Dynamics of frustrated magnetic moments in TbNiAl

We have studied the dynamics of Tb spins in the intermetallic compound TbNiAl in the paramagnetic and ordered antiferromagnetic (afm) phases by means of neutron time-of-flight and spin-echo spectroscopy. TbNiAl is a frustrated antiferromagnet (TN=47 K). In the afm phase both regular long-range-ordered Tb spins as well as frustrated Tb spins exist (the latter have a reduced moment). The quasielastic measurements show that in the ordered phase the frustrated moments relax on a time scale of 0.01 ns to 0.1 ns. The intermediate scattering function I(q,t) is q-independent and exponential in time. Below TN, in the spin-echo time window, I(q,t) reaches a plateau above zero, while above TN the relaxation is complete (the correlation function converges to zero).