Investigations of the spin dynamics of the geometrically frustrated pyrochlore $({\mathrm{Tb}}_p{\mathrm{Y}}_{1-p}{)}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, using muon spin relaxation and neutron spin echo, as a function of magnetic coverage $p$, have been carried out. Our major finding is that paramagnetic fluctuations prevail as $T\to 0$ for all values of $p$, and that they are sensitive to dilution, indicating a cooperative spin motion. However, the percolation threshold $p_c$ is not a critical point for the fluctuations. We also find that the low temperature spectral density has a $1/f$ behavior, and that dilution slows down the spin fluctuations.

• Received 30 September 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.107204