No abstract

R 2 Mo 2 O 7 compounds show a ferromagnetic metal-insulator spin-glass transition tuned by the radius of the rare earth ion R 3+ . We have studied Gd 2 Mo 2 O 7 located on the verge of the transition, by neutron diffraction on a 160 Gd isotopic sample, 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 K͒. The ordered magnetic moments at 1.7 K are parallel and equal to 5.7͑5͒ B and 0.8͑2͒ B for Gd and Mo, respectively. The relaxation rate measured by SR evidences strong spin fluctuations below T C and down to the lowest temperature ͑6.6 K͒. A spin reorientation occurs in the range 20 K Ͻ T Ͻ T C . The ferromagnetic state is strongly unstable under pressure. T C sharply decreases ͑down to 38 K at 1.3 GPa͒ 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.