Magnetic order in TbNiAl and TbCuAl intermetallic compounds

Polycrystalline samples of the ternary intermetallics TbNiAl and TbCuAl are investigated by means of magnetization and neutron diffraction measurements. When Cu replaces Ni in the crystal structure, the magnetic behaviour is shown to be totally different, although both elements are non-magnetic. Analysis of the neutron spectra by the Rietveld refinement method gives evidence that TbNiAl orders antiferromagnetically whereas TbCuAl becomes ferromagnetic. Frustrated spins at the Tb atoms are found only in the TbNiAl compound. Their alignment changes at a second magnetic transition. The magnetic structures and the transition temperatures are discussed.