Observation of stacking engineered magnetic phase transitions within moiré supercells of twisted van der Waals magnets

Twist engineering of magnetic van der Waals (vdW) moiré superlattices provides an attractive way to achieve nanoscale control over the spin degree of freedom on two-dimensional flatland. Despite the very recent demonstration of moiré magnetism featuring exotic spin order, a comprehensive understanding of the microscopic magnetic interactions, spin dynamics, and magnetic phase transitions within and across individual magnetic moiré supercells remains elusive. Taking advantage of a scanning single-spin microscopy platform, here we report observation of two distinct magnetic phase transitions with separate critical temperatures within moiré supercells of small-angle twisted trilayer chromium triiodide CrI3. By measuring temperature dependent spin fluctuations at the coexisting ferromagnetic and antiferromagnetic phases, we explicitly show that the Curie temperature of the ferromagnetic state in twisted CrI3 is higher than the Néel temperature of the antiferromagnetic one by ~10 K. Our mean-field calculations attribute such a spatial and thermodynamic phase separation to stacking order modulated interlayer exchange coupling at the twisted interface of the moiré 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 moiré quantum matter but also its dynamic magnetic responses, which may find relevant applications for designing transformative vdW magnetic devices.