Local-Ising-type magnetic order and metamagnetism in the rare-earth pyrogermanate Er2Ge2O7

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 ${R}₂{\mathrm{Ge}}₂{\mathrm{O}}₇$, 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 ${\mathrm{Dy}}₂{\mathrm{Ge}}₂{\mathrm{O}}₇$ and ${\mathrm{Ho}}₂{\mathrm{Ge}}₂{\mathrm{O}}₇$ 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 ${\mathrm{Er}}₂{\mathrm{Ge}}₂{\mathrm{O}}₇$ (space group $P{4}₁{2}₁2$) and find the lowest yet Ne\'el 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 $P{4}₁^{{}^{\ensuremath{'}}}{2}₁{2}^{{}^{\ensuremath{'}}}$, and a refined Er moment of $m=8.1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_B$ near the expected value for the ${\mathrm{Er}}³⁺$ 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 $\ensuremath{\sim}0.35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ to a lower symmetry $P{2}₁^{{}^{\ensuremath{'}}}{2}₁^{{}^{\ensuremath{'}}}2$ 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.