Correlations in Magnetic Sub-Domains as an Unconventional Phase Diagram for van der Waals Ferromagnets.

Traditional magnetic phase diagram represents a transition between the ferromagnetic and paramagnetic states of a material under the influence of varied temperature, magnetic field, and pressure. So far, the ferromagnetic phase has been considered predominantly as a single type of magnetization texture extending macroscopically in the bulk of a crystal, existing as a ground state determined by the interactions between localized magnetic moments arranged in a lattice. Here, it is demonstrated that an unconventional magnetic order composed of vertically correlated planar magnetic sub-domains occurs intrinsically in mechanically exfoliated layers of van der Waals ferromagnet CrBr. Based on the visualization of the magnetic textures through magnetic force microscopy in conjunction with the ab initio calculations of the crystal structure in the magnetic phase and micromagnetic simulations, the origin of the magnetic sub-domains is attributed to stacking faults isolating a van der Waals ferromagnetic well from the bulk film due to modifications in the interlayer exchange coupling. This enables to create a phase diagram describing the magnetic states unique to van der Waals ferromagnets in terms of the degree of correlation between the magnetic sub-domains, dependent on the exchange coupling constants and tuneable by magnetic field and temperature.