, 10–30% of the excited electrons undergo intersystem crossing (ISC) to the spin singlet states and relax into the ms = 0 ground state. This dark transition results in the reduction of the number of emitted photons relative to the ms = 0 state. Furthermore, the transition via the shelving states produces spin flip from the ms = 1 states to the ms = 0 state. The spin-sensitive fluorescence and spin-flip transition allow optical readout of the spin

Zeeman splitting [17, 18] (e.g. 5.6 MHz splitting per 1 Gauss field).

states as well as optical initialization of the qubit state [15–18].

*Verlag GmbH & Co. KGaA. Reproduced with permission.*

it is in the ms = 0 state, almost 100% cycling transition occurs upon optical

Existing magnetometers are insufficient to satisfy both requirements especially when trying to measure minute magnetic fields at the length scale of 100 nm or below. For instance, SQUID (Superconducting Quantum Interference Devices), atomic vapor cell and Hall bar are very sensitive magnetometers but their spatial resolutions are typically limited to tens of micrometers [11, 12]. On the other hand, scanning probe type tools such as SP-STM (Spin-Polarized Scanning Tunneling Microscope) and MFM (Magnetic Force Microscope) exhibit very high spatial resolution but their sensitivity is relatively low and not quantitatively defined [13, 14]. Moreover, magnetic films coated at the tips may produce unwanted stray field affecting the magnetic samples to be measured.

Here, we introduce a novel magnetometer enabling non-invasive, extremely sensitive magnetic sensing and imaging at the nanometer scale. It is based on diamond NV (nitrogen-vacancy) center which is an atomic size point defect in the diamond crystal providing high spatial resolution. It is also a spin qubit (i.e. quantum bit) possessing remarkable magnetic and quantum properties satisfying high field sensitivity [15, 16]. Since it can also operate over a wide range of temperature from room temperature down to cryogenic temperatures and is chemically inert and non-toxic, the NV center already has been applied in various experiments including magnetic imaging of solid-state materials and biomedical samples [17, 18]. In this chapter, we will discuss basic working principles of diamond NV centers (Section 2) and their sensing mechanisms (Section 3). Furthermore, we will provide two examples of imaging applications; scanning probe type imaging (Section 4.1) and wide field-of-view optical imaging (Section 4.2) (**Figure 1**).

#### **Figure 1.**

*Comparison of various magnetometers in terms of spatial resolution and magnetic field sensitivity. The diamond NV center is a promising candidate to realize the goal of highly sensitive sensing with nanometer-scale resolution. The data are adopted from [19]; MRFM [8]; SQUID [20–22]; Hall probe [23, 24]; BEC [25]; Vapor Cell [26].*
