Spin reorientation of the Fe moments in Eu${}_{0.5}$Ca${}_{0.5}$Fe${}_2$As${}_2$: Evidence for a strong interplay of Eu and Fe magnetism

Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of Eu${}_{0.5}$Ca${}_{0.5}$Fe${}_2$As${}_2$. Upon 50 % dilution of the Eu sites with isovalent Ca${}^{2+}$, the Eu sublattice is found to be still long-range ordered below $T_{Eu}$ = 10 K, in the A-typed antiferromagnetic (AFM) structure. The moment size of Eu${}^{2+}$ spins is estimated to be as large as 6.74(4) ${\mu }_B$ at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at $T_{SDW}$ = 192(2) K and displays an in-plane AFM structure above $T_{Eu}$. However, at 2.5 K, the Fe${}^{2+}$ moments are found to be ordered in a canted AFM structure with a canting angle of 14(4){\deg} out of the $ab$ plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides a direct evidence of a strong interplay between the two magnetic sublattices in Eu${}_{0.5}$Ca${}_{0.5}$Fe${}_2$As${}_2$.