Puzzling Magnetism of $Gd_3Cu_4Sn_4$

A polycrystalline, single phase sample of $Gd_3Cu_4Sn_4$ ($Gd_3Cu_4Ge_4$ type, Immm, a = 1473.5(1), b = 694.1(1), c = 447.2(1) pm) was synthesized by arc-melting of the elements and subsequent annealing of the sample at 1220 and 970 K, respectively. The magnetic properties of this compound were studied thoroughly by AC and DC magnetic susceptibility measurements and $\text{}^{155}Gd$ Mössbauer spectroscopy. These results reveal two antiferromagnetic phase transitions close to 13 and 8.6 K, respectively. The thermal variation of the magnetic hyperfine fields observed at two inequivalent Gd sites point to two magnetic substructures, which order independently.