Mapping magnetic phase diagram in two-dimensional CrSBr using nonlinear optics (Part I)

Magnetism in two-dimension (2D) can exhibit significantly different properties from its counterpart in three-dimension. Nontrivial magnetic textures can emerge in 2D due to the reduced dimensionality and the modified magnetic anisotropy and exchange pathways. In the van der Waals antiferromagnet (AFM) CrSBr, puzzled by the small spin wave gaps, i.e., ~ 30 GHz (corresponding to the thermal energy at T_mag ~1.5K), and the contrasting high magnetic onset temperatures, i.e., T_N,2L = 140 K in its thin layer form, we have performed second harmonic generation rotational anisotropy (SHG RA) measurements to track the symmetry changes of the magnetic phases as a function of temperature down to 80 K (60K below T_N,2L, but much higher than T_mag) and applied magnetic fields along three crystalline axis directions, in both monolayer and bilayer CrSBr. Our findings suggest nontrivial ground states beyond the reported in-plane ferromagnetism and A-type AFM for monolayer and bilayer CrSBr samples, respectively. These results further highlight the rich magnetic phase landscape in the 2D limit.