Long-range antiferromagnetic order in the frustrated XY pyrochlore antiferromagnet Er₂ Ge₂ O₇

A cubic Er2Ge2O7 pyrochlore was prepared under high-pressure and high-temperature conditions and its magnetic ground state was investigated by measurements of specific heat, dc and ac magnetic susceptibility as functions of temperature, pressure, and magnetic field. We found that Er2Ge2O7 undergoes a long-range antiferromagnetic transition at TN ≈ 1.4 K, which can be further enhanced by applying external physical pressure. On the other hand, application of external magnetic fields suppresses the antiferromagnetic order to zero temperature around Hc ≈ 2.3 T, where a magnetic-field-induced spin-flop transition was observed. Hc increases accordingly with increasing TN under external pressure. A comparison of the magnetic ground states and structural variations along the isostructural series Er2B2O7 (B = Sn, Ti, Ge) together with the high-pressure study on Er2Ge2O7 indicated that the magnetic properties of these highly frustrated XY pyrochlore antiferromagnets are very sensitive to the minute structural changes that determine the anisotropic exchange interactions and the local crystal-electric-field environments of Er3+ ions.