  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 $RE_2$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 $P4_{1}2_{1}2$ ) 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 $P4_{1}^{'}2_{1}2^{'}$ 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 $P2_{1}^{'}2_{1}^{'}2$ 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.
