Local-Ising type magnetic order and metamagnetism in the rare-earth pyrogermanate Er${}_2$Ge${}_2$O${}_7$

Abstract

The recent discoveries of proximate quantum spin-liquid compounds and their potential application in quantum computing informs the search for new candidate materials for quantum spin-ice and spin-liquid physics. While the majority of such work has centered on members of the pyrochlore family due to their inherently frustrated linked tetrahedral structure, the rare-earth pyrogermanates also show promise for possible frustrated magnetic behavior. With the familiar stoichiometry $R{E}_2$Ge${}_2$O${}_7$, these compounds generally have tetragonal symmetry with a rare-earth sublattice built of a spiral of alternating edge and corner sharing rare-earth site triangles. Studies on Dy${}_2$Ge${}_2$O${}_7$ and Ho${}_2$Ge${}_2$O${}_7$ have shown tunable low temperature antiferromagnetic order, a high frustration index and spin-ice like dynamics. Here we use neutron diffraction to study magnetic order in Er${}_2$Ge${}_2$O${}_7$ (space group $P{4}_1{2}_{1}2$ ) and find the lowest yet Neél temperature in the pyrogermanates of 1.15 K. Using neutron powder diffraction we find the magnetic structure to order with $k=(0,0,0)$ ordering vector, magnetic space group symmetry $P{4}_1^{{}^{\prime }}{2}_1{2}^{{}^{\prime }}$ and a refined Er moment of $m=8.1{\mu }_B$ - near the expected value for the Er${}^{3+}$ free ion. Provocatively, the magnetic structure exhibits similar 'local-Ising' behavior to that seen in the pyrocholres where the Er moment points up or down along the short Er-Er bond. Upon applying a magnetic field we find a first order metamagnetic transition at $\sim$ 0.35 T to a lower symmetry $P{2}_1^{{}^{\prime }}{2}_1^{{}^{\prime }}2$ structure. This magnetic transition involves an inversion of Er moments aligned antiparallel to the applied field describing a class I spin-flip type transition, indicating a strong local anisotropy at the Er site - reminiscent of that seen in the spin-ice pyrochlores.

11 pages, 8 figs