Critical fluctuations and noise spectra in two-dimensional Fe3GeTe2 magnets

Critical fluctuations play a crucial role in determining spin orders in low-dimensional magnetic materials. However, experimentally linking these fluctuations to scaling theory-and thereby uncovering insights into spin interaction models-remains a challenge. Here, we utilize a nitrogen-vacancy center-based quantum decoherence imaging technique to probe critical fluctuations in the van der Waals magnet Fe3GeTe2. Our data reveal that critical fluctuations produce a random magnetic field, with noise spectra undergoing significant changes near the critical temperature. To explain this phenomenon, we developed a theoretical framework showing that the spectral density exhibits 1/f noise characteristics near the critical temperature, transitioning to white noise behavior away from this regime. By experimentally adjusting the sample-to-diamond distance, we identified the crossover temperature between these two noise types. These findings offer an approach to studying phase transition dynamics through critical fluctuations, enabling precise determination of critical exponents associated with long-range correlations. This methodology holds promise for advancing our understanding of critical phenomena across diverse physical systems. The critical behaviour of phase transitions encodes information about the nature of the phase transition, however, extracting critical exponents associated with the correlation length and relaxation time is challenging. Here, taking the well-studied van der Waals magnet, Fe3GeTe2, Li, Ding, Wang, and coauthors use NV-centre based magnetometry to study critical fluctuations.