Quantum Noise Spectroscopy of Criticality in an Atomically Thin Magnet

arXiv:2407.05614v3 Announce Type: replace-cross Abstract: The universality class of a phase transition dictates the behavior of low-frequency and long-wavelength critical fluctuations in a material. Studying these dynamics in thin magnetic insulators, however, remains challenging; an experimental probe must be sensitive to low-frequency, long-wavelength fluctuations while navigating the sample's small volume, inherent inhomogeneities, and fabrication limitations. Using nitrogen-vacancy (NV) centers in diamond, we demonstrate T$_2$ (spin-decoherence) noise magnetometry as a direct and quantitative probe of critical dynamics in a tri-layer sample of CrSBr, a two-dimensional van der Waals magnet. We observe a sharp increase in the decoherence rate of the NV spin ensemble at the sample's critical temperature, highlighting our probe's sensitivity to critical fluctuations. We demonstrate that the behavior of the NV spin decoherence rate below the critical temperature is inconsistent with fluctuations due to XY criticality in 2D CrSBr. Next, by simulating the NV spin decoherence dynamics using noise models for an Ising-type transition and fitting to our data, we quantitatively measure the dynamical and the correlation length critical exponents ($z$ and $\nu$) and demonstrate that the observed fluctuations strongly deviate from those predicted by the short-range Ising universality class. Together, these observations indicate that the transition is Ising-like but modified by long-range dipolar interactions, emphasizing the importance of easy-axis anisotropy and long-range interactions for understanding magnetism in 2D CrSBr. Our work establishes T$_2$ noise magnetometry as a quantitative probe of critical dynamics in 2D materials, opening a new avenue for studying strongly correlated phenomena at the atomically thin limit.