Observation of stacking engineered magnetic phase transitions within moir\'e supercells of twisted van der Waals magnets

Twist engineering of magnetic van der Waals (vdW) moir\'e superlattices provides an attractive way to achieve precise nanoscale control over the spin degree of freedom on two-dimensional flatland. Despite the very recent demonstrations of moir\'e magnetism featuring exotic phases with noncollinear spin order in twisted vdW magnet chromium triiodide CrI3, the local magnetic interactions, spin dynamics, and magnetic phase transitions within and across individual moir\'e supercells remain elusive. Taking advantage of a scanning single-spin magnetometry platform, here we report observation of two distinct magnetic phase transitions with separate critical temperatures within a moir\'e supercell of small-angle twisted double trilayer CrI3. By measuring temperature dependent spin fluctuations at the coexisting ferromagnetic and antiferromagnetic regions in twisted CrI3, we explicitly show that the Curie temperature of the ferromagnetic state is higher than the N\'eel temperature of the antiferromagnetic one by ~10 K. Our mean-field calculations attribute such a spatial and thermodynamic phase separation to the stacking order modulated interlayer exchange coupling at the twisted interface of the moir\'e superlattices. The presented results highlight twist engineering as a promising tuning knob to realize on-demand control of not only the nanoscale spin order of moir\'e quantum matter but also its dynamic magnetic responses, which may find relevant applications in developing transformative vdW electronic and magnetic devices.