Dynamics of frustrated magnetic moments in antiferromagnetically ordered TbNiAl probed by neutron time-of-flight and spin-echo spectroscopy

We have studied the dynamics of Tb spins in the intermetallic compound TbNiAl in the paramagnetic (pm) and ordered antiferromagnetic (afm) phases by means of neutron time-of-flight and spin-echo spectroscopy. It is a remarkable and very unusual characteristic of TbNiAl that its afm phase (below ${T}_{N}=47 \mathrm{K})$ contains regular long-range ordered spins as well as frustrated spins. The latter are identified by a strongly reduced moment measured by neutron diffraction. The new quasielastic measurements show that the frustrated moments relax on a time scale of 0.01 ns to 0.1 ns. Their autocorrelation function $I(q,t)$ is q independent and exponential in time. While in the pm phase the spin relaxation is complete, i.e., $I(q,t)$ goes to zero in the time range of the measurement, in the afm phase $I(q,t)$ stays above zero.