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 c-axis and are incommensurate with the crystal structure (k = (1/2, 1/2, tau)). In the compounds containing the heavy rare-earths magnetic order is strictly commensurate. No ordered Ni moment was found.