Magnetic structures of geometrically frustrated SrGd${}_2$O${}_4$ derived from powder and single-crystal neutron diffraction

Abstract

We present the low-temperature magnetic structures of SrGd${}_2$O${}_4$ combining neutron diffraction methods on polycrystalline and single-crystal samples containing the ${}^{160}$Gd isotope. In contrast to other members of the Sr$L{n}_2$O${}_4$ family ($Ln$ = lanthanide) this system reveals two long-range ordered magnetic phases, which our diffraction data unambiguously identify. Below $T_{N1}$ = 2.73~K, a ${\mathbf{q}}_1$ = (0 0 0) magnetic structure is stabilized where ferromagnetic chains along the $c$~axis (space group $Pnam$) are coupled antiferromagnetically with neighboring chains. On cooling below $T_{N2}$ = 0.48~K, an additional incommensurate component modulated by ${\mathbf{q}}_2$ = (0~0~0.42) evolves and aligned along either of the perpendicular axes for the two different Gd sites, resulting in a fan-like magnetic structure. The identification of the particular Gd sites with the magnetic order observed with neutron diffraction is facilitated by a detailed analysis of the crystal fields acting on the sites. The observed ordering phenomena underline the complex multiaxial anisotropy in this system.