Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{ 2}$Ge$_{ 2}$O$_{ 7}$ and Yb$_{ 2}$Ge$_{ 2}$O$_{ 7}$ show that both systems are antiferromagnetically ordered in the $\Gamma_5$ manifold. The ground state is a $\psi_{ 3}$ phase for the Er sample and a $\psi_{ 2}$ or $\psi_{ 3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{ 2}$B$_{ 2}$O$_{ 7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{ \pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].
Categories: cond-mat.str-el