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 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.