The magnetic structure of (La${ }_{ 1.8}$Eu${ }_{ 0.2}$)CuO${ }_4$ has been studied by magnetization measurements of single crystals, which show antiferromagnetic long-range order below $T_N$ = 265 K and a structural phase transition at $T_s$ = 130 K. At $T_s < T < T_N$, the Cu spin susceptibility exhibits almost the same behavior as that of La${ }_2$CuO${ }_4$ in the low-temperature orthorhombic phase, which indicates the existence of finite spin canting out of the CuO${ }_2$ plane. At $T < T_s$, the magnitude of the weak-ferromagnetic moment induced by the spin canting is suppressed approximately by 70{ %}. This significant suppression of the weak-ferromagnetic moment is carefully compared with the theoretical analysis of weak ferromagnetism by Stein { \it et al.} (Phys. Rev. B { \bf 53}, 775 (1996)), in which the magnitude of weak-ferromagnetic moments strongly depend on the crystallographic symmetry. Based on such comparison, below $T_s$ (La${ }_{ 1.8}$Eu${ }_{ 0.2}$)CuO${ }_4$ is in the low-temperature less-orthorhombic phase with a space group of $Pccn$. We also discuss the possible magnetic structure of the pure low-temperature tetragonal phase with space group $P4_2/{ ncm}$, which is relevant for rare-earth and alkaline-earth ions co-doped La${ }_2$CuO${ }_4$.
Categories: cond-mat.supr-con