Study of ferromagnetic-spin glass threshold in R2Mo2O7 by high-pressure neutron diffraction and μSR

We present a comparative study of (Tb0.8La0.2)(2)Mo2O7 and Gd2Mo2O7 situated at the verge of a Mott transition from ferromagnetic metal to insulating spin glass, which can be tuned by the rare-earth ionic radius. We probe the spin correlations and fluctuations versus temperature and pressure thanks to neutron scattering and mu SR. The ambient pressure state of ferromagnetic character shows striking differences between the two compounds, both in the static and dynamic properties, showing the influence of the Tb crystal field anisotropy. Under pressure, both compounds transform into a spin glass state. We also studied the chemically ordered spin glass Tb2Mo2O7 with pressure and temperature. We observe mesoscopic ferromagnetic correlations between Tb moments, together with short-range correlations. All persist down to very low temperature (40 mK), showing that the spin glass order corresponds to the magnetic ground state. Under pressure, the lengthscale of the mesoscopic correlations is strongly reduced, whereas the short-range correlations are unchanged.