Ferromagnetic - spin glass transition induced by pressure in the geometrically frustrated pyrochlore (Tb_(1-x)La_x)₂Mo₂O₇

We have studied (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$ pyrochlores by neutron diffraction and $\mu$SR at ambient and under applied pressure. Substitution of Tb for La expands the lattice and induces a change from a spin-glass like state ({\itshape x}=0) to a non collinear ferromagnet ({\itshape x}=0.2). In the ferromagnetic structure, the Tb moments orient close to their local anisotropy axis as for an ordered spin ice, while the Mo moments orient close to the net moment. The $\mu$SR dynamical relaxation rate shows a cusp-like peak at the Curie temperature T$_C$ and a broad anomaly at T$^\star$$<$T$_C$, suggesting a second transition of local or dynamical nature. Under pressure, the long range order breaks down and a spin glass-like state is recovered. The whole set of data provide a microscopic picture of the spin correlations and fluctuations in the region of the ferromagnetic-spin glass threshold.