Abnormal Surface Magnetic-Dipole-Contributed Second Harmonic Generation in Centrosymmetric 2D Antiferromagnets

Understanding the mechanism of nonlinear second harmonic generation (SHG) in 2D magnetic materials is of great physics interest, with profound implications for both fundamental research and technological applications. Here, we report the first observation of abnormal thickness dependence of SHG in centrosymmetric 2D antiferromagnets: clear SHG in ultrathin 2D Cobalt Monoxide (CoO) nanoflakes and a systematic decrease of SHG with increasing thickness. As centrosymmetric crystals generally have no SHG signal, the observed SHG in ultrathin CoO nanoflakes originates from the antiferromagnetic spin orders that break the time-reversal symmetry and from defect states that break the spatial inversion symmetry. Temperature-dependent and polarization-resolved SHG studies reveal that the abnormal SHG in ultrathin CoO nanoflakes mainly originates from surface antiferromagnetic spin-ordered surface magnetic-dipole-contributed SHG, stronger than bulk magnetic-dipole-contributed SHG and surface electric-dipole-contributed SHG. Our work further suggests that polarization-resolved SHG provides a sensitive probe for studying surface magnetic properties in 2D magnetic materials.