Scanning magnetic imaging of geological samples based on fiber-coupled diamond quantum magnetometry

Rock magnetic microscopy (RMM) has emerged as an advanced methodology for imaging the magnetization distribution of geological thin sections under ambient conditions, providing precise information with high spatial resolution and high magnetic field sensitivity; however, several factors---including the large size and complexity of sensors, their low magnetic field sensitivity and small field of view, and the need for cryogenic conditions---have impeded the widespread adoption of current RMM setups. Here, we present a room-temperature, fiber-coupling, scanning scheme for RMM based on the negatively charged nitrogen-vacancy (NV) centers in diamond. A $200\ifmmode\times\else\texttimes\fi{}200\ifmmode\times\else\texttimes\fi{}50\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}{\mathrm{m}}^{3}$ diamond chip is glued to a multimode fiber ferrule for scanning imaging of the magnetic field. To ensure the precision of the magnetic field measurements, the diamond surface is coated with a reflective gold film. A spatial resolution of $180\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$ and a magnetic field sensitivity of $52\phantom{\rule{0.2em}{0ex}}\mathrm{nT}/{\mathrm{Hz}}^{1/2}$ in the near-dc frequency range below 10 Hz are achieved. This performance corresponds to a sensitivity of ${10}^{\ensuremath{-}12}\phantom{\rule{0.2em}{0ex}}\mathrm{A}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{2}/{\mathrm{Hz}}^{1/2}$ for magnetic moment measurements, approaching that of the superconducting rock magnetometer. The stray magnetic field distribution on a banded iron formation thin section is successfully scanned, revealing the inhomogeneous distribution of magnetization within the iron-rich bands. This study demonstrates the potential to extend the application scope of NV magnetometry in the frontiers of high-resolution geological research.