Study of ferromagnetic–spin glass threshold inR2Mo2O7 by high-pressureneutron diffraction and µSR

Abstract

We present a comparative study of(Tb0.8La0.2)2Mo2O7 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 andfluctuations versus temperature and pressure thanks to neutron scattering andµSR. The ambient pressure state of ferromagnetic character shows striking differencesbetween the two compounds, both in the static and dynamic properties, showing theinfluence of the Tb crystal field anisotropy. Under pressure, both compoundstransform into a spin glass state. We also studied the chemically ordered spin glassTb2Mo2O7 with pressure and temperature. We observe mesoscopic ferromagnetic correlations betweenTb moments, together with short-range correlations. All persist down to very lowtemperature (40 mK), showing that the spin glass order corresponds to the magnetic groundstate. Under pressure, the lengthscale of the mesoscopic correlations is strongly reduced,whereas the short-range correlations are unchanged.