Microscopic study of a pressure-induced ferromagnetic -: Spin-glass transition in the geometrically frustrated pyrochlore (Tb1--xLax)2Mo2O7

We have studied (Tb1-xLax)(2)Mo2O7 pyrochlores by neutron diffraction and mu SR at ambient and under applied pressure. (Tb,La) substitution expands the lattice and induces a change from a spin-glass-like state (x=0) to a noncollinear ferromagnet (x=0.2). In the ferromagnetic structure, the Tb moments orient close to their local anisotropy axes as for an ordered spin ice, while the Mo ones orient close to the net moment. The temperature dependence of the mu SR relaxation rates and static local fields suggests a second transition of dynamical nature below the Curie transition. Under pressure, the long range order breaks down and a spin-glass-like state is recovered. The whole set of data provides a microscopic picture of the spin correlations and fluctuations in the region of the ferromagnetic-spin-glass threshold.