Quantum Noise Spectroscopy of Critical Slowing Down in an Atomically Thin Magnet

Mark E. Ziffer, Francisco Machado, Benedikt Ursprung, Artur Lozovoi, Aya Batoul Tazi, Zhiyang Yuan, Michael E. Ziebel, Tom Delord, Nanyu Zeng, Evan Telford, Daniel G. Chica, Dane W. deQuilettes, Xiaoyang Zhu, James C. Hone, Kenneth L. Shepard, Xavier Roy, Nathalie P. de Leon, Emily J. Davis, Shubhayu Chatterjee, Carlos A. Meriles, Jonathan S. Owen, P. James Schuck, Abhay N. Pasupathy
(2024)
arXiv:2407.05614

Abstract

Low frequency critical fluctuations in magnetic materials encode important information about the physics of magnetic ordering, especially in the associated critical exponents. While a number of techniques have been established to study magnetic critical fluctuations in bulk materials, few approaches maintain the required microscopic resolution, temporal range, and signal sensitivity to quantitatively analyze critical fluctuations in magnetic phases of 2D materials. Using nitrogen-vacancy (NV) centers in diamond as quantum probes, we implement T2 (spin decoherence) noise magnetometry to quantitatively study critical dynamics in a tri-layer sample of the Van der Waals magnetic material CrSBr. We characterize critical fluctuations across the magnetic phase transition in CrSBr by analyzing the NV spin echo coherence decay on time scales that approach the characteristic fluctuation correlation time τc at criticality, allowing us to study the temperature dependence of critical slowing down. By modelling the spin echo decoherence using theoretical models for critical dynamics, we are able to extract the critical exponent ν for the correlation length. We find a value for ν which deviates from the Ising prediction and suggests the role of long-range dipolar interactions in modifying the critical behavior of magnetic fluctuation modes in CrSBr at the 2D limit. We further compare the divergence of correlation length in CrSBr to the predicted exponential divergence for 2D XY criticality, and find evidence suggesting the possibility of such behavior in a temperature window near TC where static magnetic domains are absent. Our work provides a first demonstration of the capability of decoherence based NV noise magnetometry to quantitatively analyze critical scaling laws in 2D materials.