Understanding the behaviour of spins, charge, and phonons in quantum materials is at the heart of condensed matter physics. In the past few decades, a wide range of new materials showing exciting new physical phenomena has been discovered and explored. Current characterization techniques do not provide the combined spatial resolution and sensitivity required to map their properties at the nanoscale (< 10 nm). Recently, a new technique has emerged for measuring physical properties (magnetic, optical, electrical…) at the nanometer scale based on optical detection of the electron spin magnetic resonance of nitrogen vacancy (NV) centers in diamond, Fig. Aa. Negatively charged NV centers, constituted of a substitutional nitrogen adjacent to a vacancy site, are bright, perfectly photostable emitters (650-800 nm) that exhibit high-contrast optical detected magnetic resonance (ODMR). The NV spin-triplet ground state features a zero-field splitting D = 2.87 GHz between states ms = 0 and ms = ±1, Fig. 1b. Intersystem crossing to metastable singlet states takes place preferentially for NV centers in the ms = ±1 states, allowing optical readout of the spin state via spin-dependent fluorescence [ref]. The application of an external magnetic field breaks the degeneracy of the ms = ±1 state and leads to a pair of transitions whose frequencies depend on the magnetic field component along the N-V symmetry axis. In this project we use diamond based microscopes (near-field in Fig. 1c, far-field in Fig. 1d) for mapping condensed matter phenomena at the nanometer scale.


Figure 1: NV based magnetic imaging at the nanoscale. (a) NV center in the diamond lattice. (b) Energy-levels of the NV center. The NV spin is pumped into the |0〉 state by off-resonance optical excitation, the |±1〉 excited states can decay non-radiatively through metastable singlet states, and the ground-state spin can be manipulated by microwave excitation. (c) NV-spin scanning probe microscope:  NV contained nanodiamond (ND) attached to the apex of an AFM tip integrated with a confocal microscope. (d) NV far-field magnetic microscope: a green light excites a large region of target sample (few mu2 to few hundreds of um2) using high NA objective. The NV emitted red  light is collected using sCMOS camera and its magnetic resonance peaks are monitored vs applied microwave (CW/pulsed) and external magnetic field.

Using the NV microscopes, we seek to study chiral spin textures, surface spin current, spin orbit torque, and spin dynamics in low dimensional spin system, thin magnetic films/multilayers, magnetic nanoparticles, topological insulators, multiferroics, and oxide heterostructures. These materials have attracted a huge interest in the last few years due to their novel properties and integration to spintronics and magnetic devices. Interesting systems to study include: magnetic skyrmions/domain wall structures, dynamic magnetic excitation (spinwave) in nanomagnets, magnetic relaxation and surface effects of individual magnetic nanoparticles, surface spin current generated  in topological insulators, and magnetic flux generated in high Tc superconductors and islands.