Low-temperature anomaly of a spin-glass type in ferromagnetic R ₂Mo ₂O ₇ with R = Sm and Nd

We have studied the magnetic and thermodynamic properties and the muon spin relaxation of the ferromagnetic pyrochlore compounds R ₂Mo ₂O ₇ (where R, = Sm and Nd). Apart from the already known ferromagnetic transition due to Mo moment ordering, there is another low-temperature anomaly at 18.6 K for Sm ₂Mo ₂O ₇ and at 18 K for Nd ₂Mo ₂O ₇. This low-temperature anomaly is observed in all the measured properties and resembles a spin-glass transition. In the ferromagnetic phase, we also find that the ac susceptibility exhibits a strong frequency dependence, indicating that some sorts of spin-glass components coexist with the ferromagnetically ordered moments. This metastability of the ferromagnetic phase is also borne out by the unusual spin relaxation behavior seen in the muon spin-relaxation measurements. Our results demonstrate that ferromagnetic R ₂Mo ₂O ₇ has very complex spin dynamics, which consists at least of two competing components: The ferromagnetic and the spin-glass components. This competition is seen to exist from the ferromagnetic transition down to the lowest measured temperature. With lowering temperature, the spin-glass component appears to gain in strength, at least in the time window of the bulk magnetization experiment. We discuss possible origins for the unusual spin dynamics.