EuCd₂P₂ has been reported to show a giant, magnetic-field-sensitive resistance peak at 18 K, which is well above the antiferromagnetic (AFM) transition at 11 K in this material [1]. The physics origin of the resistance peak remains an open question, which leaves the space to explore magnetic structures above the Neel temperature. We observe a clear hysteresis in EuCd₂P₂ via magneto-optical Kerr effect microscopy, indicating an unexpected ferromagnetic (FM) order. The hysteresis is observed both below and above the AFM transition. Remarkably, the hysteresis becomes inverted above the Neel temperature, which coincides with the emergence of the resistance peak. The out-of-plane moment also correlates with the off-diagonal linear magneto-optical effect [2] observed in this material.

[1] Z. Wang, J. D. Rogers, X. Yao, R. Nichols, K. Atay, B. Xu, J. Franklin, I. Sochnikov, P. J. Ryan, D. Haskel, and F. Tafti, Colossal Magnetoresistance without Mixed Valence in a Layered Phosphide Crystal, Adv. Mater. 33, 2005755 (2021).

[2] V. V. Eremenko, Yu. G. Litvinenko, N. K. Kharchenko, and V. M. Naumenko, Magneto-Optics and Spectroscopy of Antiferromagnets (Springer New York, New York, NY, 1992).