  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.
