Imaging the Néel Vector in Few-Layer CrPS₄ with Second-Harmonic Generation

Second-harmonic generation (SHG) is a powerful technique for probing the crystallographic symmetry. Recently, SHG has also been shown to exhibit sensitivity to time-reversal symmetry, allowing studies of magnetic order in layered antiferromagnetic (AFM) materials. Here, we report SHG investigation of chromium thiophosphate (CrPS₄), a layered A-type AFM material with a finite/zero net magnetization for odd/even-layer-number crystals, respectively. The SHG intensity of bulk CrPS₄ across its Néel temperature reveals that both crystallographic and magnetic symmetry breaking contribute to SHG at low temperatures. From the polarization analysis, we identify the tensor element linked to the magnetic order. Interestingly, the polarization response is sensitive to the history of the magnetic field applied to the even-number-layer crystals, suggesting that the SHG is sensitive to the Néel vector direction. Polarized SHG microscopy of a terraced CrPS₄ shows that the AFM order of even-number-layer regions is determined by the ferromagnetic order of the neighboring odd-number-layer regions.