  In two-dimensional (2D) ferromagnets, anisotropy is essential for the
magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing
anisotropies are present, the phase transition becomes nontrivial. Here,
utilizing highly sensitive susceptometry of scanning superconducting quantum
interference device microscopy, we probe the spin correlations of ABC-stacked
CrBr3 under zero magnetic field. We identify a plateau feature in
susceptibility above the critical temperature (Tc) in thick samples. It
signifies a crossover regime induced by the competition between easy-plane
intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The
evolution of the critical behavior from the bulk to 2D shows that the
competition between the anisotropies is magnified in the reduced dimension. It
leads to a strongly frustrated ferromagnetic transition in the bilayer with
fluctuation on the order of Tc, which is distinct from both the monolayer and
the bulk. Our observation potentially offers a 2D localized spin system on
honeycomb lattice to explore magnetic frustration.
