Imaging equilibrium and non-equilibrium critical phenomena in mesoscopic magnets*

We demonstrate a technique by which the dynamical magnetic susceptibility of a material can be measured via dephasing spectroscopy of nearby single-spin qubits. This probe measures a complementary regime of frequency and momentum compared to existing techniques, and can be applied to mesoscopic sample volumes. We use the technique to image and characterize the critical magnetic fluctuations of chromium trihalide nanoflakes near their magnetic melting temperatures. We further show how dephasing spectroscopy can be combined with external stimulus of the target material to study its critical dynamics out of equilibrium. This work can be extended to a variety of bulk materials, two-dimensional magnets, films, and devices.