Frustrated magnetic moments in RNiAl intermetallic compounds

Magnetic order in rare-earth-based intermetallic compounds of the series RNiAl (R = Pr, Nd, Tb, Dy, Ho) is investigated by means of neutron diffraction and susceptibility measurements. Nearest neighbours of the rare earths form a triangular lattice in the hexagonal crystal structure. Due to antiferromagnetic coupling in the a b plane this gives rise to frustration of one-third of the magnetic moments of the rare earths R. We always find long-range magnetic ordering of the frustrated moments as well as of the non-frustrated ones in different spin structures. At cryogenic temperatures, the spin structures of the compounds with the light rare earths are sinusoidally modulated along the hexagonal oaxis and are incommensurate with the crystal structure (k = (1/2, 1/2, τ)). In the compounds containing the heavy rare-earths magnetic order is strictly commensurate. No ordered Ni moment was found.