Fig. 2: Temperature dependence of SHG.

Upon cooling from room temperature, the SHG intensity increases dramatically. Below ~100 K, a broad hump (most apparent in EuSn₂AsP) and large fluctuations (most apparent in EuSn₂P₂) are observed. We identify these features with the emergence of fluctuating short-range ferromagnetic correlations within the Eu planes (blue). Within ~2 K of T_N, a pronounced peak appears, which we attribute to critical slowing down at the magnetic phase transition. Below T_N, the SHG intensity decreases as long-range three-dimensional antiferromagnetic order freezes in (green). The inset shows a close-up view of the low-temperature region.