Ultrasensitive Nano-MRI Using Nitrogen Vacancy Centers in Diamond
Nitrogen vacancy (NV) centers are a fluorescent defect in diamond which are a leading candidate for nanoscale magnetic resonance imaging (MRI) in both biological samples and electronic devices. Their spin-state dependent fluorescence intensity allows sensitive optical detection of magnetic resonance (ODMR) of the NV’s spin 1 system. This unique magneto-optical property enables the measurement of local magnetic fields experienced by the NV center with exceptional sensitivity by measuring changes to the resonance due to these fields. This allows sensitivity to magnetic fields from nearby resonating spins. The atomic size of NV centers promises magnetic field and target spin imaging with unprecedented resolution. Recently, we have used nanodiamonds as spin labels for a single DNA molecule to develop a platform for the study of single molecule dynamics. Our current work focuses on using NV centers to optically detect the ferromagnetic resonance of a proximal ferromagnet. Our longer term goal is to develop protocols for ultrasensitive atomic scale imaging of nuclear spins and electron spin labels in biological samples, and magnetization dynamics in interesting condensed matter systems such as skyrmions and nanoscale spintronic devices.
Magnetic Resonance Force Microscopy
Magnetic resonance force microscopy (MRFM) is a powerful spin detection tool because it combines the spectroscopic resolution of magnetic resonance imaging (MRI) with the force sensitivity of scanned probe microscopy. We use the MRFM probe to study weakly interacting paramagnetic spin systems, which are a promising candidate for quantum computation. We are interested in the spin dynamics of these systems and typically study nano-scale volumes (< 1 μm3) and spin ensembles containing ~ 100 spins. Our recent work involves measuring fast spin dynamics using silicon nitride membranes as resonating force detectors, which provide a complimentary measurement space to traditional cantilevers.
Cantilever Magnetometry is a sensitive and reliable probe of sample magnetization. Cantilever magnetometry is capable of measuring magnetism in smaller samples than traditional techniques like SQUID or neutron scattering. The magnetic sample exerts a torque on the cantilever as it oscillates, and this torque manifests as a change in cantilever frequency. We have developed robust models for understanding the strength of ferro-, para-, and diamagnetism in order to relate the cantilever frequency change to a sample magnetization and magnetic anisotropy. Our current work focuses on understanding the magnetic properties of graphite and graphene systems.
Ferromagnetic Resonance Force Microscopy
Scanning probe FMR, or FMR force microscopy (FMRFM), is based on magnetic resonance force microscopy (MRFM) in which magnetic resonance is sensitively detected through the magnetic dipole force exerted on a cantilever by means of a micromagnetic tip. FMRFM has demonstrated the sensitivity necessary to study nanoscale ferromagnetic structures. Local properties in thin ferromagnetic films are difficult to probe due to strong spin-spin interactions causing the the lowest energy excitations to reflect global properties in the film - the so-called uniform mode. In contrast, we have used the strong dipolar field from a high-coercivity micromagnetic tip to localize discrete modes directly beneath the tip. By scanning the tip we have demonstrated scanned probe FMR imaging of the effective internal fields of a ferromagnetic film by observing shifts in the localized mode.