Nanoscale magnetic ordering dynamics near criticality in a high Curie-temperature ferromagnet

Continuous phase transitions and critical points, often crucial for revealing exotic states of matter in condensed matter physics, are characterized by critical behavior such as divergent susceptibilities, long-range correlations, and spin dynamics that span from kHz to GHz as the system approaches the critical point. Probing the relevant length and time scales has proven technically challenging with conventional measurement techniques. In this study, we employ scanning nitrogen-vacancy (NV) center-based magnetometry and relaxometry to investigate the critical behavior of a high-Tc ferromagnetic oxide near its Curie temperature. Cluster analysis of nanoscale magnetic textures demonstrate robust power scaling behavior falling into to a 3D universality class near the critical point. Simultaneously, temperature-dependent magnetic fluctuations, measured via all-optical relaxometry, suggest that the spin degrees of freedom belong to the XY universality class. Our findings capture both the static and dynamic aspects of critical behavior, offering insights into the universal properties governing phase transitions in magnetic materials.