Imaging and control of critical spin fluctuations in two-dimensional magnets

Strong spin fluctuations are expected near the thermodynamic critical point of a continuous magnetic phase transition. Such critical spin fluctuations are highly correlated and in principle can occur at any time- and length-scales; they govern critical phenomena and potentially can drive new phases. Although theoretical studies have been made for decades, direct observation of critical spin fluctuations remains elusive. The recent discovery of two-dimensional (2D) layered magnets, in which spin fluctuations significantly modify magnetic properties as compared to their bulk counterparts and integration into heterostructures and devices can be easily achieved, provides an ideal platform to investigate and harness critical spin phenomena. Here we develop a fast and sensitive magneto-optical imaging microscope to achieve wide-field, real-time monitoring of critical spin fluctuations in single-layer CrBr3, which is a 2D ferromagnetic insulator. We track the critical phenomena directly from the fluctuation correlations and observe both slowing-down dynamics and enhanced correlation length. Through real-time feedback control of critical spin fluctuations, we further achieve switching of magnetic states solely by electrostatic gating without applying a magnetic field or a Joule current. The ability to directly image and control critical spin fluctuations in 2D magnets opens up exciting opportunities to explore critical phenomena and develop applications in nanoscale engines and information science.