Magnetic structure and resistivity minimum in GdCuAs₂

The electrical resistivity of GdCuAs₂ single crystals exhibits an anomalous Kondo-like resistivity minimum above the antiferromagnetic ordering temperature T_N1 ≈ 10.6 K, which is unusual for highly localized 4f- moment (Gd³⁺) system. Using X-ray resonant magnetic scattering, we determined the magnetic structure of GdCuAs₂, where Gd moments are antiferromagnetically aligned along the crystallographic a axis and (++−−) arrangement in the c direction and ferromagnetically arranged in the b direction. The antiferromagnetic order appears first at q = (δ, 0, 0.5) below 10 K, with an incommensurate modulation along the a axis, and locks into a commensurate position at q = (1/3, 0, 0.5) below T_N2 ≈ 6 K. Our high-resolution X-ray diffraction measurements show a two-peak structure at Q = (2, 0, 6) above the resistivity minimum, suggesting a lower symmetry crystal structure than the reported tetragonal structure, and the Q = (2, 0, 6) peak becomes a sharp one-peak structure below the resistivity minimum, implying a magnetoelastic coupling above TN. Our findings suggest a complex interplay between the crystal structure and antiferromagnetic structure through a magnetoelastic cou- pling, associated with the anomalous resistivity minimum above the T_N1.