Spin structures with frustrated moments in RNiAl intermetallic compounds

Magnetic order in the ternary equiatomic intermetallic compounds PrNiAl, DyNiAl and HoNiAl is investigated by means of neutron powder diffraction and dc- and ac-susceptibility measurements. DyNiAl shows two magnetic phases: It is ferromagnetic between T₁ = 15 K and T_c = 31 K and a tilted antiferromagnet below T₁. HoNiAl is a ferromagnet in a narrow temperature region just below T_c = 14.5 K, but furthermore exhibits two tilted antiferromagnetic phases (T₁ = 5.5 K, T₂ = 12.5 K). The corresponding magnetic structures differ in the ordering of some of the magnetic moments which are coupled by frustrated bonds. The existence of those frustrated moments, which is induced by the topology is a common phenomenon in the hexagonal RNiAl series. They are manifested with reduced mean values of their moments in the elastic neutron spectra. Another interesting observation is the scaling of the second temperature T₁ at which the frustrated spins change their order: For all heavy rare earths investigated so far (R = Tb, Dy, Ho) we find T₁:T_N,C≈1:2. PrNiAl displays an incommensurate sinusoidal (TSW) structure with XY spin behaviour below T_N = 6.9 K, and only one ordered phase is found. The observed ordering temperatures roughly follow the de Gennes scaling factor.