Pressure-induced ferromagnet to spin-glass transition in Gd2Mo2O7

R2Mo2O7 compounds show a ferromagnetic metal-insulator spin-glass transition tuned by the radius of the rare earth ion R3+. We have studied Gd2Mo2O7 located on the verge of the transition, by neutron diffraction on a Gd-160 isotopic sample, mu SR and x-ray diffraction using the synchrotron radiation. All measurements were done both at ambient and under applied pressure. At ambient pressure, a ferromagnetic state is observed below the Curie temperature (T-C=70 K). The ordered magnetic moments at 1.7 K are parallel and equal to 5.7(5) mu(B) and 0.8(2) mu(B) for Gd and Mo, respectively. The relaxation rate measured by mu SR evidences strong spin fluctuations below T-C and down to the lowest temperature (6.6 K). A spin reorientation occurs in the range 20 K < T < T-C. The ferromagnetic state is strongly unstable under pressure. T-C sharply decreases (down to 38 K at 1.3 GPa) and Bragg peaks start to coexist with mesoscopic ferromagnetic correlations. The ordered moments decrease under pressure. At 2.7 GPa long range magnetic order completely breaks down. In this spin-glass state, Gd-Gd spin correlations remain ferromagnetic with a correlation length limited to the fourth neighbor, and Gd-Mo spin correlations turn to antiferromagnetic. The unique combination of three microscopic probes under pressure provides a detailed description of the magnetic transition, crucial for further theories.