X-ray resonant magnetic scattering study of the spin-flop transition in Gd$_5$Ge$_4$

Gd$_5$Ge$_4$ crystallizes in the orthorhombic space group Pnma. Below 127~K the Gd moments order antiferromagnetically in a layered structure with a magnetic unit cell same as the chemical unit cell. The ferromagnetic Gd-rich slabs are stacked antiferromagnetically along b-axis The magnetic moments are primarily aligned along the c-axis. X-ray resonant magnetic scattering was used to study a fully reversible spin-flop transition in a single crystal Gd$_5$Ge$_4$ and to elucidate details of the magnetic structure in the spin-flop phase. The Gd moments at the three Wyckoff sites flop from c-axis antiferromagnetically aligned to a-axis antiferromagnetically aligned in a critical field $H_{sf} = 9$ kOe applied along c-axis at $T = 10$ K. The magnetic space group changes from $Pnm'a$ to $Pn'm'a'$ at all three sites.